Investigation on low temperature pyrolysis of pentachlorophenol contaminated soil

In this dissertation, investigation of PCP pyrolysis in soil at a relatively low temperature range 150-400oC and the behavior of PCDD/Fs formation, dechlorination and destruction during

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論文名稱: Investigation on Low-Temperature Pyrolysis of Pentachlorophenol-Contaminated Soil 指導教授姓名： 張木彬 博士

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Abstract

High energy cost and potential formation of dioxins during incineration/combustion of pentachlorophenol (PCP) have limited their application on simultaneous removal of highly contaminated soil of PCP, polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) at AnShun

in Taiwan In this dissertation, investigation of PCP pyrolysis in soil at a relatively low temperature range (150-400oC) and the behavior of PCDD/Fs formation, dechlorination and destruction during pyrolysis of PCP-contaminated soil have been examined in detail Most PCP (>90%) and PCP byproducts can be removed from soil at 350oC for 40 min The PCP decay rates from soil increased exponentially from 0.20 to 1.96 min-1 as temperature was increased from

150oC to 400oC Very low levels of PCDD/Fs were found in soil (0.38-2.48 ng TEQ/kg) and gaseous phase (0.0015-0.0044 ng TEQ/Nm3) during pyrolysis of PCP-contaminated soil 70% of PCP removal from the soil was achieved, resulting in 1436230 ng/kg, the highest PCDD/F formation at 250oC; however, the highest toxic concentration was measured around 4.20  0.62

ng TEQ/kg at 300oC with 80% PCP removal from the soil Further analysis has revealed that OCDD is the most dominant congener supposed to form from the pyrolysis of PCP, while OCDF

is the second prevailing congener, possibly due to 2,3,4,5-TeCP reaction which is a main byproduct of PCP pyrolysis Detection of less chlorinated dioxins and furans over 300oC indicates the dechlorination of highly chlorinated dioxins and furans, especially OCDD at 350oC and 400oC Desorption from soil was supposed as a main mechanism for the distribution of PCDD/Fs in the gas phase, and not much difference in dioxins and furan levels was observed at

350oC and 400oC in the gas phase In order to test the nZVI reactivity with PCP, a thermally enhanced pump-and-treatment method coupled with nZVI was proposed to remove PCP from soil and to detoxify aqueous phase and soil The results indicated that total PCP removal in soil and

aqueous phases increased with increasing nZVI dose The PCP distribution in aqueous phase was enhanced when PCP-contaminated soil was remediated with nZVI In addition, decrease in pH resulted in decreasing PCP distribution in aqueous phase but increasing PCP dechlorination Dechlorination rate was enhanced from 2.26 to 6.84 h-1 as the temperature was increased from

25oC to 85oC Dechlorination and PCP residual in soil were increased to 42% and decreased to 6%, respectively, at 85oC and pH1 The dechlorination of PCP preferred to occur at ortho> meta> para positions in the respect of OH group Based on the results of nZVI reactivity with thermal enhancement, the combination of pyrolysis and nZVI was proposed to investigate the PCP removal from soil Consequently, the decay rate constant (k) of pyrolysis combined with nZVI increased exponentially from 0.59 to 3.67 min-1 which were 4 times higher than that without nZVI in the temperature range of 150°C -300°C The activation energies of PCP removal from soil with and without nZVI are 23.80 and 36.98 kJ/mol, respectively PCP degradation increases linearly with increasing nZVI dose The rate decay constant increased from 0.21 min-1 to 1.56 min-1 as nZVI dose was increased from 0% to 10% at 200oC The order of PCP dechlorination during pyrolysis coupled with nZVI is the same as that in the absence of nZVI but dechlorination process during pyrolysis with nZVI occurred more completely into the final product as phenol Increasing temperature to 300oC resulted in the predominant TeCP ((0.4  0.1) %) in soil and none byproducts was detected in soil as either temperature or time was increased above 300oC and 30 min, respectively Especially, both PCP and byproducts were not detected in gaseous phase This study provides relevant information for risk assessment for PCP contaminated soils when low thermal pyrolysis is applied for remediation of PCP contaminated soil

Key words: Pentachlorophenol, low thermal, nano scale zero valent iron, soil, byproducts, dioxin formation

為 測 試 nZVI 對 於 PCP 的 活 性 ， 熱 促 進 處 理 方 法 (thermally enhanced

pump-and-treatment method)與 nZVI 被用來移除土壤中的 PCP 和去除土壤中和液相之

毒性。結果顯示土壤中與液相之 PCP 去除效果隨 nZVI 劑量增加而上升。利用 nZVI 復育

土壤時 PCP 在水中的分布會增加。而 pH 降低會導致 PCP 在液相中分布減少但 PCP 脫氯

Acknowledgements

First of all, I would like to express my great appreciation to my advisor, Professor Moo Been Chang, for his guidance, patience and encouragement during the past 4 years This dissertation could not be finished without his continuous support I also wish to thank Mrs Chang for her kindness

Special thanks to Assistant Professor Kai Hsien Chi for his help, good advice and experiences on doing research and writing paper

The author is greatly appreciated to Mr Pao Chen Hung, Mr Shu Hao Chang and Nguyen Thanh Dien for their assistance in dioxin analysis and nZVI synthesis

I gratefully acknowledge all faculty members of Graduate Institute of Environmental Engineering who have provided the basic knowledge, support and encouragement during my studies at NCU

I would like to thank all Taiwanese students of Graduate Institute of Environmental Engineering, especially my labmates in Professor Chang group for the friendship and willingness to help me during the experiment

I would also like to thank my Vietnamese friends at NCU for their funny stories and daily encouraging conversations

I would like to thank the financial support from National Science Council (NSC E-008-019-M Y3), National Central University and Vietnam National University, Hochiminh City

98-2221-Finally but perhaps most importantly, I would like to delicate this dissertation to my parents and my husband who are always by my side to help me getting balance my life and show me the value of love, hard work and perseverance

NCU, 16/07/2012 Ngo Thi Thuan

Table of Contents

Abstract

Chapter1 Introduction

1.1 Background and motivation ……… 1

1.2 Objectives and Scope……… 3

Chapter 2 Literature Review 2.1 History of production and use of PCP ……… 6

2.2 History of An Shun site ……… 6

2.3 Chemical and physical properties of PCP ……… 7

2.4 Chemical and properties of PCDD/Fs ……… 8

2.5 Technologies for PCP remediation from soil ……… 9

2.6 Thermal remediation technologies ……… 10

2.6.1 Incineration/ combustion ……… 10

2.6.2 Potential PCDD/Fs formation from chlorophenols during thermal…… 11

2.6.3 Mechanism of PCDD/F formation from chlorophenols ……… 13

2.7 Low-thermal technologies ……… 15

2.7.1 Thermal desorption ……… 15

2.7.2 Low-temperature pyrolysis ……… 16

2.8 Removal of chlorinated phenols by nZVI ……… 19

2.8.1 nZVI characteristics ……… 19

2.8.2 nZVI synthesis ……… 19

2.8.3 General mechanisms of pollutant remediation by nZVI ………… 21

2.8.4 Literature review on remediation of chlorinated compounds with nZVI… 21

Chapter 3 Materials and Experimental Methods

3.1 Materials ……… 55

3.2 Soil preparation ……… 55

3.3 Experimental systems ……… 56

3.3.1 Thermal system ……… 56

3.3.2 nZVI system for testing nZVI reactivity with PCP contaminated soil 57

3.3.2.1 nZVI synthesis ………57

3.3.2.2 Experimental setup for PCP degradation with nZVI ………57

3.4 Analytical methods ……… 58

3.4.1 Analytical methods for PCP determination in soil ……… 58

3.4.2 Analytical methods for PCDD/F ……….……… 59

3.4.3 Analytical methods for chloride determination ……… 60

3.4.4 Instruments for analysis of nZVI reactivity ……….60

Chapter 4 Results and Discussion 4.1 Analytical methods for PCP analysis ……… 66

4.2 Degradation of PCP contaminated soil with low thermal pyrolysis (200-400 o ) …… 66

4.2.1 Kinetics of PCP removal from soil ………66

4.2.2 Analysis of the temperature impact on the fate of PCP during pyrolysis ….69

4.2.3 Analysis of the temperature impact on byproduct releases……….70

4.2.4 Analysis of time impact on byproduct releases ………71

4.2.5 Formation and degradation of PCDD/Fs in soil……… 71

4.2.6 Formations and degradation of PCDD/F congeners in soil……….73

4.2.7 Formation and degradation of PCDD/Fs in gaseous phase ………75

4.2.8 Proposed pathways leading to PCDD/F formation and removal ………… 76

4.2.9 Possible overall pathways of PCP removal from soil ……… 77

4.3 PCP degradation in slurry soil with nZVI……… 79

4.3.1 Characteristics and reactivity of nZVI ……….79

4.3.1.1 Effect of synthesis environment ……… 79

4.3.1.2 Effect of acid washing after reaction ……… 81

4.3.2 Effect of nZVI dose ……….81

4.3.3 Effect of initial pH on PCP removal from slurry soil……… 81

4.3.4 Effect of temperature ……… 83

4.3.5 Effect of time ……… 84

4.3.6 Identification of byproducts ……… 84

4.4 PCP degradation in soil with nZVI coupled with thermal……….85

4.4.1 Temperature effect on PCP removal from soil ………85

4.4.2 Effect of nZVI dose ……… 88

4.4.3 Analysis of PCP byproducts ……… 89

4.4.3.1 Temperature effect on byproduct releases ……… 89

4.4.3.2 Time effect on byproduct releases ………90

4.4.4 Pathways of pyrolysis of PCP contaminated soil in the presence of nZVI 91

Chapter 5 Conclusions and Perspectives

5.1 Conclusions ……… ……… 116 5.2 Perspectives ……… 119 References ………120

Table Caption

Table 2-1 Potential sources of occupational exposure to PCP and PCP impurities …………32

Table 2-2 Physical and chemical properties of chlorophenols……… 33

Table 2-3 Gibbs energy of chlorophenol formation ………34

Table 2-4 The TEF scheme for dioxins and furan……….35

Table 2-5 The factors affecting on PCDD/F formation from chlorophenols………36

Table 2-6 Paper review of PCDD/F formation from chlorophenols……… 37

Table 2-7 Literature review of POPs thermal remediation……… 39

Table 2-8 Review of characteristics of iron particles synthesized with different methods… 41

Table 2-9 Factors affecting reactivity of nZVI……….42

Table 2-10 Paper reviews on rate constants in the degradation of several chlorinated phenols by various metals/ metal combinations……… 43

Table 2-11 Summaries of possible reaction pathways for remediation of nZVI with pollutants 45

Table 2-12 Literature reviews of nZVI applications in the remediation of various pollutants in water and soil………46

Table 3-1 Main physical-chemical properties of soils collected……… 65

Table 4-1 Temperature effect on byproduct releases in gaseous phase and soil during pyrolysis

of PCP-contaminated soil……… 98

Table 4-2 The measurement of rate constant (k) and activation energy (Ea) of PCP

contaminated soil during pyrolysis with and without nZVI……… 114

Table 4-3 The pseudo-first-order constants of PCP by nZVI at 200oC and kinetic curve

regression correlation coefficients (R2)……….114

Table 4-3 Yield (%) of chlorinated phenol congeners from pyrolysis of PCP contaminated soil

in the presence of 5% nZVI……… 115

Figure Caption

Figure 1-1 Scope of research………5

Figure 2-1 General structures of PCDD and PCDF………24

Figure 2-2 Pilot-scale diagram of rotary kiln incinerator……… 24

Figure 2-3 Formation of the phenoxy radical in the presence of oxygen ……… 25

Figure 2-4 PCDD congener distribution under pyrolytic and oxidative conditions ………… 25

Figure 2-5 Mechanism of the formation of PCDD/PCDF via phenoxy radicals………26

Figure 2-6 Mesomeric structures and spin densities of the phenoxy radical……… 26

Figure 2-7 Adsorption of chlorophenols on transition metals ….……… 27

Figure 2-8 Mediated formation of PCDF from keto carbon centered radicals on CuO surface……… 27

Figure 2-9 Mediated formation of PCDD from chlorophenol oxygen centered radicals on CuO surface……… 28

Figure 2-10 Pilot scale of low thermal system……….28

Figure 2-11 Effect of metal oxides on dechlorination of OCDD……….29

Figure 2-12 The total suppressing efficiencies of metal compounds on PCDD/Fs researched under 280oC for 2h………29

Figure 2-13 Standard reduction potentials ……… 30

Figure 2-14 Eh-pH diagram for the simple ions of iron at 25oC and 1 bar……… 30

Figure 2-15 Typical nZVI particle surface area……… 30

Figure 2-16 The core-shell model of zero-valent iron nanoparticles……… 31

Figure 2-17 Depicting various mechanisms for the removal of metals and chlorinated compounds by catalyzed nZVI……….31

Figure 2-18 The average absorbed power of Chlorobenzene solution with microwave irradiation time at 2.45 GHz, MW input power 250 W……… 31

Figure 3-1 Experimental setup for pyrolysis of PCP contaminated soil………62

Figure 3-2 Experimental setup for PCDD/F sampling during PCP contaminated soil pyrolysis ……… 62

Figure 3-3 Temperature profile in reactors ………63

Figure 3-4 Experimental setup for nZVI synthesis……….63

Figure 3-5 Procedure for PCDD/F clean up in soil………64

Figure 3-6 13C-labeled recoveries of PDDD/F extraction and cleanup procedure……….65

Figure 4-1 Effect of solvent with Soxhlet extraction……… 92

Figure 4-2a Effect of various extraction methods on PCP recovery ……… 92

Figure 4-2b Time effect on PCP recovery from extraction……… 92

Figure 4-3 Residual PCP concentrations as a function of time at various temperatures………93

Figure 4-4 Temperature effect on fate of neat PCP and PCP-contaminated sandy soil during

pyrolysis at  = 30 min……… 93

Figure 4-5a Effect of temperature on byproduct yield in soil from pyrolysis of PCP

contaminated sandy soil at  = 30 min……… 94

Figure 4-5b Effect of temperature on byproduct release to gas from pyrolysis of PCP

contaminated sandy soil at  = 30 min……… 94

Figure 4-6a Effect of time on byproduct yield in the soil from pyrolysis of PCP contaminated

Figure 4-6b Effect of time on byproduct release to the gas from the pyrolysis of PCP

contaminated sandy soil at 350oC……….94

Figure 4-7 PCDD/F formation from pyrolysis of clean soil and PCP-contaminated soil…… 95

Figure 4-8a Effect of temperature on dioxin formation from pyrolysis of PCP-contaminated

soil ………95

Figure 4-8b Effect of temperature on PCDD/F release into gas phase during pyrolysis of PCP-

contaminated soil……… 95 Figure 4-9 Effect of temperature on congener formation in pyrolysis of PCP-contaminated

soil……….96

Figure 4-10 Effect of temperature on congener release into gas phase during pyrolysis of PCP-

contaminated soil……… 96

Figure 4-11 Mechanisms leading to OCDD and OCDF formation in pyrolysis of

PCP-contaminated soil……… 97 Figure 4-12 Pathways of PCP removal from contaminated-sandy soil during pyrolysis in the

temperature range of 200-400oC……… 99 Figure 4-13 XRD of nZVI in (a) air condition, (b) N2 condition……… 100

Figure 4-14 Testing reactivity of nZVI synthesized in water and ethanol condition………….100

Figure 4-15 Effect of acid washing on PCP recovery after 10 h reaction ……… 101

Figure 4-16 Effect of nZVI dose on PCP degradation and PCP distribution in soil and water 101

Figure 4-17 Effect of pH value on PCP distribution in soil and liquid……… 102

Figure 4-18 Effect of pH and nZVI dose on PCP removal from soil and liquid………102

Figure 4-19 Effect of temperature on PCP removal and PCP dechlorination from soil……….103

Figure 4-20 Effect of nZVI age on PCP dechlorination in soil……… 103

Figure 4-21 GC/MS chromatogram of PCP and byproducts in liquid phase at different

temperatures and pH values………104

Figure 4-22 Temperature effect on PCP removal from soil with nZVI and pyrolysis……… 105

Figure 4-23 Residual PCP concentrations during pyrolysis with nZVI as a function of time at

various temperatures……… 105

Figure 4-24a Activation energy of PCP degradation during pyrolysis in the temperature range

Figure 4-24b Activation energy of PCP degradation during pyrolysis coupled with nZVI in the

temperature range of 150-300oC……… 106

Figure 4-25 Effect of nZVI dose on PCP degradation during pyrolysis at 200oC………… 106

Figure 4-26 Effect of nZVI dose on PCP degradation during pyrolysis……….107

Figure 4-27 Temperature effect on byproduct releases during pyrolysis of PCP-contaminated

soil in the presence of 5% nZVI……… 107

Figure 4-28 Dechlorination process with nZVI during pyrolysis of PCP contaminated soil in

the temperature range of 150-250oC………108

Figure 4-29 Variations with time in the byproduct yield during pyrolysis of PCP contaminated

soil in the presence of 5% nZVI at (a) 200oC and (b) 250oC……… 109

Figure 4-30 GC/MS chromatogram of product release from pyrolysis of PCP contaminated

soil into (a) soil and (b) gaseous phase………110

Figure 4-31a GC/MS chromatogram of temperature effect on product release from pyrolysis

with 5% nZVI of PCP contaminated soil into soil ……… 111

Figure 4-31b GC/MS chromatogram of temperature effect on product release from pyrolysis

with 5% nZVI of PCP contaminated soil into gaseous phase……… 112

Figure 4-32 Pathways of PCP removal from contaminated-sandy soil during pyrolysis in the

temperature range of 150-400oC……… 113

Chapter 1 Introduction

An-Shun Site of Southern Taiwan, one example of dioxin formation from PCP, has been known for extremely high concentrations of pentachlorophenol (PCP), mercury (Hg) and polychlorinated dibenzo dioxins and furans (PCDD/Fs) in soil The high content of PCP (0.312 – 110 mg/kg), PCDD/Fs (36,000 ng I-TEQ/kg) and Hg (0.212 – 12,000 mg/kg) in soil around the factory is due to chlorination of the non-substituted phenol to produce chlorophenols It is well-known that when hydrophobic organic contaminants (HOCs) such

as dioxins and PCP are released into soil, they are persistent in soil The removal techniques

of these persistent organic contaminants are governed by the soil characteristics and the physicochemical properties of the contaminants A variety of technologies have been reported for individual or simultaneous removal of PCP and dioxins They include thermal, chemical treatments and electrolysis developed recently Advanced oxidation processes (AOPs) based

on the chemistry of hydroxyl radicals or ultrasound combined with elemental iron (US/Fe0) seem to be of good potential for eliminating hydrophobic organic contaminants However, their applications are still limited to water and application in contaminated soil remediation has been found to be inefficient mainly due to: (i) sorption of hydrophobic organic pollutant

to natural organic matter with a very slow desorption rate and (ii) the high reactivity of hydroxyl radicals with soil constituents Some studies have done on the biodegradation of PCP Due to their persistent characteristic, reductive dechlorination requires strict anaerobic conditions and a very long acclimation period On the other hand, a practical difficulty faced under field conditions is to provide a favorable redox potential and to keep rigorous conditions for microorganisms’ growth Electrochemical technology has been also employed

transformation are limited by soil characteristics including soil texture, soil humus and soil acid–base properties Hence, this technique is still under researched

Thermal treatments, considered as conventional techniques, are commonly used as a technique for simultaneous removal of organic persistent pollutants (POPs) with high concentration Incineration or combustion, normally related to the use of high temperature in the oxic environment, is proved as an efficient technique The combustion of POPs normally requires high temperatures (>1000OC) and excess oxygen However, energy cost and possible toxic dioxin formation in gas-phase from PCP as well as chlorophenol combustion have limited the use of these techniques On the other hand, incineration at the high temperature results in the “dead soil” which cannot be used to vegetate anymore

Low temperature thermal technique, known as desorption, volatilization or thermal stripping, is an ex-situ remedial technology that uses heat to separate pollutants physically from soil However, they are not designed to decompose contaminants The vaporized pollutants are generally treated in the secondary treatment units (e.g after burning, catalytic oxidation, condenser or carbon adsorption unit) before discharging to the atmosphere Recently, catalytic combustion is preferable to incineration, especially for chlorinated compounds due to some advantages: (1) significantly decreasing the temperature range to operate this process; (2) dechlorination ability to less toxic products and thus easy for further treatment However, it can still generate significant quantities of PCCD/PCDF, with CO2

release and products such as CO, Cl2, and COCl2 that are difficult to trap

Extensive research has been conducted to study the formation of PCDD/Fs in thermal processes In general, two mechanisms have been put forward primarily to explain dioxin synthesis - the precursor mechanism and the De Novo synthesis In the precursor mechanism, chlorophenols are major precursors for dioxin formation which involves phenoxy radicals

formation in the presence of oxygen and then their dimerization In the formation of PCDD/Fs via De Novo synthesis, on the other hand, carbon can be oxidized to form CO or CO2 and generate dioxins by oxygen (Eq 1-1, 1-2) Gaseous oxygen plays a crucial role in both cases

1.2 Objectives and scope

Three major objectives of this work are: 1) to investigate kinetic removal of PCP during low temperature pyrolysis; 2) to find out the best conditions of pyrolysis treatment and additives for PCP; 3) to understand the pathways of PCP removal from soil via pyrolysis

There are four reasons to initiate this research Firstly, as mentioned above, PCP and

dioxins, OCDD as a representative of dioxins are observed with relatively high concentrations

at An Shun site that needs to be remediated as soon as possible Secondly, it is suspected that

PCP is a main precursor of PCDD/F formation during condensation of thermal treatment,

known as the dimerization pathway of chlorophenoxy radicals In addition, the less

chlorinated phenols as byproducts of PCP during thermal treatment are very reactive, possibly affecting on PCDD/PCDF ratio and the formation of more toxic products in gas phase, such as

2,3,7,8-TeCDD formation from 2,4,5-TCP Thirdly, understanding the nZVI as dechlorination catalysts is necessary to enhance PCP degradation at lower temperature process Finally, the

pyrolytical process for PCP at relatively low temperature range (150-4000C) need to be optimized to reduce their remaining in contaminated soil, the energy cost and off-gas release

The research includes three parts:

1 Low thermal was chosen to evaluate the capability of PCP removal from soil under 200-400oC conditions Therefore, the first part of the dissertation is to investigate:

- Effect of temperature, time and flow rate on PCP removal

- Kinetics of PCP removal in the temperature range of 200-400oC

- Analysis of byproducts of PCP remediation in the soil and the gaseous phase during pyrolysis

- Elucidation of removal pathways

2 The second part involves the feasibility of nZVI application in PCP remediation of soil Therefore, the following parameters were investigated:

- nZVI characteristics and reactivity in PCP removal in water under different synthesis conditions

- nZVI doses on PCP removal

- Effect of pH, temperature, time on nZVI reactivity with PCP removal in water and slurry soil

- Analysis of byproducts and pathways of PCP removal

3 The final part involved the combination of thermal and nZVI to enhance PCP and byproduct removal The same parameters including temperature, time and nZVI doses were investigated and made comparison to indicate the effectiveness of method

Figure 1-1 Scope of research

Temperature variation

Time variation

nZVI characteristics:BET, XRD nZVI dose

Reaction time Temperature

Pathways Byproducts in soil and gaseous phase Evaluation of PCDD/F formation and degradation

Chapter 2 Literature Review

Pentachlorophenol (PCP) is mainly used as a general wood preservative, herbicide, pesticide and broad-spectrum biocide in a wide variety of agricultural and industrial applications (Fisher., 1991) Chlorophenols (CPs) in general are industrially produced by direct chlorination of phenol with chlorine gas, as well as other reactions, such as hydrolysis and hydro dechlorination or by chlorination of less chlorinated phenols in the presence of aluminum or iron trichloride About 96.5% of PCP is present in soil matrix around sawmills and wood preserving facilities PCP have been detected in groundwater, surface water, waste water, air and soils as a result of their improper disposal, leaching from landfills and the incineration of chlorinated wastes (Table 2-1)

An Shun site is located in coastal area of Tainan city and the soil’s characteristic is mainly sand and low fraction of clay The Tainan An-Shun plant was established by Japanese

in 1938 In the beginning, they produced sodium hydroxide (NaOH), hydrochloric acid (HCl), chlorine (Cl2), calcium hypochlorite, bromine and salt In 1964, they expanded their production with sodium pentachlorophenate manufacturing In 1982, An-Shun plant was shut down In 1994, dioxin contamination was detected in soil by National Tsing Hua University Right now, the significantly high concentrations of PCP, dioxin and mercury due to salt production are still found around this area This information has been modified in the final version of the dissertation

2.3 Chemical and physical properties of PCP(Corsby et al., 1981)

In general, the toxicity of CPs increases with the degree of chlorination but decreases with the degree of their dissociation As a result, at low pH, where the non-dissociated form of PCP

is dominant, its toxicity is greater The environmental prevalence of CPs is attributed to their recalcitrance that results from the carbon-halogen bond, which is cleaved with great difficulty and the stability of the aromatic structure PCP is moderately persistent in the soil environment and is difficult to treat

As any other chlorophenols or phenol, the hydroxyl group of PCP takes part in nucleophilic reactions Electron withdrawal by the ring-chlorines causes PCP to be unusually acidic [pKa 4.70 in water, roughly comparable to propionic acid, pKa 4.9] and a relatively weak nucleophile, while stabilizing its salts (sodium pentachlorophenate is a stable item of commerce) Although high degree of chlorination makes the aromatic ring sufficiently electropositive to form stable charge-transfer complexes with electron donors, the ring chlorines are as resistant to nucleophilic displacement under normal conditions as those of the chlorinated aromatic hydro carbon Oxidation of PCP produces pentachlorophenoxyl radicals which combine reversibly to form "dimers"

The adsorption of light energy allows PCP to undergo a number of reactions under very mild conditions; the long-wave absorption maximum is near 300 nm in organic solvents and below pH5 In either water or organic solvents, PCP undergoes photochemical reduction to isomeric tri- and tetrachlorophenols The pentachlorophenoxide anion (max 320 nm) also can displace chloride from PCP in sufficiently concentrated solution with eventual cyclization to OCDD in water at ambient temperature

Some common physical properties of PCP and other chlorophenols are summarized in

Table 2-2 & 2-3. The pure substances are white crystalline solids while the commercial material generally is seen as gray flakes Pure anhydrous PCP melts near 190oC (its

monohydrate melts at 174oC), but technical products may melt at 187-189oC or less due to impurities, PCP salts are highly melting

PCP partially decomposes to HCB, OCDD and tar at its boiling point, the vapor pressure

of 760 torr is achieved at 300.6oC but even at ambient temperatures PCP must be considered

to be relatively volatile NaPCP is nonvolatile, it’s sharp to form OCDD PCP is soluble in most sovents but only slightly soluble in water However, its solubility, volatility and partitioning must be considered in relation to its ionization At pH 2.7, PCP is only 1% ionized, while at 6.7, the pH of many natural waters, it is 99% ionized The 18 mg/L aqueous solubility (20oC) at the slightly acidic pH generated by its dissolution (pH 5) increases rapidly with increasing pH to over 2*105 mg/L as NaCP at pH 10

2.4 Chemical and physical properties of PCDD/Fs

Polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) comprised by cyclic ethers, planar and electropositive rings are rather stable chemically, environmental persistence and high bioaccumulation (Figure 2-1) There are 75 PCDD and

135 PCDF congeners and 17 congeners of 2,3,7,8-chlorine substituents are the most toxic congeners to human and animals Dioxins have been characterized by EPA as likely to be human carcinogens and are anticipated to increase the risk of cancer at background levels of exposure

The toxicity of dioxins is expressed based on International Toxic Equivalents (I-TEQ) Each congener has characterized by a toxic equivalency factor (TEF), which indicates the degree of toxicity and the list of TEF for each congener is listed in Table 2-4 2,3,7,8-TCDD

is the most toxic congener, it has been given a TEF of 1.0 The TEFs of the other dioxin congeners range between 0.5 and 0.0001 The sum of toxic equivalents (TEQ) congeners is calculated as the sum of the concentration of each congener multiplied by its TEF:

∑

Ci: the concentration of each congener Basically, dioxins have high melting points, low vapor pressures and low water solubility resulting in being adsorbed strongly on the surfaces of particulate matter These characteristics vary according to different congeners In general, water solubility and vapor pressure decrease and octanol solubility increases with increasing the number of chlorine atom substituents

2.5 Technologies for PCP remediation from soil

When hydrophobic organic contaminants (HOCs) such as PCP are released into soil, they are persistent in organic matter of soil The removal techniques of these persistent organic contaminants are governed by the soil’s characteristics and the contaminant’s physic-chemical properties A variety of technologies have been reported for individual or simultaneous removal of PCP They include thermal, chemical treatments and electrolysis developed recently Advanced oxidation processes (AOPs), which based on the chemistry of hydroxyl radicals (Liou et al., 2004) or ultrasound combined with elemental iron (US/Fe0) (Dai et al.,

However, their applications are still limited in water and contaminated soil remediation purposes has been found to be low efficient mainly due to: (i) sorption of hydrophobic organic pollutant to natural organic matter with a very slow desorption rate and (ii) the high reactivity

of hydroxyl radicals with soil constituents Some studies have done on the biodegradation of

requires strict anaerobic condition and a very long acclimation period On the other hand, a practical difficulty faced under field conditions is to provide a favorable redox potential and to keep rigorous conditions for microorganisms’ growth Electrochemical technology has been

Eq 2-1

system and PCP migration and transformation are limited by soil characteristics including soil texture, soil humus and soil acid–base properties Hence, this technique is still under

commonly used as a technique for simultaneous removal of organic persistent pollutants (POPs) with high concentration

2.6 Thermal remediation technologies

2.6.1 Incineration/ combustion

Incineration or combustion, normally related to use high temperature in the oxic environment, is proved as an efficient technique When incineration/combustion is used in the remediation of contaminated soils, temperature exceeding 1000°C is usually applied The high-energy demands make this technique costly, although when soils with high organic contents are treated, the energy gained from their combustion will help to reduce the total energy demands Therefore, mainly highly polluted areas with total organic contents exceeding 20-25% are likely to be considered for this kind of remediation The presence of oxygen produces two opposite effects: on one hand, it favored the formation of free radicals, leading to higher rates of pyrolytic reactions and therefore to an increase in the yields of hydrocarbons; on the other hand, there was an oxidative destruction, which favored oxidative reactions of pyrolytic products, leading to a decline in their yields At low temperatures, the first effect prevailed, whereas at higher temperatures the second was the predominant one (Aracil and Conesa, 2010)

The commercial pilot of incineration is the rotary kiln at 700o-1000oC for dioxins and PCBs to enhance the evaporation of pollutants from soil (Figure 2-2) (Acharya et al., 1995)

The treated soil is quenched and the off gas is conducted into secondary combustion chamber

at higher temperature at 2000oC to destroy the pollutant vapor completely The main disadvantage of this system is high energy supply Modern incinerators above are commonly

described to destroy pesticides, PCBs, PCDD/Fs and similar chemicals with dioxins very efficiently However, recent tests suggested that incinerators achieve destruction efficiencies that are lower than those achieved by certain non-combustion technologies In addition, some incinerators burning POPs (pesticides such as chlorophenols and PCBs) and other waste are associated with the spread of undestroyed and newly formed POPs (dioxins and furans) into the surrounding environment Lee et al (2008) reported that the total I-TEQ concentrations at outlet of gas stream were higher than the stationary limit when they treated PCDD/Fs in soil with a primary furnace (750oC and 8500C) and a secondary furnace at 1200oC while air pollution control devices (APCDs) are operated at the temperature range of 150oC-400oC Therefore, energy cost and possible toxic dioxin formation in gas-phase from chlorophenol combustion have limited the usage of these techniques In the other hand, incineration at the high temperature results in the “dead soil” Soil organic matter is fully decomposed and the microorganisms that support for plant life cannot grow in the dead soil without the extra aid of fertilizer

2.6.2 Potential PCDD/F formation from chlorophenols during thermal treatment

Chlorinated phenols are the main precursors of PCDD/Fs formation during combustion They can be formed in the homogenous as well as in the heterogeneous phase There are numerous researches on thermal formation of PCDD/Fs during thermal processes Knowledge

of dioxin formation from chlorophenols is necessary to prevent this source The basic questions regarding to the PCDD/Fs formations are addressed: what are the process parameters controlling the PCDD/F formation in heterogeneous and homogenous systems and these factors are summarized in Table 2-5

Surface: In general, most of fly ash or materials similar to fly ash such as carbon, graphite are responsible for promoting PCDD/F formation However, apart from fly ash, PCDD/Fs are also discovered to be formed at surface of a SiO2, Al2O3, Al2O3, glass wool, SiO2NaOH Both

gaseous and solid compounds appear to be capable of providing the necessary chlorine atoms for PCDD/F formation

Temperature: temperature of PCDD/F formation is in the range of 150 – 600oC It is believed that dioxin formation occurred in gaseous phase and surface mediated process However, the gas-phase reactions only occur above 600oC (Sidhu et al., 1995; Sidhu and Dellinger, 1997) while the surface mediated processes were observed in the 200 to 500oC range (Lomnickia and Dellinger, 2003) Dioxins on fly ash can be formed from dibenzo dioxin, phenol and chlorobenzen at 1500C The optimal temperatures for maximum formation vary with different surfaces, pollutants and the precursor concentrations (Huang & Buekens,

350-375oC for activated carbon/fly ash

Catalysts: during PCDD/F synthesis from chlorophenols, the presence of catalysts, especially transition metal oxides, is the most important factor to promote the PCDD/F reaction occurring faster Both CuCl2 and FeCl3 catalyze formation reactions of PCDD/F on carbon surface CuCl, CuCl2, CuO and CuSO4 have also been identified as catalysts while alkali earth metal oxide had promotion effect on suppressing for PCDD formation from pentachlorophenol (Qian et al., 2005) Especially, copper chloride promotes ring condensation reaction via Ullman reaction to form diphenyl ethers, finally PCDD/F formation via cyclization (Ryu, 2008) Experiments with carbon on fly ash or a model support generally yield PCDD/F for 2-4 h, eventually followed by a decrease of the PCDD/F concentration Such a decrease points to depletion of one of the reactants and shows that a formation and destruction are simultaneous reaction, the balance depending on the rates of both pathways

Atmosphere: Oxygen also enables formation of phenoxy radicals (Figure 2-3) In the

pyrolysis of ortho-chlorophenol in Argon atmosphere at 410oC (10 min), less than 0.1% of the ortho-chlorophenol condensed, about 12% in air and 95% in oxygen atmosphere However,

there are contradictory reports on this oxygen effect on increase and decrease of PCDD formation Lomnicki and Dellinger (2003) reported that higher chlorinated congeners disappeared at temperature above 350oC under pyrolytic conditions due to the higher rate of desorption of chlorophenol intermediates than that of chlorination while more highly chlorinated PCDDs were detected above 350oC than below that temperature under oxidative conditions (Figure 2-4)

Moisture: Experiments with and without water have yielded contradictory results regarding PCDD/F formation Possible effect of water could be as follows:

+ Presence of an additional hydrogen source which can increase dechlorination

+ Presence of an additional oxygen sources

+ Presence of a source of •OH radicals which play important role in formation

+ Competition with possible precursors for adsorption at surface

+ Water could change the equilibrium in the Deacon reaction and consequently

2.6.3 Mechanisms of PCDD/F formation from chlorophenols

PCDD/Fs are formed from precursors (chlorophenols, chlorobenzenes or chlorinated biphenyls) via condensation process much faster than from de novo synthesis The formation

of intermediates or their condensation to PCDD/Fs and ratios of PCDD to PCDF is highly dependent on temperature, the oxygen concentration and the substitution pattern as well residence time There are numerous review papers regarding to dioxins and furans formation from chlorophenols during thermal treatments and are listed in Table 2-6 Therefore,

knowledge of the mechanism on the chlorophenol condensation to PCDD and PCDF is possible to explain thoroughly this temperature dependence and PCDD/F congener pattern

related to chlorophenols

Weber et al (1998) identified intermediates, analyzed PCDD/PCDF isomers and concluded that the intermediates (polychlorinated dihydroxybiphenyls) of PCDF are most likely formed by the dimerization of two phenoxy radicals at the hydrogen substituted carbons

in ortho-positions under simultaneous movement of the hydrogen atoms to the phenolic oxygen atoms Whereas PCDDs are formed in the gas phase due to the radical mechanism in the first condensation step to form ortho-phenoxyphenols (POPs), both a chlorine atom and the hydrogen atoms capable for substitution (Figure 2-5).

The phenoxy radical is not localized only at the oxygen atom This phenomenon was confirmed by spin densities in the phenoxy radical which were derived by quantum mechanical calculation (Figure 2-6). This explains that oxygen atom at the phenoxy radical is not the only reactive center The reactive sites are also at ortho- and para- positions If oxygen

of the phenoxy radical attacks a chlorophenol, it can substitute a chlorine atom in a radical substitution and a phenoxyphenol is formed In principle, if the first step the chlorine atom is substituted in ortho-position, another chlorine atom (also in ortho-position) can be substituted

to form a dibenzodioxin via the phenoxyphenol radical

Surface catalyzed chlorination reaction also has an important role on the PCDD/F formation Chlorophenols can be adsorbed on the surface and undergo a series of surface-catalyzed reactions (Lomonicki and Dellinger, 2003) In general, chlorophenols adsorb on the surfaces through H2O elimination at surface oxide and hydroxyl sites resulting in surface phenolate formation (Huang & Buekens, 2000; Bandara et al., 2001) Some authors propose that chlorophenolates withdraw electrons from the metal site leading to carbanion formation

(Voncina & Solmajer, 1998) However, chlorophenols are known to be electron donors (Bandara et al., 2001), it is more likely that the electron transfer is from chlorophenolate to the metal cation site (Mn+) forming surface-associated chlorophenoxyl radical and M (n+)-1 site It

is very interesting that the formation of phenoxyl radical was observed when experiment of

2-chlorophenol (Lomonicki and Dellinger, 2003) and PCP (Zhu & Shan, 2009) were carried out The most favorable positions for the electron to be localized are the oxygen atom and the ortho and para positions in the phenol ring (Weber and Hagenmaier, 1999; Mulholland et al., 2001) If ortho and para positions in the ring are substituted, the equilibrium between different mesomers is shifted toward the oxygen-centered phenoxyl radicals If ortho and para position are not substituted, the carbon centered keto form of the radical appears to be more stable on the surface (Figure 2-7)

Lomonicki and Dellinger (2003) suggested that two different surface mechanisms are involved in PCDD/F formation, PCDFs are formed according to the Langmuir –Hinshelwood pathway and PCDDs are produced as a result of an Eley-Rideal mechanism Keto form of carbon-centered radical is a precursor in PCDF formation As a result of surface radical-radical interaction, they undergo recombination and tautomerization leading to the adsorbed species, then eliminate hydrogen and ring close via a cyclic transition state that results in PCDF formation (Figure 2-8) Meanwhile, oxygen centered radicals of chlorophenol on metal surface can react with gas phase chlorophenol molecular desorbed from the surface, following hydrogen transfer to the surface and finally PCDD formation involves ring closure via HCl elimination (Figure 2-9)

2.7 Low- thermal technologies

2.7.1 Thermal desorption

Thermal desorption can be considered as a thermal separation technique, in which contaminated soils are heated to temperatures between 100 and 600oC to vaporize contaminants with boiling points in that range Comparing with incineration, thermal desorption is energy demanding and less costly However, the vaporized contaminants are not destroyed in this process, but they can be collected and destroyed by other methods This

process only works well on volatile compounds, while compounds with higher boiling point require higher temperatures

Low temperature thermal technique, known as desorption, volatilization or thermal stripping, is an ex-situ remedial technology that uses heat to separate pollutant physically from soil However, they are not designed to decompose contaminants The vaporized pollutants are generally treated in the secondary treatment units (e.g after burning, catalytic oxidation, condenser or carbon adsorption unit) before discharging to the atmosphere Recently, catalytic combustion is preferable to incineration, especially for chlorinated compounds due to some advantages: (1) significantly decreasing the temperature range to operate this process; (2) dechlorination ability to less toxic products and thus easy for further treatment However, it can still generate significant quantities of PCCD/PCDF, with CO2

release and products such as CO, Cl2, and COCl2 that are difficult to trap

2.7.2 Low-temperature pyrolysis

Low temperature pyrolysis process has been introduced and developed recently The first reduction of PCDDs/PCDFs in fly ash at 200oC-300oC in reducing atmosphere was discovered by Eicemann and Rghei (1982) Numerous studies have further proved the dechlorination and destruction of PCDD/Fs in fly ash at relatively low temperature (200oC -

600oC) (Table 2-7) A full scale system of thermal dechlorination at low temperature to

remove PCDD/Fs in fly ash has been developed (Figure 2-10) Low oxygen content is maintained in this system Temperature and retention time are two important factors for this process The suitable ranges of temperature and retention time should be 250oC - 400oC, 1-3h respectively and 99.9% dioxins were destructed in this condition In order to prevent further dioxin formation, the flue gas must cool below 60oC immediately (Ishida et al., 1998)

Many metal oxides are used to prove the dioxin dechlorination and destruction capacity of fly ash at low temperature under nitrogen or argon atmosphere Cu2O among many oxides (Cu2O and Cu(OH)2, Fe2O3, ZnO, PbO, and SnO) has the strongest dechlorination effect than other metal oxides, followed by element Cu and Cu(OH)2 Fe2O3 and SnO also showed the dechlorination potential for PCDDs and PCDFs (Figure 2-11) Model fly ash containing

Ca(OH)2 exhibited the highest destruction potential at 250oC, but a low dechlorination potential of dioxins (Weber et al., 2002) However, Qian et al., (2005) indicated that CuO, ZnO, MnO2, TiO2 and Co3O4 were observed to promote PCDD/Fs formation from PCP while alkali earth metal oxide such as CaO and BaO were supposed to have higher suppressing efficiencies (Figure 2-12) The different dechlorination capacity of CuO and destruction

capacity of CaO on PCDD/Fs in two types of fly ash were observed The removal efficiencies and behavior characteristics of PCDD/Fs after thermal treatment were different even at the same treatment conditions, because each congener has its own thermodynamic characteristics with different composition in fly ash (Song et al., 2008) In addition, dechlorination properties

of fly ash are also markedly influenced by physicochemical properties of copper and carbon The copper with a well-defined structure (dendritic or spheroidal) is much less suitable for the dechlorination than nanosize, activated forms Carbons with disordered structures are convenient for these reactions due to their facile gasification and graphitic forms of carbon practically do not react at all (Pekfirek et al., 2003) Improvement of reductive dechlorination

of PCDD/Fs in fly ash from municipal solid waste incinerator (MSWI) by sodium hypophosphite (NaH2PO2) was carried out in a lab scale experiment at about 250–450oC under oxygen deficient conditions Sodium hypophosphite generates reducing agents (PH3and H2) at about 300oC, which can increase the detoxification efficiency and accelerate the dechlorination reaction (Wang et al., 2006)

For soil treatment under low temperature, a dechlorination chemical is used to promote dechlorination process by providing hydrogen or a reducing radical containing hydrogen donor PCB-contaminated soil is mixed with sodium bicarbonate (NaHCO3) as one of examples This mixture is then heated to 300°C - 350°C in a reactor PCB removal of 99.79% was achieved and organic matter played the role of hydrogen donors during this process

aromatic compounds in soil The hydration of quicklime is highly exothermic reaction, and relatively high temperature can be reached in quicklime-amend soil (Sedlak et al., 1991) However, this effect is very minor on PCB destruction in soil Metallic calcium with solvent

or the combination of calcium and many other metals were proposed (Mitoma et al., 2004) The usage of metallic calcium and catalytic reduction by Rh/C in alcohol as a proton source and hydrogen, which was generated by a side reaction, causes an increase in the activity of Rh/C catalyst and promotes the decomposition efficiency (Tingyu et al., 2000) In addition, the presence of CaO was proved to decrease significantly the tar yield during pyrolysis (Mitoma et al., 2006)

Another study of hexachlorobenzene dechlorination was also carried out under temperature and oxygen deficient conditions on difference of supported solids such as SiO2, CaO, CaSiO3, and cement and treated fly ash (tFA) All the tested supports except SiO2

low-showed a HCB dechlorination potential The effect of Cu addition (0.2–5.0%) on HCB dechlorination might result from the Ullmann coupling which was not notable in enhancing the dechlorination reaction and dechlorination potential mainly depends on the support characteristic rather than the transition metal content The dechlorination/polymerization induced by the electron transfer mode was thought to be the dominant pathway while the hydrogen transfer mode was minor (Gao et al., 2007) A synergic effect of CaO and -Fe2O3

in closed systems at 300 – 350oC on dechlorination of hexachlorobenzene (HCB) was also

observed HCB was dechlorinated efficiently when CaO and -Fe2O3 coexisted (Ma et al., 2005)

2.8 Removal of chlorinated phenols by nZVI

2.8.1 nZVI characteristics

Iron is an essential element of living organism Iron belongs to group 8 and period 4 of the period table Iron is very active and easily oxidized in atmospheric conditions The standard reduction potential of iron species is drawn in Figure 2-13 In water system, ZVI particles can exhibit complexion with water depending on the solution chemistry ZVI (Fe2+/Fe) has a standard reduction potential (Eo) of -0.44 V When the pH solution is above 8, iron oxides are predominant species The oxide surface becomes negatively charged and can form complexes with other cations (Figure 2-14)

The major factor which defines the capability of nanoparticles as extremely effective material is their small sizes (1-100 nm) (Figure 2-15) The nZVI particles synthesized using

the sodium borohydride method are mostly spherical in shape and exist as chain-like aggregates (Figure 2-16) The mixed iron oxide and pure iron like the core-shell model depends on the fabrication processes and environmental conditions The oxide shell may consist mainly of maghaemite (-Fe2O3) and Fe2O3 with richer -Fe2O3 for smaller particles due to higher surface-to-volume ratio and rapid surface oxidation (Figure 2-16).

2.8.2 nZVI synthesis

The methods of preparation result in different sizes and shapes of nano-scale particles

(Table 2-8). Researchers has classified in two categories: Physical and chemical Physical methods include inert gas condensation, severe plastic deformation, high energy ball milling, ultrasound shot peening while chemical methods have reverse micelle, controlled chemical

co-precipitation, chemical vapor condensation, pulse electro deposition, liquid flame spray, liquid phase reduction and gas phase reduction (Li et al., 2006) Among these methods, liquid phase reduction which refers to reduction of Fe(II) or Fe(III) by BH4- has been widely used to synthesis nZVI The major advantages of ferric and ferrous salts with strong reductant addition are simplicity and chemical homogeneity and thus it can be employed by almost any laboratory without any special equipment Generally, the synthesis process of nZVI by NaBH4

reduction involves 4 steps: (1) super saturation of the solution; (2) nucleation of the nZVI cluster (3) growth of nZVI nuclei and (4) agglomeration (Schwarzer & Peukert, 2002) Synthesis conditions including the reductant delivery rate, a high concentration of reductant, the preparation medium, stirred conditions affect on surface area, crystallinity, reactivity and morphology of nZVI Reductant delivery rate and concentration of the reductant were key factors for deciding the growth and agglomeration time The higher the precursor concentration results in the higher surface area and reactivity of nZVI The precursor concentration not only affected the reactivity but also nZVI characteristics, while delivery rate affect reactivity only The particle size may dramatically decreases and the BET surface area increases under short reaction time and high concentrations of precursor and reductant

studies investigated nZVI synthesis in open air and in the presence of ethanol to prevent oxides/hydroxides iron of Fe2O3, Fe3O4, FeOOH formation (Liu et al., 2005a, Liu et al., 2005b) However, nZVI particles consist of a zero-valent core and an oxide shell with a thickness of about 3-5 nm is normally observed Wang et al (2006a) stated that an iron-boron noncrystalline alloy present on the nZVI surface but this layer can be removed by water washing (Nurmi et al., 2005)

Some factors affect on nZVI reactivity and decay rate constants of chlorophenol with ZVI reaction are summarized in Table 2-9 and Table 2-10

2.8.3 General mechanisms of pollutant remediation with nZVI

In general, there are three pathways for remediation of halogenated organic compounds

(Gunawardana et al., 2011):

- Dechlorination: degradation of the contaminant and formation of intermediates

- Sorption: accumulation of the contaminant molecules by adsorption or complex formation, without their degradation

- Precipitation: immobilization of the contaminant due to the formation of insoluble compounds

- Co-precipitation: nonspecific removal of the contaminant via entrapment in the matrix

of precipitating or recrystallizing iron oxides/hydroxides on the ZVI surface

However, co-precipitation and adsorption are other important mechanisms of pollutant removal but precipitation has been ignored because this mechanism has primarily reported in relation to the removal of heavy metals All mechanisms regarding to pollutant removal with nZVI are depicted and summarized in Figure 2-17 and Table 2-11

2.8.4 Literature review on remediation of chlorinated compounds with nZVI

Most of PCP remediation with nZVI was investigated in water Table 2-12 highlighted some recent researches on nZVI application for chlorinated aromatic compound remediation

in soil and aqueous phases

Kim and Carraway (2000) suspected that pentachlorophenolates can be chemisorbed on oxide surface via inner-sphere coordination and that cannot remove from oxide surface by washing with water Therefore, the total amount of unreacted PCP, both that in aqueous solution and that sorbed to ZVI-related surfaces were washed with 1 mL of concentrated HCl

after reaction As a result, mass balance is achieved with washing 1 mL of concentrated HCl and the loss due to sorption on ZVI surface is stronger than dechlorination Bimetallic Pd/Fe was observed to inhibit the rate of PCP dechlorination However, when Ni element was introduced as the second catalyst coated on nZVI surface for PCP dechlorination, 60% PCP removal efficiency was obtained and that was higher 10% PCP removal efficiency compared with nZVI only (Cheng et al., 2010) The high removal efficiency of PCP from water (96.2%) was obtained when nZVI was immobilized on the organobentonite Dechlorination occurred completely to the final dechlorinated product phenol (Li et al., 2011) Dechlorination of PCP

in water was also investigated with different surfaces (Fe(0), Mg(0), Pd/Fe, Pd/Mg) Neither iron system nor unmodified Mg was able to completely remove this chlorinated phenol congeners Both Pd/Fe and Pd/Mg systems were able to degrade PCP and tetrachlorophenol but no evidence of chlorinated phenol intermediates and mass balance cannot obtain in Pd/Mg system (Morales et al., 2002)

So far, the remediation of PCP contaminated soil with nZVI is not well addressed Some studies have investigated the remediation of nZVI on PCP contaminated soil (Liao et al., 2007; Reddy and Karri, 2008); however, no result has been investigated on byproducts and pathways of PCP removal from soil In addition, the retention of PCP is strongly related to the natural organic matter content of soil to be treated The sorption ability of PCP has been proved to increase with high humid acid content in soil (He et al., 2006) Consequently, higher natural organic matter of soil indicates that PCP will retain more favorably in soil, resulting in lower removal efficiency The soil mineral fractions also play a role of adsorbent

If the contaminated soil with low soil organic matter exhibits an alkaline property, the majority of the PCP in the contaminated soil exists in ionized Pentachlorophenolate form, which interacts mainly with mineral surface fraction (Dela Cruz et al., 2011) In addition, while PCP is retained in organic matter and strongly attached to soil colloid particles, nZVI is