Arsenic China

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Arsenic in Drinking Water and Its Removal Liu Zhenzhong1,2, Deng Huiping2, Zhan Jian1 1. School of Architecture, Nanchang University, Nanchang Jiangxi 330031, China; 2. School of Environment Science and Engineering, Tongji University, Shanghai 200092, China

Abstract: Superfluous arsenic in drinking water can do harm to human health. In this paper, a broad overview of the available technologies for arsenic removal has been presented on the basis of literature survey. The main treatment methods included coagulation-sedimentation, adsorption separation and ion exchange, membrane technique, which have both advantages and disadvantages. It concluded that the selection of treatment process should be site specific and prevailing conditions and no process can serve the purpose under diverse conditions as each technology has its own limitations. In order to gain good results, some methods should be improved.

Key words: arsenic removal, coagulation-sedimentation, adsorption, ion exchange, membrane technique

1

Introduction

Groundwater in the part of the world has been affected by the arsenic pollution. The arsenic contamination in water body was not only caused by the physical geography chemistry change, such as the soil and rocks containing arsenic weathering, geology variance, the arsenic mine drenching, the dissolved groundrock, but also caused by human activities, such as mining, metallurgy, waste release of china industry, leather process, chemistry process, dyeing industry, use of pesticide and insecticide (Chakravarty et al, 2002; Smedley and Kinniburgh, 2002). Common arsenic species in the environment include arsenate (As(V)), arsenite (As(III)), dimethylarsinic acid (DMA), and monomethylarsenic acid (MMA) (Gu et al, 2005). Arsenic (As(V) and As(III)) in the inorganic forms were more toxic than those in the organic forms. Most of arsenic was presented in the inorganic forms in the nature water. When it was in the oxidizing environment, the main form was arsenate(As(V)); when it was in the reducing environment, the main form was arsenite (As(III))(Lien et al, 2005). As(V) includes H3AsO4, H2AsO4−, HAsO42−, AsO43−, in which the negatively charged arsenate accounted for more; while As(III) includes H3AsO3, H2AsO3−, in which the uncharged H3AsO3 accounted for more (Cul-

len and Reimer, 1989). The high arsenic groundwater is mainly distributed in Bangladesh, India, American, German, Japan, Argentina, Mexico, Brazil, Poland and China (Zhao, 2002). Arsenic was classified as a Group A carcinogen by the United States Environmental Protection Agency. There have been extensive epidemiological studies showing that chronic ingestion of high levels of inorganic As caused skin cancer (NRC, 1999), at the same time, some documentation of As exposure also caused cancers of the nasal cavity, trachea, bronchus, lung, liver, bladder, colon, kidney, prostate, brain, the lymphatic and hematopoietic tissues as well as the nervous system (Chen and Lin, 1994; Naqvi et al 1994; Remembrance, 2003). According to a recent report by the United States National Academy of Science and United States National Research Council, even at 3 µg/L of As, the risk of bladder and lung cancer is between four and seven deaths per 10,000 people. At 10 µg/L, the risk increases to between 12 and 23 deaths per 10,000 people (NRC, 2001). In addition, As can cause high blood pressure and diabetes. Triggered by the risk concerned, most of countries have changed their drinking water standard. The US EPA announced its ruling in October 2001 to lower the maximum contaminant level (MCL) from 50 µg/L to 10 µg/L with a compliance date of January 22, 2006 (An et al, 2005; EPA, 2000a). In China, the new standard of drinking water standard which was promulgated by National Standard Committee and Sanitation Ministry lowered the arsenic level from 50 µg/L to 10 µg/L after consulted with the developed countries standard. The new standard will be implemented in July 2007. The improved drinking water standard posed new challenges to water treatment technique.

2

Arsenic removal from water

Mainly, arsenic species were presented in natural waters as As (V), As(III), the As valent has a great impact on the removal effect and the incidence of endemic (Korte and Quintus, 1991). The dissociation constants of the two arse-

Corresponding author: Liu Zhenzhong ( [email protected])

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nic species and its redox reaction were reported as follows (Luis, 2005): Dissociation of As(V) H 2 AsO 4 ↔ H 2 AsO −4 + H + pK1 = 2.2 (1) H 2 AsO −4 ↔ HAsO 24 − + H +

pK 2 = 7.0

(2)

HAsO 24 − ↔ AsO 24 − + H +

pK3 = 11.5

(3)

pK1 = 9.2

(4)

Dissociation of As(III) HAsO3 ↔ AsO 23 − + H +

Redox reaction H 3AsO 4 + 2H + + 2e − ↔ H 3AsO 3 + H 2O Eh = +0.56V (5)

It was not favorable for arsenite sedimentation because of its high solubility. Figure 1 demonstrated the relationship between arsenic species and Eh-pH. Arsenite was more difficult to remove than arsenate because of its less negatively charged on the surface. Therefore, it was necessary to oxidize arsenite to arsenate. A number of treatment technologies have been reported to the date for arsenate and arsenite removal from waters. Many technologies such as coagulation and sedimentation, sorption and ion exchange, membrane separation were applied for arsenic separation.

Fig. 1

2.1

The relationship of arsenic and Eh-pH

Coagulation- sedimentation

Coagulation-sedimentation was a common technique to remove the contamination from water. This process can capture soluble As, transforming it into insoluble reaction products (Edwards, 1994). As may be converted to an insoluble form by precipitation, co-precipitation and adsorption onto ferric or aluminum hydroxides as the ferric and alum salts were applied. In general, iron salts were more effective at removing arsenite than aluminum salts (Cheng,

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et al, 1994; Sancha, 2000; Scot, et al, 1995). This may be due to some of the aluminum remaining soluble, and passing through filtration steps, while iron salts completely form particulate iron hydroxide. Meng et al (2000) studied the influence of the SiO43−, SO42−, CO32− on the As removal when the ferric chloride was used. SO42− and CO32− have little impact on As at pH 4–10. While SiO43− has decreased the As removal efficiency because of its high competition on adsorption site and produced the soluble polymer with the ferric. In addition, Meng et al (2001) removed the arsenic in the well using the ferric chloride and found that PO43− and SiO43− were the main anion to decrease the removal efficiency. Liu et al (2005) investigated the effectiveness and mechanism of permanganate enhancing arsenite (As(III)) co-precipitation with ferric chloride. Permanganate significantly enhanced As(III) removal for ferric co-precipitation (FCP)process. With Fe(III) dosage increasing from 2 mg/L to 8 mg/L, As removal increased from 41.3% to 75.4% for FCP process; for permanganate oxidation-ferric co-precipitation (POFCP) process, however, corresponsive As removal increased from 61.2% to 99.3%. Yuan et al (2006) adopted the ferrate as coagulation and evaluated the performance of ferrate for arsenic removal by experiment. The results showed that the efficiency of As removal can be achieved by 98%. The Optimum pH was 5.5–7.5. The oxidative and coagulation time was 10 min and 30 min respectively. The salinity and hardness did not interfere with removal arsenic. This method was very easy and effective comparing with the ferric method and KMnO4Ferric method. Coagulation-sedimentation removing the arsenic adapted to the large-scale water plant which need not increase the other treatment facilities with less investment and convenient operation. However, the disadvantage of the method was that the process must follow the clarified tank to remove the produced colloid arsenic and high dependence on the pH. It was not good for the arsenite removal efficiency to depend on the constitute in the water. At the same time, large quantity chemicals were supplied and the volume of the sludge was huge, other matters were also introduced into the water.

2.2

Membrane technique

The ordinary membrane separation removing arsenic

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included nanofiltration (NF) and reverse osmosis (RO). The large diameter ions including SO42−, Cl− and Na+ can be removed by NF which was a process of relatively low pressure. While all the ions can be removed by reverse osmosis which was a process of higher pressure such as seawater desaltation. Compared with the coagulationsedimentation, membrane separation had a higher efficiency to remove arsenic. A number of studies have been performed to examine the removal of arsenic by NF membranes (Chang, 1994; Urase et al, 1998). The results showed that NF processes were effective for the removal of arsenic. Removal, however, depended upon operating parameters, membrane properties and the characteristics of the source water. Reverse osmosis was an effective arsenic removal technology proven through bench and pilot scale studies according to a report prepared for the US EPA (2000). Various pilot studies reported arsenic removal ranging between 40%–99% without specifying the arsenic species removed. Ning (2002) reported As(III) reduction at 73%. Bench-scale studies with RO membranes showed As(V) reductions at 88%–96%. Generally, the membrane process was highly effective for arsenic removal. Membrane also provided an effective barrier to suspended solids, all inorganic pollutants, organic micropollutants, pesticides and so on. In another word, it removed all the ions presented in the water, though some minerals were essential for proper growth, remineralization was required after the treatment. In order to prevent membrane contaminant, some pretreatment was necessary. The water should be acidic and pH need be corrected. The process was expensive compared to other methods.

2.3

Adsorption

Some materials with big specific surface area and high surface energy which have strong adsorption ability can separate and remove the contaminant to purify water in the process of adsorption. This adsorption action may be chemistry effect such as surface chemistry coordination or complex or physical effect such as staticelectric attraction. Adsorption was one of the most effective methods to remove the arsenic in the water; the common adsorbents included activated alumina, activated carbon, function resin and metal oxide, etc.

2.3.1

Activated alumina (AA)

Because AA was a common, commercially produced adsorbent material for water treatment, there was more literature on the use of the AA for As removal relative to other adsorbents. Activated alumina can remove the arsenic effectively in the drinking water at pH 5.5. The principle was that the soluble arsenic (AsO43− and AsO33−) in the water can be adsorbed on the surface of the AA[am-Al(OH)3] and occupied the aluminous octahedron crystal lattice sites (Xiao et al, 2001). This kind of adsorption action can further increase the instability of the AA uncrystalloid and be favorable to adsorb more As ion to decease the soluble As in the water. Compared with other metal oxides such as hydrogen iron oxide, the kinetic sorption of AA was slower than that of hydrogen iron oxide. The former spent two days to reach half equilibrium but the latter just used a few hours to become equilibrium (Clifford, 1990). The optimum pH to adsorb As(III) and As(V) was respectively 5.5 and nearly neutral, and the big adsorption capability was respectively 15 mg/L and 3.5 mg/L (Jiang, 2001; Manning et al, 1998). In addition, some interrupting ions such as SO42−, Cl−, PO43− and F− can decrease the arsenic removal. This method can be used in the concentrating supply water plants and in the family purify equipment. However, common AA with low sorption capability adapted to thin pH range and acidic water, As(III) need be oxidated to As(V). After a period of the operation, regeneration can lose 10%–50% sorption capability. At the same time, aluminum quantity would increase in the treated water because of the losing of the aluminum. It was known to all that the persistent adsorbed aluminum caused the diseases such as agedness imbecile.

2.3.2

Metal (hydrogen) oxide

Iron oxide with high surface energy and surface area has strong adsorption capability with respect to many inorganic ions and organic matters. Lots of literatures have reported that heavy metal ions and organic contaminate can be removed by iron (hydrogen) oxide. Several iron oxides removed arsenic effectively such as amorphous hydrous ferric oxide, crystalline hydrous ferric oxide (ferrihydrite), α-FeOOH, hematite, magnetitie and goethite (Goldberg, 2002; Jackson and Miller, 2000; Jain and Loeppert, 2000; Manning et al 1998; Raven et al, 1998). The iron hydroxide

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and its polymer with high adsorption and fast kinetic which have the biggest capability of adsorption were the most effective. However, some shortcomings with iron hydroxide still existed, for example, some anions have an adverse influence on the arsenic removal such as SO42−, Cl−, F−, PO43− and SiO3−. The latter two have relatively stronger impact than the former three. In addition, the arsenic removal was influenced by pH markedly. The removal efficiency decreased quickly as pH was over 8.5. Arsenate removal was much better than arsenite by iron hydroxide. On the other hand, the robustness and mechanical strength of the granular iron hydroxide was not very good and needed improvement, the headloss pressure was produced quickly with time and became more significant after backwashing, the larger the particles, the less adsorption capacity. While the regeneration of granular iron hydroxide seemed feasible, it generates an alkaline solution with high levels of arsenate, which requires further treatment and disposal (Gu et al, 2005; Selvin et al, 2000). As far as iron hydroxide had poor adsorption to arsenite, Lakshmipathiraj et al (2006) combined the advantages of Mn and Fe, synthesize a suitable adsorbent, Mn-substituted iron oxyhydroxide (MIOH), which could remove both arsenite and arsenate from aqueous solutions with considerable efficiency. Mn-substituted iron oxyhydroxide (MIOH) efficacy was studied for the removal of arsenite and arsenate from aqueous solutions. The maximum uptake of arsenite and arsenate was found to be 4.58 and 5.72 mg/g respectively. Adsorption was best described by Langmuir isotherm and activation energies as calculated from a model-free isoconversional method were found to be on the order of 15–24 and 45–67 kJ mol−1 for arsenate and arsenite, respectively.

2.3.3

Iron oxide coated sands

In light of the deficiency of granular iron hydroxide, some iron oxide was coated on the sands to remove the contamination in the water. It was low cost operation for iron oxide coated sands beds and it adapted to the small equipment and family usage. In the trial, the iron oxide coated sands volume was 50 mL, diameter was 11 mm, velocity was 1 cm/m, detention time was 50 min, the initial concentration was 1 000 µg/L, according to WHO drinking sanitary standard, the arsenic goal level was 10 µg/L. The equipment operated for ten times, the penetrating volume of the iron oxide coated sands for arsenite and arsenate was

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respectively 163–184 BV and 149–165 BV (Joshi and Chaudhuri, 1996). The iron oxide coated sands has low saturation capacity of arsenite and arsenate. When the coated iron oxide quantity was 0.1%–0.2% or 1%–2%, the saturation quantity was respectively 0.041 mg/L and 0.043 mg/L (Thirunavukkarasu, 2003). The shortcoming of the process was that the granular size must be large enough to reduce the stream resistance which decreased the sorption capacity of the unit volume and quality. The larger volume sorption unit and frequently regeneration operation was needed because of the low adsorption capacity. Because sands belonged to the silicon oxide, which combined with the iron oxide and hydroxide less strongly, the iron oxide was subject to break off.

2.3.4

Activated carbon

Activated carbon, either granular or powered, was widely used as an adsorbent for water and advanced wastewater treatment. It was capable of adsorbing a wide of organic contaminants and heavy metals and was designated as the best available technology by the US ( Gu et al, 2005; James, 1985). Arsenic adsorption onto virgin activated carbon was minimal and regeneration was difficult, so it cannot be directly applied for arsenic treatment (Daus et al, 2004; Deng et al, 2005). Literature has, however, shown that the adsorption on activated carbon can be significantly increased by treatment with various metal compounds (Huang and Vane, 1989; Reed et al, 2000). Some iron compounds were impregnated into activated carbon, resulting in enhanced As sorption (Gu et al, 2005; Huang and Vane, 1989). Enhanced arsenic adsorption was similarly observed with copper-treated and zirconium-treated activated carbon (Birgit et al, 2004; Manju et al, 1998). Birgit (2004) adopted Zr treated activated carbon to remove arsenic in the wastewater, and the efficiency was high, but it was not suitable for drinking water because of its toxic. Then Birgit coated Cu and Fe to the carbon, both of arsenite and arsenate removal effect were improved markedly. Huang et al (1989) reported that activated carbon traited ferrous salts improved the arsenate removal efficiency because of the producing ferrous arsenate complex. Gu et al (2005) posed that ferrous ion can impenetrate into activated carbon inner holes and be oxided to ferric ion by oxidants. Then ferric ion can combine multifunction group to enhance the sorption capacity of arsenic (Fig. 2).

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Fig. 2

An illustrative model for preparation of As-GAC and arsenic adsorption

The sorption capacity increased after activated carbon was traited. However, some shortcomings existed when metal-impregnated activated carbon was used. The metal ion was subjected to leak from the media in the process of backwashing, then the second contamination of water source was caused and the sorption capacity decreased. In order to improve the combination ability with metal ions, activated carbon was substituted with the ion exchange resin and fiber, to remove arsenic in water.

2.3.5

Ion exchange resin and fiber

Ion exchanged resin has many kinds according to the material, making method and purpose. As far as the function group was concerned, there were strong (weak) acid canion exchanged resin, strong (weak) base anion exchange resin, redox resin, heat regeneration resin and chelating resin (Shao, 1989). Ion exchange technique was regarded as one of the best available technology for arsenic removal (EPA, 2000a). Current commercial ion exchanger was prone to be saturated because of its poor selective to arsenic and suffered from competing with other anions. Due to the lack of As-selectivity, current ion exchange required frequent regeneration and lots of regeneration brine was used, which in turn resulted in a large quantity of volumes wastewater containing arsenic (Clifford, 1999). Generally, a polymeric ligand exchanger (PLE) was composed of (a) a cross-linked hosting resin that can firmly

bind with a transition metal such as copper and iron, and (b) metal ions that was immobilized to the functional groups of the hosting resin. While sharing many common features with standard ion exchangers, a ligand exchanger employs transition metal ions as its terminal functional groups. As a result, ligand exchange involves concurrent Lewis acid-base (LAB) interactions (metal-ligand complexation) and electrostatic interactions between the fixed metal ions and target anionic ligands. While conventional anion exchanger selectivity for various anions was governed by electrostatic interactions, the affinity of a PLE was predominated by both the ligand strength and ionic charge of the ligands (An et al, 2005). In the 1980s, Chanda et al (1988) impregnated iron to weak base macroporus chelating resin to remove As (III) and As (V), the results showed that As (III) and As (V) were prior to other anions reacting with the resin at a low level of As. When equilibrium level reached, As (III)was lower than As (V) for one order of magnitude. Chanda also studied the sorption principle of iron impregnated chelating resin and thought iron was not toxic metal, and the adsorption activity was prior to other metal ions such as copper and nickel ion. In order to effectively select anions, the hosting resin must have positive charge, which was anion exchanger. Such polymers have high affinity to metal ions and prevented to strip off from the hosting resin. An (2005) prepared a polymeric ligand exchanger (PLE) by loading

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Cu2+ to a commercially available chelating ion exchange resin. Results from batch and column experiments indicated that the PLE offered unusually high selectivity for arsenate over other ubiquitous anions such as sulfate, bicarbonate and chloride. Because of the enhanced arsenate selectivity, the PLE was able to treat ten times more bed volumes (BVs) of water than commonly used SBA resins. Lenoble et al (2004) loaded manganese dioxide on a polystyrene matrix anionic commercial resin in chloride form, called R-MnO2, which was tested for As(V) retention and for As(III) simultaneous oxidation and removal. Equilibrium was reached in 2 h and isotherms showed that R-MnO2 maximal capacities towards As(III) and As(V) were, respectively, 0.7 and 0.3 mmol/g. Various mechanisms involved in As(III) and As(V) as follows (Fig. 3).

Fig. 3

Various mechanisms involved in As(III) retention by R-

MnO2

Luis (2005) loaded nanoparticle iron hydroxide on the polymer resin producing four polymers (HCIX-M, HCIX-G, HAIX-M,HAIX-G) to remove arsenic. Lewis acid-type functional groups existed between iron hydroxide and hybrid polymeric sorbents. Lewis acid-base LAB, electricstatic action and complexation combined two matters together. Luis proposed the main reason and process to adsorb arsenic by iron hydrogen as follows

(≡

= FeOH −2 ↔ H + + = FeOH pK1

(5)

= FeOH ↔ H + + = FeO − pK 2

(6)

FeOH +2 )(CI − ) +

H 2 AsO −4

(≡ FeO +2 )(H 2 AsO −4 ) + CI −

EL + LAB

⎯⎯⎯⎯ →

(7)

LAB (≡ FeOH +2 ) + H 2 AsO 2 ⎯⎯⎯ → ( ≡ FeOH)(HAsO 2 ) + H + (8)

Luis found the removal effect of macroporous resin was better than gel resin. Perhaps macroporous resin has more

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surface area than gel resin which can adsorb more iron inside. Strong base anion resin with positive charge removed more arsenic than strong acid canion. Compared the iron-loaded resin with granular ferric hydroxide, Cumber found the former removed more arsenic than the latter, but the sorption kenetic was slow. Diffusion inside the granular maybe velocity limited process. Because the adsorption velocity of ion exchange resin was low, ion exchange fiber was used to remove arsenic. Ion exchange fiber, a kind of surface adsorption and separation material in fiber type, was 20–300 µm in diameter which composed of lots of single silk. Compared with traditional ion exchange resin, ion exchange fiber has high stable chemistry, short distance of transfer, fast adsorption and disadsorption, comprehensive purifying, good washing and regeneration capacity, low energy consumption and small fluid resistance (Xu et al, 2005). Liu et al (2002) prepared a new type of ion exchange fiber to remove arsenate from water. The batch sorption experiments showed that fibrous sorbent had high sorption capacity and good kinetic property for arsenate ion. The sorption kinetic data can be described by Lagergren pseudo-second order rate equation very well. Greenleaf (2006) loaded nanoparticle hydrated iron oxide (HFO) on ion exchange fiber to sorb arsenic. Compared with the corresponding resin, the efficiency was higher, sorption and desorption velocity was more fast. Other anions have less influence on it. While the marked shortcoming of ion exchange fiber was its high cost.

3

Conclusion

The literature survey has indicated that each of the discussed techniques can remove arsenic under specified conditions. Different techniques have different advantages and disadvantages. Specific technique can be selected according to specific conditions. The existing methods should be perfected to gain the best effect.

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