Phsicochemical Treatment of Hazardous Wastes - Chapter 12 doc

Table 12.1shows the removal efficiency of these organic compounds in aqueous tion by high-energy electron beam, as a function of solute concentration,absorbed dose, pH, and scavengers in

applica-in sewage sludge (Kurucz et al., 1991) It can be applied to treat wastewaterfrom numerous industries such as the food, health, pharmaceutical, pulpand paper, and textile sectors The groups of compounds from these waste-waters may contain benzene; substituted benzenes such as toluene, m-xylene,and o-xylene; phenol; halogenated ethenes such as trichloroethylene (TCE)and tetrachloroethylene (PCE); halogenated methanes such as trihalom-ethanes (THMs); carbon tetrachloride; and methylene chloride Table 12.1

shows the removal efficiency of these organic compounds in aqueous tion by high-energy electron beam, as a function of solute concentration,absorbed dose, pH, and scavengers in potable water and raw and secondarywastewaters This innovative treatment process is not only limited to simpletoxic organic chemicals but is also applicable to complex mixtures of organicpollutants under varying water quality

solu-12.2 Chemistry of Aqueous Electrons

12.2.1 Formation of Radical Species

The irradiation of pure water with high-energy electrons generated by anaccelerator results in the rapid formation of electronically excited states and/

or free radicals in 10–14 to 10–9 s These reactive species will react with organic

462 Physicochemical Treatment of Hazardous Wastes

and inorganic compounds in aqueous solutions The reactions result in eitheroxidation or reduction of compounds and lead to the formation of endproducts such as CO2, H2O, and inorganic salts After 10–12 s, an ionizationspecies such as , OH•, and molecular fragments resulting fromdissociation of excited-state molecules are in thermal equilibrium with themedium After 10–7 s of irradiation, the radiolysis products begin to diffuse,resulting in a fraction of them reacting to form radical products Because theenergy required to produce a chemical change is only a few electron-volts(eV) per molecule, a high-energy electron is capable of initiating severalthousand reactions These reactions can be summarized by the followingequation:

a Bench-scale studies using 60 Co.

Source: Kurucz, C.N et al., Radiat Phys Chem., 45(2), 299–308, 1995 With permission.

e aq– H O3 aq+

High-Energy Electron Beam 463

H2O [2.7]•OH + [2.6] + [0.6]•H + [2.7]H3O+ + [0.45]H2 + [0.7]H2O2

(12.1) Removal efficiency by high-energy electron beam is usually expressed by G

values The G value is defined as the number of excited states, radicals orother products formed or lost in a system when 100 eV of energy is absorbed(Kurucz et al., 1995b) The G values for each species is shown in the brackets

in Equation (12.1) Removal efficiency of solutes can be quantitativelyexpressed in terms of two constants, G D and k′ G D describes the percentremoval of solute at a given dose It is defined by the disappearance of thesolute in aqueous solution and is determined experimentally using the fol-lowing equation (Kurucz et al., 1991):

G D = [∆C•NA/D(6.24 × 1017)] (12.2) where ∆C is the difference in organic solute concentration (M) at dose D andzero dose after irradiation; D is the dose in krad; NA is Avogadro’s number(6.02 × 1023); and 6.24 × 1017 is the conversion factor from moles to 100 eVL–1

12.2.2 Hydroxyl Radical

Of the chemical species formed in Equation (12.1), the most reactive are theoxidizing species such as hydroxyl radical (•OH) and the reducing speciessuch as aqueous electron ( ) and hydrogen atom (•H) The concentration

of the reactive radicals formed in the irradiated solutions can be determinedaccording to the fact that 1 krad of irradiation adsorbed will form 1.04 M

reactive species when G equals 1 for a solute Thus, for a G of 2.7, theconcentration of •OH from Equation (12.1) is 0.28 mM at an absorbed dose

of 100 krad Based on these calculations, the total reactive radical speciesconcentration formed usually ranges from 0.40 to 2.23 mM for •OH Thepresence of both oxidizing and reducing species is unique to this processand distinguishes it from other treatment processes

Hydroxyl radicals, •OH, can undergo several types of reactions with ical species in aqueous solution The types of reactions that are likely to occurare hydroxylation, hydrogen abstraction, electron transfer, and radical–rad-ical recombination Hydroxylation reaction occurs readily with aromatic andunsaturated aliphatic compounds, which result in the formation of hydrox-ylated radicals:

464 Physicochemical Treatment of Hazardous Wastes

•OH + CH3–CO-CH3→•CH2COCH3 + H2O (12.4)

When hydroxyl radical is reacting with inorganic species, electron transfer

will occur after aqueous solutions are irradiated with high energy electrons

For example, halogen ions (X–) will react with hydroxyl radical as follows:

H2O2 can also be generated in oxygenated aqueous solutions by the reactions

of and •H with O2 The reactions result in the formation of reduced

oxygen, the superoxide radical ion, and/or the conjugate acid:

This reaction occurs with a second rate constant of 1.2 to 1.4 × 1010 M–1 s–1

and suggests that the addition of H2O2 would increase the OH•

concentra-tion, which would increase the removal efficiency of organic pollutants

12.2.4 Aqueous Electron

Aqueous electron, , is a powerful reducing reagent with an E°= 2.7 V It

reacts with many hazardous halogenated and nonhalogenated organic

com-e aq–

e aq–

e aq–

e aq–

pounds during their removal from aqueous solutions The reaction of anaqueous electron, , is a single electron transfer process:

TABLE 12.2

for Reactivity with Aqueous Electrons

12.3 Irradiation of Toxic Organic Chemicals in Aqueous Solutions

High-energy electron beam irradiation can be used to effectively removeand/or destroy toxic organic chemicals in aqueous solutions by oxidation

or reduction It has been used to treat many compounds in water treatment(trihalomethanes) (Cooper et al., 1993b), groundwater contamination (chlo-rinated methanes and ethenes) (Kurucz et al., 1993), and hydrocarbons leak-ing from underground storage tanks (benzene and substituted benzenes)(Nickelsen et al., 1992a,b), as well as many other hazardous organic chemi-cals that are regulated

To gain a better understanding of the electron beam process it is necessary

to study the kinetics at various solute concentrations, the pH effects,absorbed doses, and the effect of scavengers in various water qualities Waterquality plays an important role in the removal efficiency of toxic chemicalsand has led to experiments investigating the destruction of selected organiccompounds suspended in different water matrices Many studies have beenconducted in pure aqueous solutions and have not taken into account thepresence of inorganic and organic matter that naturally exist in water andmay affect removal rates of the solute in question by interacting with thetransient reactive species formed during irradiation In the presence andabsence of known scavengers, such as methanol, oxygen, bicarbonate/car-bonate ions, and dissolved organic carbon, the degradation rates changesignificantly By examining the rate constants for each transient species with

a known scavenger and comparing it with the rate constant of the sametransient species with the solute to be removed in a specific water quality,

it is possible to predict which will be the preferred reaction, thereby mining the removal efficiency of the solute

deter-The following sections discuss the research results from Cooper and hiscolleagues at the Miami High-Energy Electron Beam Facility In a typicalexperiment, chemicals are injected into different water quality streams atvarious concentrations, the pH is varied, and samples of the solution arecollected and analyzed after it has passed through the beam at variousabsorbed doses

12.3.1 Saturated Halogenated Methanes

Halogenated methylenes, including carbon tetrachloride, methylene ride, and four trihalomethanes (THMs), namely, chloroform, bromodichlo-romethane, dibromochloromethane, and bromoform, have been studied Theformation of THMs in drinking water results from chlorination during dis-infection THMs in drinking water are classified as probable human carcin-ogens Carbon tetrachloride is used in the production of refrigerator fluid,propellants for aerosol cans, and other end uses and is found in groundwater;

chlo-it can cause serious health effects and is now regulated by the U.S mental Protection Agency at a maximum contaminant level (MCL) of 5 µg/

Environ-L Methylene chloride is used widely in industry as a process solvent in themanufacture of photographic films and pharmaceuticals and as an agent infoam and paint-stripping operations If consumed, methylene chloride canaffect the central nervous system and is therefore regulated at an MCL of 5µg/L in the United States

Chloroform removal was found to be dependent on the water quality Inpotable water, at two solute concentrations of 75 and 750 µg/L, 99%removal was observed at 800 krads (Cooper et al., 1993a) With the similarinitial concentrations and irradiation doses in secondary and raw waste-waters, the removal efficiency was 85 and 90%, respectively However, for

99% bromoform removal, the effect of water quality was insignificant whenthe initial concentration was between 100 and 1500 µg/L and the irradia-tion dose was above 300 krads Three experiments were carried out inpotable water of three different compositions: (1) the addition of individualTHMs, (2) a mixture of THMs, and (3) the addition of chlorine to form

THMs in the water In each case, G D decreased as the relative percentage

of bromide and the dosage increased; in other words, at the low tration, removal is relatively independent of solute concentration How-ever, at the highest concentration, an increased dose was required to meetthe same solute removal A possible explanation for this is that radical–rad-ical recombination of the OH• and the increases with increasing dosage;therefore, the relative concentration of the reactive species available for theTHMs is lowered Thus, the factors that will most affect the removal effi-ciency of THMs are those that affect the concentration of in solution,because THM removal depends more on the concentration of the thanthe •OH This was the general trend noted among other low-molecular-weight organic pollutants, such as CCl4 and methylene chloride

concen-Two studies were conducted for CCl4 removal (Cooper, 1993a) One wasconducted with three different CCl4 concentrations at one pH, and the otherwas conducted with three different pHs at one solute concentration Removalefficiency was similar to that of the THMs pH was found not to be acontributing factor in the removal of CCl4 by high-energy electron beamirradiation When the pH of water was adjusted to make it potable, thebicarbonate/carbonate equilibrium was disturbed As the pH increased, sodid the alkalinity, resulting in increased scavenging of the •OH, thus reduc-ing the removal efficiency of CCl4 as well as the THMs

Similar experiments for methylene chloride (CH2Cl2) removal were formed The results were similar to those for THM removal Due to the highsolubility of CH2Cl2, the experiments were carried out at higher concentra-tions Such concentrations would require a higher dose to meet treatmentobjectives in the low microgram per liter range

per-High-energy electron beam degradation of halogenated saturated anes led to the formation of by-products (Mak et al., 1997) To determinewhich by-products may be formed, the mechanism for the destruction of thecompound was investigated For example, the irradiation of chloroform(CHCl3) and the formation of by-products have been studied by Mak et al.(1997) The formation of oxidized organic compounds such as formaldehydeand formic acid has been observed However, when chloroform is irradiatedand no halogenated compounds have been detected

meth-A mechanism for the decomposition of 0.07-M CHCl3 at low irradiationdosages is proposed in Equation (12.16) to Equation (12.29), with or without

dissolved oxygen The bimolecular rate constant, k, is in M–1 s–l The reactionequations were found to be in agreement with experimental data (Dickson

et al., 1986):

e aq–

e aq–

e aq–

In solutions with high oxygen concentrations, the following reactions appear

to play an important role in the decomposition of CHCl3:

•CHCl2 + O2 → •O2CHCI (12.28)

•CCl3 + O2→ •O2 CCl3 (12.29)

At high organic concentration and high radiation dosage, the removal ofCHCl3 was inhibited at high dosages when the oxygen concentration wasdepleted This observation suggests that radical–radical recombination hasoccurred, thus allowing halogenated by-products to remain In the experi-ments conducted by Mak et al (1997), formaldehyde was the only aldehydeobserved No halogenated reaction by-products such as haloacetic acids orketones were observed after irradiation

The relative percent removal of CHCl3 can be calculated from the

bimo-lecular reaction rate constants and the G values for the reaction of CHCl3

with the three transient species •OH, , and •H These calculations indicate

e aq–

e aq–

that the initiates greater than 99% of the removal of CHCl3, whereas •OH

is responsible for less than 0.2% and •H for less than 0.01% Therefore, thefactors that will affect the removal efficiency of CHCl3 from solution arethose that affect the concentration

12.3.2 Unsaturated Halogenated Ethenes

The removal of TCE and PCE from potable water and secondary and rawwastewater was studied by full-scale experiments at both low and highsolute concentrations (Cooper et al., 1993a) TCE removal was found to bedependent on both •OH and , with second-order rate constants of 4.0 ×

109 and 1.9 × 109 M–l s–l (Buxton et al., 1988), respectively In contrast, PCEremoval is dependent primarily on the at 1.3 × 1010 M–l s–l Greater than90% removal was obtained for both TCE and PCE at both concentrations inall waters (Cooper et al., 1993a); however, PCE was not as effectivelyremoved as was TCE in solution of equal solute concentration and waterquality The reason for this may be related to the number of chlorine atomspresent in both compounds Overall, the removal rates of TCE and PCE werebest in potable water and about the same for secondary and raw wastewater.Although the percent removal at lower concentration was higher in allwaters than at higher concentrations, greater than 90% removal wasobserved for both TCE and PCE in all waters

Irradiation was also successful in the decomposition of THMs to chlorideand bromide ions (Cooper et al., 1993b) Toxic organic by-products such ashaloacetic acid, aldehyde, ketones, or halogenated organic compounds wereformed after irradiation It has also been proven effective in the destruction

of halogenated ethenes such as TCE and PCE Removal rates decreased20-fold in the presence of methanol as opposed to its absence Aldehydesand formic acid were found when low solute concentrations of TCE andPCE were irradiated; however, at high concentrations no more than 5%formic acid was found Complete conversion of organic chlorine to chlorideion can be achieved

The G D decreased in all three waters as the radiation dose increased per et al., 1992a-c) As the solute concentration was increased tenfold, the

(Coo-G D also increased tenfold When G D was compared at each radiation dose,the results were remarkably similar regardless of water quality At high

concentration, the G D value for TCE was higher in secondary wastewaterthan in potable water This may be due to higher influent solute concentra-tion in the secondary wastewater Although water quality is a factor inremoval efficiency, it does not appear to be as important when compared toother treatment processes

In the presence of 3.3-mM methanol, which is a •OH scavenger, theremoval efficiency of the solutes was reduced up to 20-fold when compared

to solution with no methanol; however, it is not clear why the removal of

e aq–

e aq–

e aq–

e aq–

PCE by the •OH is so dramatically reduced when PCE removal primarilydepends on the more than •OH.

The proposed reaction mechanism for the destruction of aqueous solutions

of TCE or PCE predicts the formation of stable oxidized polar organic pounds These compounds consist of acids, aldehydes, and possibly halo-acetic acids Three possible mechanisms have been proposed for theformation of by-products due to the irradiation of aqueous solutions con-taining TCE and PCE The first is for the formation of formaldehyde, acetal-dehyde, and glyoxal, which are formed at a concentration of approximatelytwo orders of magnitude less than the influent solute concentration Second,the formation of formic acid decreased with increasing radiation dose Theformic acid concentration was found to be higher for PCE than TCE Theseresults are most probably due to the slower reaction rate constants of PCEwith and •OH, compared to TCE The third possible reaction is the for-mation of haloacetic acids when TCE and •OH react The mechanism ofdecomposition of PCE by •OH is shown in Equation (12.30) to Equation(12.42) (Dickson et al., 1986):

com-CCl2 = CCl2 + •OH → HOCCl2–CCl2• (12.30)HOCCl2–CCI2•→ COCl–CCl2• + H+ + Cl– (12.31)COCl–CCL2• + O2→ COCl–CCl2OO• (12.32)2COCI–CCl2OO → O2 + 2COCI–CCl2O• (12.33)COCI–CCl2O•→ •COCI + CCl2O (12.34)CCl2O + H2O → CO2 + 2CI– + 2H+ (12.35)

Once again the principal reaction products at high absorbed doses are themore oxidized organic aldehydes and acids.

12.3.3 Substituted Benzenes

The maximum contaminant level (MCL) of benzene is 0.5 mg/L for solidwaste and wastewater and 0.005 mg/L in drinking water Toluene,

m-xylene, and o-xylene, which are all alkyl-substituted benzenes, are also

regulated in drinking water due to their carcinogenic nature; therefore,their removal or destruction is imperative not only for the population’shealth but also for the safety of the environment Mixtures of these threecompounds, plus benzene, in potable water and secondary wastewatereffluent were studied using continuous high-energy electron beam irradi-ation in the absence and presence of methanol (Nickelsen et al., 1992a,b).Three factors were studied in the removal efficiency of these compounds:water quality, solute concentration, and dose All four compounds whenadded at a low concentration and at a dose of 787 krad were removed tobelow detection limits in potable water, whereas in secondary wastewater

90 to 96% removal was observed (Nickelsen et al., 1992a,b) At a tration 20 times higher and at the same dose of 787 krad, removal efficiencywas a little above the detection limit — for example, 93% for benzene inboth potable water and secondary wastewater Similar results wereobserved for all other compounds studied in potable water and secondarywastewater The dose required to remove 90% of the initial toluene,

concen-m-xylene, and o-xylene concentrations in potable water was about 1.5-fold

lower than the dose required to remove similar concentrations in secondarywastewater At the higher concentrations, the dose required for benzene

in secondary wastewater was higher when compared to potable water atboth low and high concentrations During the removal of benzene and itsalkyl-substituted derivatives, phenol was detected as the reaction by-prod-uct Hydroxyl radical was primarily responsible for their disappearance;however, water quality was shown to affect removal rates due to dissolvedorganic carbon (DOC) at higher organic concentrations

The G D values at all doses for both potable and secondary wastewaterwere similar at low concentrations At high solute concentrations and only

at the lowest dose, the G D values in potable water were higher when

com-pared with secondary wastewater At the two higher doses, the G D values

were similar in both waters, with the exception of benzene, where the G D value was higher in potable water The G D values for the higher concentrationwere one order of magnitude higher than those observed for low concentra-tions The removal of these compounds follows the first order with respect

to solute concentration

The effect of oxygen, bicarbonate/carbonate, and methanol on removalefficiency of these compounds was also studied All four compounds reliedprimarily on •OH for removal efficiency, with the exception of toluene (for

which 16% of its removal depended on H•), and the removal rate increasedwith increasing dose because less scavenging effect occurs at the higherdoses The oxygen content of both waters was approximately 3.7 mg/L.Both and •H rapidly reduce O2 to form O2 with second-order rate con-stants of 1.9 × 1010 and 2.1 × 1010 M–1 s–l, respectively, so dissolved oxygenwas not expected to affect removal efficiency except in the case of toluene.

In the presence of methanol, which is a •OH scavenger, all four compoundswere expected to have a reduction in removal efficiency Several by-productshave been identified upon the irradiation of benzene; its alkyl-substitutedderivatives are shown in Table 12.4, which also shows the concentration ofthese intermediates

Phenols are initial by-products in the decomposition of these compounds

At low absorbed doses, the phenol concentration increased and thendecreased below influent concentration at high absorbed doses, which sug-gests that the high-energy electron beam process can also be used to degradephenols to simpler compounds

After hydroxylation of benzene by hydroxyl radical, the intermediate ical may decompose to phenol Figure 12.1 shows the reaction pathways ofbenzene: phenol will form upon irradiation of aqueous solutions of solute,involving reaction of the compounds with •OH, forming hydroxycyclohexa-dienyl radicals Hydroxycyclohexadienyl radicals generate phenol by dis-

rad-proportionation Reaction of toluene, m-xylene, and o-xylene with •OHwould yield alkyl-substituted phenols by similar mechanisms Glyoxal isformed upon irradiation of benzene, which first forms hydroxycyclohexadi-enyl radicals and then phenol or combines with oxygen to form an unstablehydroxy hydroperoxide Subsequent water elimination and ring openinglead to the formation of mucondialdehyde, which is further oxidized tomuconic acid Continued oxidative processes ultimately lead to the forma-tion of glyoxal

12.3.4 Phenol

Phenols and substituted phenols are found naturally occurring in naturalwaters at low concentrations; however, their high concentrations due toindustrial and agricultural waste may be toxic, especially to aquatic life.Experiments were conducted on a large scale to determine the effect of soluteconcentration, absorbed dose, total alkalinity, and total solids on the removal

of phenol Three solute concentrations (10.6, 106, and 531 µmol/L) were usedover a pH range of 5 to 9 in the presence or absence of 3% Kaolin clay Asmuch as 94.2% phenol removal could be attributed to •OH, 5.4% to •H, and0.4% by (Nickelsen et al., 1992a,b) Because •OH is the primary reactingspecies, hydroxylated compounds through •OH were expected to be themajor by-products As with all compounds studied with electron beam tech-nology, the greatest percent removal was observed at the lowest initial soluteconcentration (10.6 µmol/L) and increased with increasing dose The percent

e aq–

e aq–

bmdl = below method detection limit.

Source: Nickelsen, M.G et al., Water Res., 28(5), 1227–1237, 1992a With permission.

FIGURE 12.1

Proposed mechanism for the destruction of benzene Identified dose-dependent reaction

by-products include phenol (I), catechol (II), and resorcinol (III) (From Nickelsen, M.G et al., Water Res., 28(5), 1227–1237, 1992 With permission.)

( O ) O-OH

O

+ OH

OH H

O 2

OH -H 2 O

OH H

HC O

V

H

CH +

HCH H 3 C CH +