NATURAL ARSENIC IN GROUNDWATER: OCCURRENCE, REMEDIATION AND MANAGEMENT - CHAPTER 21 ppsx

The techniques existing for the removal of As include conventional processes like ion exchange and coagulation/filtration and also emerging processes with iron oxide based adsorbents lik

Section 4: Remediation of arsenic-rich

groundwaters

Technologies for arsenic removal from potable water

W Driehaus

GEH Wasserchemie GmbH & Co KG, Osnabrueck, Germany

ABSTRACT: Arsenic (As) contamination of ground water resources used for potable water sup-ply is an emerging problem throughout the world Except for the use of alternative, uncontami-nated water sources, water treatment for As removal is often the only solution to meet the standards The effective application of As removal processes requires the knowledge of its chem-istry in natural water Before making a choice of a suitable treatment technique it is to ascertain whether As(III) is present and to evaluate the need for an oxidation technique for As(III) The techniques existing for the removal of As include conventional processes like ion exchange and coagulation/filtration and also emerging processes with iron oxide based adsorbents like granular ferric hydroxides While most of the removal techniques suitable for water works operation rely on sorption processes, they exhibit big differences in investment, chemical cost, maintenance and the type of arsenic bearing waste Different options for disposal are discussed

1 INTRODUCTION

Arsenic (As) became a serious health problem in several countries around the world This is also attributed to new findings about the toxicity of arsenic, especially for long term exposure to low levels in drinking water and food Thus, a number of countries have lowered their drinking water standards within the last 13 years (WHO 2001) The lowered standards, mostly to 10
g/L and the

“As calamity” in India and Bangladesh induced world wide activities in research of available treat-ment methods for As-removal and the developtreat-ment of new technologies The situation of drinking water supply in the affected countries and areas are different, from more centralized treatment plants and supply i.e in the UK and the USA, to smaller decentralized waterworks in Germany and France to a supply by hand pumps, missing any distribution network in the affected areas of West Bengal and Bangladesh

This paper gives an introduction to the basics of aquatic chemistry of As and an overview on As removal techniques in potable water treatment, including oxidation techniques for As(III)

2 AQUATIC CHEMISTRY OF ARSENIC

Arsenic is a semimetal and occurs in natural groundwater mostly occurring as oxyanions as triva-lent As (H3AsO3) or pentavalent As (H3AsO4) They are commonly named arsenite or As(III) and arsenate or As(V) The occurrence of organic As compounds, especially methylated species is only reported from surface water (Anderson & Bruland 1991), but they rarely, if ever occur in ground-water Elevated As concentrations at a level relevant for human health are mostly caused by inorganic As species in groundwater Figure 1 shows the stability diagram of inorganic As Natural groundwater is commonly in the field between pH 5–9 and Eh0.50 V to 0.50 V Under this conditions there are two dominating species, As(III) in more reducing environments and As(V) under oxidizing conditions In strongly reducing environments with hydrogen sulfide present form also a couple of As-sulfides, most of them being solids As this conditions are rare in

Natural Arsenic in Groundwater: Occurrence, Remediation and Management –

Bundschuh, Bhattacharya and Chandrasekharam (eds)

© 2005, Taylor & Francis Group, London, ISBN 04 1536 700 X

groundwater bodies used for drinking water supply, these species are excluded from the following description

The dissociation constants of As(III) are pKS1 9.22; pKS2 12.10; pKS3 13.40 (Pierce 1981) That means that at pH 9.2 the As(III) is by 50% dissociated At lower pH, most of As(III) exists as a neutral molecule

The dissociation constants of As(V) are pKS1 2.22; pKS2 6.96; pKS3 11.5 That means, that a pH 6.96 about 50% of As(V) exists as a monovalent anion and 50% as an divalent anion The difference in charge has important effects on the removal characteristics of both As species, because neutral, uncharged molecules can not or less effectively be removed by most treatment techniques (Jekel 1994) As(III) can be oxidized to As(V) at a relatively low Eh-potential of 0.1–0.2 V and, from the energetic point of view, dissolved oxygen is sufficient as an oxidant Unfortunately, the oxidation of As(III) by oxygen is very slow with conversion rates of a few per-cent per day Thus, even oxidizing groundwater with high oxygen conper-centrations may contain some As(III) As(III) in its typical stability range is very often associated with dissolved iron and manganese As(V) is the stable species in oxidizing water and has a great similarity to ortho-phosphate

in terms of dissociation, precipitation, adsorption and ion exchange (Jekel 1994) Thus phosphate

is identified as one of the major competing ions in As(V) adsorption

3 TREATMENT TECHNIQUES

Nearly all existing treatment technologies have been examined for their ability to remove As from water In addition, new techniques were developed to avoid the shorts of existing techniques The principle goals for As removal techniques are:

• Efficiency: output concentrations below the standard of 10 ppb should be safely achieved,

• Reliability and maintenance: Techniques should be reliable and adapted to the skills of the staff

• Residuals: Take into account the handling, treatment (if necessary) and disposal of any treat-ment residuals, they are suspected to contain high amounts of As

• Costs: Investment as well as operating costs define whether the treatment technique is economical

Figure 1 Stability diagram for inorganic As (according to Ferguson & Gavis 1972, Baldauf 1995).

The more promising treatment techniques for As removal, which are currently applied, are (USEPA 2000):

(I) Technologies based on adsorption reactions:

• Coagulation with iron or alum coagulants

• Iron and manganese removal

• Subterranean removal

• Adsorption on activated alumina

• Adsorption on iron oxide based adsorbants

(II) Other technologies:

• Ion exchange

• Membrane processes, i.e reverse osmosis and nanofiltration

Figure 2 gives schematically an overview for the classification of As-removal processes

We distinguish between the sorption processes and the separation processes Sorption processes means here, that chemically active solids are involved in the process and that the contaminant is bound by electrostatic or chemical forces to the surface Ion exchange generally requires a charged ion and thus, it fails completely in removing As(III) at neutral pH The adsorption techniques are all based on the great affinity of As species to metal oxide surfaces, to form stable surface com-plexes (chemisorption) Some surfaces have the potential to bind As(III), but mostly to a minor extend compared to As(V)

Arsenic removal by adsorption on metal oxides and hydroxides is the most important removal mechanism in natural environments (Soils, lacustrine and marine sediments) and also in technical applications The adsorption reaction is understood as a surface complexation reaction, where As forms inner-sphere complexes

Metal oxide surfaces are usually protonated and contain positive and negative electrostatic charges at the different surface sites, according to the following reactions (M stands for metals like

Fe or Al)

Figure 2 Classification of As-removal techniques.

Protonation and deprotonation of metal oxides:

Adsorption of As(V) on metal oxide surfaces:

Protonation and deprotonation reactions are pH dependent and at a distinct pH, the isoelectric point, the surface contains negative and positive charges to the same extend and the overall charge

is zero The pH of the isoelectric point varies for different metal oxides Iron(III) hydroxides and oxihydroxides have their isoelectric point generally around pH 7–8 At greater pH, the overall sur-face charge is negative, below, the overall charge is positive This has a big impact on the adsorpt-ion of As(V): At neutral pH, As(V) as an anadsorpt-ion is negatively charged It will adsorb very good to

a positively charged surface, but the adsorption is hindered by a negatively charged surface The specific adsorption of As takes place via an oxygen bridge between the metal atom and As The reaction given above shows a monodentate complex, but also bidentate complexes occur, which are using two sites for one As atom (Waychunas et al 1993)

At high pH adsorption and removal efficiency is reduced and vice versa Thus, removal techn-iques for As(V), which rely on adsorption work far better at low pH In the practical consequence one need to dose more coagulant for the same removal result, or, with fixed bed techniques, one need to change out or regenerate the sorbent more frequently

The membrane processes are separation processes and remove nearly all dissolved substances from the water They are also uneven in the removal of As(III) or As(V) Usually, As(V) is much better removed than As(III) Uncharged molecules have, in any way, the ability to pass membranes

to some extend

3.1 Oxidation techniques for As(III)

Most of the treatment techniques are much more effective in removing As(V) rather than As(III), because they partially rely on electrostatic forces Thus, there is a great discussion on oxidation techniques for As(III) With As(III) present in a raw water, usually other reduced species are pres-ent: ferrous iron and manganese(II) This co-occurrence is important for the evaluation of oxidat-ion processes because iron and manganese are also subject to treatment goals As described previously, As(III) can not be oxidized simply by dissolved oxygen, because this reactions are kinetically hindered and very slow In this way, stronger oxidants have to be applied

The following oxidants are applied successfully for As(III) oxidation (Jekel 1994):

• Chlorine, hypochlorite

• Ozone

• Permanganate

• Hydrogen peroxide

• Manganese oxide

Chlorination is one method for As(III) oxidation, either by dosing gaseous Cl2or by dosing a hypochlorite solution The required dose is between 0.3–1 mg/L of Cl The dose depends mainly

on the Cl-consumption by other reduced species like ferrous iron, manganese(II) and organics Restrictions and concerns occur about the formation of chlorination byproducts They evolve by the reaction of natural organic matter (NOM) with chlorine and are generally called THM Trihalomethanes The oxidation of As(III) by chlorine is a fast and complete reaction This and the fact, that chlorination is a very common chemical in the preparation and treatment of potable waters, makes it to the most commonly applied oxidation method for As(III)

Ozone (O3) is another potent oxidant, effective for As(III) oxidation Same as Chlorine, ozone oxidizes also other reduced species, like ferrous iron and manganese(II) The reaction mechanism

is via hydroxyl radicals Despite the different mechanism, there are similar concerns about the side effects as with chlorine, the oxidation byproducts This byproducts are produced during partial oxidation of NOM Ozone has to be prepared on site by a high voltage discharge process and can-not be stored in tanks Ozone is applied in water treatment processes for nearly all oxidation processes, also those to correct taste deteriorations by oxidizing organic compounds

The oxidants permanganate and hydrogen peroxide are used in a low number of treatment facil-ities to oxidize As(III) Especially permanganate has a good potential to be used for As(III) oxidat-ion It reacts fast, leads to an almost complete conversion and the As(III) oxidation is preferred over other specific oxidation processes like ferrous to ferric iron (Borho 1996) The application of permanganate requires a filtration step to remove the manganese oxide particles which are gener-ated during its reaction, that can simultaneously be used to remove the oxidation product arsen-ate(V) after dosing of coagulants

A single hydrogen peroxide application cannot oxidize As(III) quick enough If ferrous iron is present in the raw water, hydrogen peroxide reacts with Fe2, which is known as the Fenton’s reac-tion The Fenton’s reaction generates hydroxyl radicals, which act as oxidants for As Since some iron hydroxide is also generated by this reaction, As(III) can be oxidized and partially adsorbed to the suspended ferric hydroxide in one step Same as with Ozone, the Fenton’s reaction produces undesired byproducts by partially oxidizing NOM

The need for an As(III) oxidation has to be carefully evaluated for several reasons:

• The application of oxidants has sometimes undesired side effects

• Arsenic occurs more frequently in its oxidized form as As(V) and an oxidation is completely useless with respect to As removal performance

• Some of recently developed removal techniques are also effective in removing As(III)

• Oxidation by dosing chemicals makes the process more complex This may be unacceptable, when using maintenance-free fixed bed processes

Oxidation reactions of As(III) occur also as a side effect in conventional treatment for iron and manganese removal: As(III) is oxidized together with iron and manganese during filtration and removed by adsorption on solid ferric hydroxide This mechanism is reported from a number of conventional treatment plants in Germany (Haase, personal communication), which contain high

As amounts in the residual sludge There is the evidence of a biological aided oxidation during the oxidation and removal of ferrous iron and/or manganese(II) (Seith & Jekel 1997, Driehaus et al 1995) One promising technique for As(III) oxidation is also the filtration over manganese oxide media: That can be manganese oxide coated sand (byproduct from groundwater treatment processes) or commercially available mined manganese oxide products For the recently devel-oped As treatment by adsorption on iron based adsorbents, it is reported that they are also effect-ive for removing As(III), thus eventually avoiding the need of a special oxidation treatment (Driehaus et al 1998)

4 ARSENIC REMOVAL TECHNIQUES

4.1 Conventional techniques

4.1.1 Coagulation/filtration

Coagulation, followed by filtration is a widely used treatment, not only for As, but especially for the removal of colloidal substances There exist also the terms “flocculation” and “precipitation” for this technique, but all mean the same The coagulation or precipitation technique relies on the dosing of alume or iron salts to form precipitates of iron hydroxide which takes up arsenate by adsorption The coagulation is applied in combination with a separation technique to remove

As loaded particles from the water, i.e by filtration over granular media, or microfiltration The

residual of this technique is a backwash sludge with a high water content The sludge needs fur-ther treatment for dewatering for easy shipping and disposal

The coagulants of choice are iron salts like ferric chloride (FeCl3), ferric sulphate (Fe2(SO4)3), ferrous chloride (FeCl2) or ferrous sulphate (FeSO4) The use of alum based coagulants is not rec-ommended, because they are far less effective in removing As (Jekel 1994)

A scheme of a coagulation/filtration plant for As removal is given in Figure 3 This plant has installed four filter vessels, with two vessels used as a two layer filter for coagulation/filtration Jekel & Seith (1999) reported a coagulation process at this site being effective for a groundwater source with about 20
g/L As Arsenic was nearly completely in the oxidized form and no oxida-tion process was applied The performance of As removal as a funcoxida-tion of the coagulant dose is displayed in Figure 4 (a) for a raw water with about 50% As(III) at a pH of 6–6.2 The removal of As(III) without oxidation is, even at high coagulant doses, very low Fig 4 (b) shows the percentage

Figure 3 Scheme of the full scale As removal plant in Germany, where coagulation/filtration and adsorpt-ion techniques were examined side by side (acc Jekel & Seith 1999).

0

20

40

60

80

100

120

140

As conc Without chlorine

As conc with 1 ppm chlorine

(a)

0 10 20 30 40 50 60 70 80 90 100

(b)

Ferric iron dose, mg/L Ferric iron dose, mg/L

control value 5 ppb pH: 7.9–8.0 pH: 6.0–6.2

Figure 4 (a) Arsenic removal depends on coagulant dose and oxidation (after Jekel 1994); (b) Iron dose and percent removal at the plant shown in Figure 3 (acc Jekel & Seith 1999).

of As removal versus iron dose from the full scale pilot plant illustrated in Figure 3 An acceptable removal of 85% was achieved with a dose of 1 mg/L as Fe

The coagulant dose depends on the raw water profile, especially on the pH and on concentrat-ions of adsorption competitors like phosphate and silica For ferric salts, the minimum dose can be calculated from the As concentration At low pH between 6 and 7 a minimum dose of ferric iron

of 10–20 times of the As concentration is required At higher pH 8 the minimum dose is 40–50 times the As concentration This is only a rough estimation, and it is recommended to test the required coagulant dose by jar tests in the laboratory

The filter(s) separate the As bearing coagulant flocs from the liquid stream Filters have to be backwashed frequently to remove particles and to avoid a breakthrough of As-loaded particles The backwash water can be stored in a tank to allow the particles to settle down After sedimentat-ion the supernatant solutsedimentat-ion can be discharged or fed again in the process

The main critical step for the implementation and optimization of the conventional coagulat-ion/filtration treatment is the separation of the As loaded particles from the liquid stream The filt-ration properties of the particles depend on the chosen coagulant, the dose and the raw water parameters like pH, hardness, alkalinity

The residual from the coagulation process is an As bearing sludge with a high water content This residual needs a special treatment, at least dewatering, to be handled and transported safely

4.1.2 Iron and manganese removal, subterranean removal

Arsenic removal during iron and manganese removal can be understood as a variation of a coagu-lation process, because the raw water contains soluble iron, which is oxidized by aeration or by chemical means and precipitated This technique also needs a filtration step to remove the precip-itates from the liquid; mostly applied is the filtration over granular media There exists the same dependence with regard to pH and competing ions In general, a sufficient As removal is achieved

at a mass ratio of iron to As above 50:1 Some complication arise from the fact, that ground waters with elevated iron concentrations mostly have reduced As(III) This leads to reduced efficiency of the As removal, if it can not be oxidized during treatment

Subterranean removal is a special case of iron removal The iron containing raw water is aerated and reinjected to the ground, where iron precipitates, adsorbs As and is filtered of by the porous aquifer The subterranean removal is rarely applied, because it needs special aquifer conditions and it is thought to influence and block the aquifer’s porous structure

4.1.3 Ion exchange and membrane techniques

Ion exchange (IX) and the membrane techniques are less important for water works operation and the application is restricted to special conditions like household treatment in Point-Of-Use and Point-Of-Entry devices IX is applied as a simple filter technique and needs to be regenerated fre-quently, the capacity is restricted and other (an)-ions are also removed

Ion exchange (IX), especially anion exchange can effectively remove As, generally using strong base anion exchange resins in the chloride form However, sulfate and other anions compete with

As and can greatly reduce run length Regeneration is inevitable for ion exchange systems In nat-ural water, the runlength of IX columns are only 300–1000 bed volumes Pilot testing indicates that the brine regeneration solution could be reused as about 20 times with no impact on As removal provided that some salt was added to the solution to provide adequate chloride levels for regeneration This mode of operation would reduce the amount of waste for disposal, but increases the ultimate As concentration of the spent brine Disposal routes for the spent brine become criti-cal in assessment of the viability of IX for specific treatment situations The brine can be treated with ferric coagulant to remove the As from the liquid waste The additional complexity intro-duced by brine handling and disposal may make IX unattractive for small systems IX is widely applied and accepted in the USA for potable water treatment Thus it seems quite attractive to use this technique also for As removal, despite of its shorts as restricted capacity and generation of liq-uid wastes There are also a couple of efforts to make improvements to this process Kim et al (2003) suggested an IX process with a complete brine recycling and treatment, which might

overcome the shorts with liquid waste handlings, but could not improve the run length of less than

100 bed volumes

Membrane techniques are not selective to As and other toxic contaminants, but remove most of the dissolved substances and reduce the TDS and conductivity drastically The membrane techn-iques remove As(III) to a minor extend and produce a constant stream of waste water, where all removed substances are concentrated Reverse osmosis (RO) and nanofiltration (NF) provide effective removal of As(V) and nearly all other dissolved species These technologies are interest-ing options for point of use and point of entry applications at low flowrates, particularly when As

is just one of several water quality parameters requiring treatment For larger flows, the 15% to 30% feed flow lost as reject (concentrate waste liquid) is an important consideration, as is the 50–150 psi operating pressure required even for modern high efficiency low pressure RO operat-ing on low total dissolved solids feed water For sites with several water quality issues to address, particularly if these are related to dissolved solids, reverse osmosis or nanofiltration have the attraction of providing complete treatment in a single process step

4.1.4 Activated alumina

The adsorption techniques in the stricter sense rely on a simple filtration process over granular adsorbents like activated alumina or granular iron oxides and ferric hydroxides Dosing of chem-icals is usually not required These techniques do not produce a backwash sludge, the residual is the As loaded adsorbent itself Activated alumina is known since more than 20 years as a good adsorbent for arsenate containing waters, but needs a regeneration, due to the restricted lifetime of the media until exhaustion It is reported that the optimal pH for As removal with activated alu-mina is around 6.0 Thus a pH-adjustment by adding mineral acids or CO2could increase the treat-ment capacity drastically Figure 5 shows the treattreat-ment capacities of activated alumina with a model raw water containing As(III) or, after oxidation, As(V) Obviously is activated alumina not effective in adsorbing As(III)

Activated alumina can be regenerated by rinsing with diluted caustic soda at a concentration of 2–4%, followed by rinse with 2% sulfuric to equilibrate to neutral pH This causes some problems because it produces a significant loss in capacity and, more important, liquid waste streams highly enriched with As

Recently, various activated aluminas were pilot tested with ground water from Arizona They exhibited treatment capacities between 1000 and 4000 bed volumes at raw water pH of 7.5–9 (Chang et al., in press) The advantages of activated alumina, simple filter operation, and the shorts, restricted capacity, lead to the development of iron oxide based adsorbents in granular form, as it was expected that they have a much higher capacity, while being suitable for fixed bed operation

4.2 Emerging techniques: Iron oxide based adsorbents

The technique with iron oxide based adsorbents was in 1991–1994 developed at the Technical University of Berlin, Department of Water Quality Control, to meet the new treatment goals of the

0 20 40 60 80 100 120

0 5000 10000 15000 20000 25000 30000 35000

Bed volumes

As(III) As(V)

Figure 5 Process life of activated alumina with 100
g/L As(III) and As(V) at pH 6 (Frank & Clifford 1986).

lowered As standard in Germany (Driehaus et al 1998) The technique provides a simple filtration process over granular adsorbent medias, commonly without any dose of chemicals and without pH adjustment The following description focuses on GEH, one of several available types of iron oxide based adsorbents Other iron oxide based adsorbents are currently under development and examination (Zeng 2002)

GEH is a pure ferric hydroxide in granular form The grain size is 0.32–2 mm and the specific surface is 250–300 m2/g The maximum As adsorption density is 55 g/kg, the typical adsorption from drinking water applications is 1–10 g/kg The adsorption densities, that are calculated from batch tests at different pH values are given in Figure 6 The adsorption density at a residual con-centration of 10
g/L is plotted against pH for both As(III) and As(V) At low pH, the adsorption density of As(V) is much higher than of As (III), but at slightly alkaline pH, adsorption is nearly equal for both oxidation states of As

The adsorption density and the lifetime until exhaustion in a treatment plant increases with decreasing pH for As(V) It is quite constant for As(III) The batch tests were prepared simulating

a typical groundwater chemistry with an electrical conductivity of 480
S/cm

Natural water has some constituents which interact with the ferric hydroxide surface and lead more or less to a reduction in adsorption density for As The most important interfering substances are phosphate, dissolved organic matter and at low pH Also high silica amounts may interfere with As adsorption, reducing the adsorption kinetics and the treatment capacities (Waltham & Eick 2002) For practical evaluations, for the comparison of different media and for economic cal-culations, the treatment capacity, expressed in bed volumes, rather than the adsorption density is a useful expression Figure 7 shows a typical scheme of an installation and an installation in a 32 year old building Currently more than 80 plants with granular ferric hydroxide are installed worldwide We give an overview on operational data for 24 plants in EU states

The design flow rates of the yet installed treatment plants are between 4 and 800 m3/h and the annual supply is from 10,000 m3to 7,000,000 m3 All plants usually work in a non-continuous mode, i.e with supply by night to a reservoir The empty bed contact time (EBCT) can be expressed in two values: (a) hydraulic EBCT at design flow and (b) average EBCT which includes also the regular down time of the plant This value is calculated from the annual supply and the bed volume Figure 9 displays both EBCT values from 24 treatment plants

The hydraulic EBCT at design flow of the plants varies between 3 and 10 minutes, the mean value is 4.2 minutes The average EBCT varies between 4 and 17 minutes, with a mean of 10 min-utes The small EBCT leads to quite small plant designs and footprints, which allow to reduce installation costs

The As concentrations are between 10–40
g/L in the raw water The raw water pH values are

in the range of 6.5–8 for the drinking water applications The treatment capacities, expressed as

0,1 1 10 100

5,5 6,5 7,5 8,5 9,5

pH

q(As V) q(As III)

Figure 6 Adsorption densities with granular ferric hydroxide for As (III) and As (V) in a typical ground-water at different pH values.