Leaching of arsenic in response to organic matter contamination in groundwater treatment practice

ABSTRACT A large number of drinking water treatment units have been installed in many regions adopting the technique of arsenic removal through adsorption and co-precipitation with the naturally occurring iron in groundwater and subsequent sand filtration. This study revealed the consequence of the organic matter inclusion on the arsenic treatment process for drinking water. Laboratory investigation confirmed that the organic contamination in the treatment process impeded the arsenic removal efficiency depending on the types and concentrations of the organic matters. The impact of organic matter contamination on the arsenic removal efficiency was almost immediate and the autoclaved examination showed similar results. Nevertheless, the bioleaching of arsenic, 93 μg/L, from the accumulated sludge in the filter bed was observed under the inoperative condition, for 7 days, of the treatment unit. However, in the control observation (using organic matter plus antibiotic) the effluent arsenic concentration was found to be less than 30 μg/L. The effluent iron concentration in the bioleaching process was not worth mentioning and found to be less than 0.22 mg/L. In this study, the chemical and biological consequences of the organic matter contamination on the arsenic removal practice is elucidated, which might contribute in designing safe options for drinking water

Leaching of arsenic in response to organic matter contamination in groundwater treatment practice

Khondoker Mahbub Hassan 1) *, Kensuke Fukushi 2) , Fumiyuki Nakajima 3) , Kazuo Yamamoto 3)

1) Department of Urban Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan (e-mail: hassan@env.t.u-tokyo.ac.jp) *Corresponding author

2) Integrated Research System for Sustainability Science (IR3S), The University of Tokyo, 7-3-1 Hongo,

Bunkyo-ku, Tokyo 113-8654, Japan (e-mail: fukushi@ir3s.u-tokyo.ac.jp)

3) Environmental Science Center, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

(e-mail: yamamoto@esc.u-tokyo.ac.jp ; nakajima@esc.u-tokyo.ac.jp)

ABSTRACT

A large number of drinking water treatment units have been installed in many regions adopting the technique of arsenic removal through adsorption and co-precipitation with the naturally occurring iron in groundwater and subsequent sand filtration This study revealed the consequence of the organic matter inclusion on the arsenic treatment process for drinking water Laboratory investigation confirmed that the organic contamination in the treatment process impeded the arsenic removal efficiency depending on the types and concentrations of the organic matters The impact of organic matter contamination on the arsenic removal efficiency was almost immediate and the autoclaved examination showed similar results Nevertheless, the bioleaching of arsenic, 93 μg/L, from the accumulated sludge in the filter bed was observed under the inoperative condition, for 7 days, of the treatment unit However, in the control observation (using organic matter plus antibiotic) the effluent arsenic concentration was found to be less than 30 μg/L The effluent iron concentration in the bioleaching process was not worth mentioning and found to be less than 0.22 mg/L In this study, the chemical and biological consequences of the organic matter contamination on the arsenic removal practice is elucidated, which might contribute in designing safe options for drinking water

Keywords: Arsenic removal, groundwater, organic matter

INTRODUCTION

Arsenic, the world’s most hazardous chemical is found to exist within the shallow zones of groundwater of many countries in various concentrations Arsenic contamination in water has posed severe health problems around the world With newer-affected sites discovered during the last decade, a significant change has been observed in the global scenario of arsenic contamination, especially in Asian countries Before 2000, Bangladesh, West Bengal in India and sites in China were the major incidents of arsenic contamination in groundwater Between

2000 and 2005, arsenic-related groundwater problems have emerged in different Asian countries, including new sites in China, Mongolia, Nepal, Cambodia, Myanmar, Afghanistan, DPR Korea, and Pakistan (Mukherjee et al., 2006) There are reports of arsenic contamination from Kurdistan province of Western Iran and Vietnam where several million people may have a considerable risk of chronic arsenic poisoning During 1998, 41 of the 64 districts in Bangladesh were identified as having concentrations of arsenic in groundwater exceeding 50 μg/L (Sengupta et al., 2003) and about 50% of the installed hand tubewells were reported to have high arsenic concentrations (Rahman and Ishiga, 2003) It is apparent from the current arsenic research in China that the epidemic area is still expanding Recent updates on chronic arsenicism in PR China (Xia and Liu, 2004) state that, up to now, chronic arsenicism via drinking-water isfound in Taiwan, Xinjiang, Inner Mongolia, Shanxi, Ningxia, Jilin, Qinghai,

Received November 21, 2008, Accepted January 27, 2009

and Anhui provinces, and in certain suburbs of Beijing In a long-term survey on 140,150 water samples from hand-tubewells in West Bengal it was found that 48.2% had arsenic concentrations of >10 μg/L (WHO guideline value for drinking water) and 23.9% had >50 μg/L (Mukherjee et al., 2006) Arsenic contaminations of the Red River Delta in Hanoi city and the surrounding rural districts of Vietnam were first reported in 2001 (Berg et al., 2001) Analysis of raw groundwater pumped from the lower aquifer for the Hanoi water supply showed arsenic levels of 240–320 μg/L in three of eight arsenic treatment plants In Cambodia, the natural arsenic originates from the upper Mekong basin, and is widespread in soils Within the lower Mekong delta, 5.7% of all groundwater samples exceeded 50 μg/L, while 12.9% exceeded 10 μg/L (Stagner et al., 2005) A small-scale health survey conducted

in Myanmar in 2002 reported that 66.6% of the water samples from wells have arsenic levels

of >50 μg/L (Tun, et al., 2002) In 1987, skin manifestations of chronic poisoning of arsenic were first diagnosed among the residents of Ronpibool district of Thailand (Choprapawon and Rodcline, 1992) The people of this district use water, which drains from the highly-contaminated areas of the Suan Jun and Ronna Mountains having 0.1% arsenopyrite

Mobility of arsenic is primarily controlled by sorption onto metal oxide surfaces and the scope of this sorption is highly influenced by the presence of organic matter Ligand exchange-surface complexation, between carboxyl/hydroxyl functional groups of organic matter and metal hydroxides, was found as the dominant interaction mechanism, under circumneutral pH conditions (Gu et al., 1994) Therefore, they tend to compete with arsenic anions for adsorption to the solid surfaces (Xu et al., 1988) Redman et al (2002) proposed that aqueous organic-metal complexes may, in turn, associate strongly with dissolved arsenic anions, presumably by metal-bridging mechanisms, diminishing the tendencies of such anions

to form surface complexes However, organic decomposition due to microbial action may lead to anaerobic conditions and hence anaerobic bacteria can greatly affect the mobilization

of arsenic from the associated solid phase by either an indirect or a direct mechanism (Zobrist

et al., 2000) The former is the reductive dissolution of iron hydroxide minerals, leading to the release of associated arsenic into solution The latter is the direct reduction of arsenate associated with a solid phase to the less adsorptive arsenite The reaction is energetically favorable when coupled with the oxidation of organic matter because the arsenate/arsenite oxidation/reduction potential is +135 mV (Oremland and Stolz, 2003) Organic matter is ubiquitous in natural waters and typically found at TOC (total organic carbon) concentrations between 1 and 50 mg/L (Redman et al., 2002) The water with 50 mg/L of TOC is not appropriate for drinking water treatment Due to lack of other options/sources, groundwater with high organic contamination is even used in drinking water treatment in the rural areas of Bangladesh Moreover, the concentration of organic matter is sometimes unnoticed in the installation process of the treatment unit

Several techniques were reported for the removal of arsenic from groundwater including physicochemical and biological treatments and membrane filtrations (Dang et al., 2008) Based on the established biological iron oxidation from groundwaters (Dimitrakos et al., 1992), arsenic removal by adsorption and co-precipitation onto the flocs of iron hydroxides and subsequent sand filtration has become a very popular technique Moreover, there was an indication of arsenite oxidation by iron oxidizing bacteria, leading to improved overall removal efficiency (Katsoyiannis and Zouboulis, 2004) Adopting this technique, different types of arsenic and iron removal units (AIRU) were designed and installed in many regions (Figure 1) Organic matters, present in groundwater and also from unsanitary operation and maintenance of the AIRU might hamper the arsenic removal efficiency The water quality in

real field situation includes inorganic anions, cationic metals and both the inorganic forms of arsenic (arsenate and arsenite) along with organic contaminants All these constituents are potentially important factors influencing the arsenic removal efficiency, depending on their mutual interactions in the aquatic environments of treatment units Many researchers have focused on the quantitative influence of several inorganic competing ions for the removal of arsenic from groundwater (Meng, et al., 2001, Stollenwerk, et al., 2007) However, the harmful consequence of arsenic mobility due to the organic contamination in the AIRU treatment process is still unrecognized and needs to be investigated properly to ensure effective remediation strategies In this study, the consequence of organic matter inclusion in feed water of the AIRU on the removal performance of arsenic and iron has been elucidated Both the chemical and the biological phenomena related to this issue were addressed

Figure 1 Typical arsenic and iron removal units (AIRU)

MATERIALS AND METHODS

Field study

Preliminary minor-scale field inspection was made in Bangladesh to presume the level of organic matter contaminations in the existing arsenic and iron removal units and their removal performances The collection procedure of water samples, the water quality parameters tested and the method used for analysis are summarized in Table 1

Table 1 Summary of the procedures and methods for sampling and analyses of selected water quality parameters in this study

Water Quality

Parameters

Sampling

Storage time / Temperature Method Trace Metals:

Fe, As

-Acidifying with HNO3 (pH <2) Plastic < 7 days ICPMS

Organic Matter:

TOC

-Acidifying with HCl (pH <2) and refrigeration Glass < 7 days / 4ºC

High-temperature combustion catalyzed oxidation

ICPMS = Inductively Coupled Plasma Mass Spectrometry; TOC = Total Organic Carbon

Household-type AIRU Community-type AIRU

Gravel

Treated

water

Sand filter Pea-gravel

Raw water (As-Fe)

Treated water

150 cm

30 cm

Sand filter Strainer

Lab-scale AIRU

Dia, Φ = 25 mm

Pump Raw Water Stirring

Treated water

Household-type AIRU Community-type AIRU

Gravel

Treated

water

Sand filter Pea-gravel

Raw water (As-Fe)

Treated water

150 cm

30 cm

Sand filter Strainer

Raw water (As-Fe)

Treated water

150 cm

30 cm

Sand filter Strainer

Lab-scale AIRU

Dia, Φ = 25 mm

Pump Raw Water Stirring

Treated water

Laboratory study

Laboratory-scale AIRU was designed and tested under variable dosages of organic matters In order to minimize the hazardous and toxic waste generation during the research activity in the laboratory, a small cross-sectional area (diameter, φ = 25mm) was taken into consideration for the development of the AIRU (Figure 1) The raw water was kept stirred to avoid any deposition of the particles in the upper tank Then, the raw water was fed to the sand filter reactor using a pump and hence the concentrations of arsenic and iron were similar to the input dose The sand (effective size, D10 = 0.4 mm; uniformity coefficient, UC = 1.7) filter bed having a depth of 300 mm was used The filtration rate was kept around 0.6 ~ 0.7 m/hr, which

is the highest permissible filter loading rate for slow sand filtration (Montogomery, 1985) It

is evident from several studies that the overall arsenic removal performance in the AIRU treatment process was mainly contributed by arsenate [As(V)] in the influent water, due to its high adsorption capacity with iron hydroxide solid phase (Katsoyiannis and Zouboulis, 2004) Thus, the contaminated groundwater was prepared artificially using arsenate (H3AsO4) and ferrous sulphate (FeSO4 7H2O) in Milli-Q water and then the pH was adjusted to 7 by using sodium hydroxide (NaOH) reagent Several studies identified that the organic matter contamination in groundwater of the shallow reducing aquifer is frequently combined with high concentrations of nitrogen and phosphorus (Bhattacharya, et al., 2001, Stollenwerk, et al., 2007) The source of this contamination was suspected to be the wastewater from pit latrines

as well as the grey water, which was highly biodegradable Thus, in laboratory study, the simulated wastewater was prepared, using tryptone (T), yeast extract (Y) and glucose (G) in pure water, following standard methods (2005) for plate count (G:Y:T ≡ 1:2.5:5 wt/wt) excluding agar, which would give a preferable environment for the growth of bacteria Laboratory reagent-grade chemicals from Becton, Dickinson and Company (BD), USA were used in the above preparation Moreover, humic acid, constituting the major part of organic contents in groundwater, was used separately to find its impact on the arsenic and iron removal performances in AIRU Humic acid reagent-grade chemicals from Aldrich, USA was used in this study Each organic contamination dosage was maintained for 10 bed volumes (volume of permeate/volume of filter bed) of effluent water and within this period, the removal efficiency was found to become almost stable The sampling was done at the 10th bed volume of effluent water in the AIRU In the real field situation, the AIRU is usually operated

in both the “continuous flow” and the “intermittent flow” modes and occasionally it is kept inoperative for afew days The microbial decomposition of organic matter would most likely happen in the inoperative condition of the AIRU In the biodegradation process of organic matter, the aerobic oxidation is precedingthe other reactions, because oxygen reducers would derive more energy from the substrate than the iron and arsenic reductions The depletion of dissolved oxygen due to microbial action may lead to an anaerobic condition within the accumulated sludge in the filter bed and hence would cause the bioleaching of arsenic Thus, the bioleaching of arsenic, in the presence of organic matter, was observed under the inoperative condition of the laboratory-scale AIRU Subsequently the water samples were collected from the outlet of the effluent pipe for the laboratory analyses In another observation, antibiotic (tetracycline hydrochloride, 8 mg/L) was added to control the microbial activity in the bioleaching process

Analytical techniques

Laboratory analyses for total arsenic and iron concentrations in water samples were carried out by inductively coupled plasma mass spectrometry (ICP-MS, HP4500, Yokogawa), which allowed detection of arsenic and iron species with limits of 0.3 μg/L and 0.1 μg/L, respectively For the ICP-MS analysis, the water samples were digested in a closed

microwave system (Multiwave 3000, Perkin-Elmer) in 10% HNO3 solution and in temperature range of 165 ±5 ºC for 20 minutes following the standard methods (2005) After digestion, the water samples were adjusted to 1% HNO3 concentration and filtered through 0.45 μm PTFE filter for the analysis of arsenic and iron using ICP-MS For the isobaric interference of ArCl+ in the arsenic analysis, the EPA recommended corrections (EPA Method 200.8) were applied The organic matter concentrations in stock solution and in other water samples were determined by using a total organic carbon analyzer (TOC-VCSH, Shimadzu), which allowed detection limit of 0.4 mg/L as TOC The value of pH was measured using a pH meter (HACH Co., USA) Dissolved oxygen (DO) concentrations within the accumulated sludge in sand filter bed as well as in liquid phase of the laboratory-scale AIRU were observed using a microelectrode with a 10 μm tip diameter and a micromanipulator having a vertical resolution of 10 μm (OX10, MM33-2, Unisense) Standard methods (2005) were followed in all laboratory analyses of the test samples

RESULTS AND DISCUSSION

Field inspection of the treatment units

Influent and effluent water samples from the six AIRUs (I~VI) at the field level were collected from arsenic-contaminated groundwater areas in Bangladesh Laboratory analyses

of the field water samples identified that most of the groundwater sources had significant organic contamination (Table 2) In AIRUs (I~IV), in spite of having adequate iron concentrations of 3.4~6.9 mg/L in the influent water to enable adsorption and co-precipitation

of arsenic, the effluent arsenic concentrations were beyond 50 μg/L, which is the acceptable limit of Bangladesh standards for drinking water In these cases, high concentration of organic matters, 25.8~51.4 mg/L as TOC, were observed in the influent water Several studies reported that the dissolved organic carbon (DOC) concentrations in groundwater from reducing aquifers in Bangladesh range from 1 to 15 mg/L (BGS and DPHE, 2001, Bhattacharya et al., 2002, Anawar et al., 2003) The unusually high concentration of organic matter in the studied groundwater from shallow aquifer might be due to the anthropogenic contamination from pit latrines as well as the grey water in the rural environment High organic concentrations in the influent water might hamper the arsenic treatment process, and ultimately high concentration of arsenic would appear with the effluent water

Table 2 Effect of influent organic matter contaminations on the arsenic removal performance through field-AIRUs in Bangladesh

Effluent Water Quality

AIRU-I was a household-type unit while all other AIRUs (II~VI) were community-type units

Field-AIRUs Influent Water Qality

On the other hand, for AIRUs (V and VI) in a different locality, which were free from organic contamination and having iron concentrations of 2 mg/L and 5 mg/L, respectively in the influent water could satisfactorily treat the arsenic concentration less than 40 μg/L in the

effluent water The removal efficiency of TOC was found to be extraordinarily high in the community-type AIRUs (II~IV) because of its far-extended hydraulic retention time in comparison with the household-type AIRU-1 Another study on the AIRU treatment process

in Bangladesh, suggested that the removal efficiency was significantly influenced by the raw water concentrations of both arsenic and iron and in this context a relationship was established (Ahmed, 2001) Following this relationship, the removal efficiency of arsenic in our studied AIRUs (II and VI), having the same concentration of iron in the raw water, were estimated to be 74% and 77%, respectively However, in our study they were found to be 53% and 80%, respectively Considerably less arsenic removal efficiency in AIRU-II from its estimated value might be due to the presence of high concentration of organic matter in the raw feed water Clear evidence was not obtained from this limited field survey The small-scale field investigation was carried out only to presume the level of organic contamination in the existing AIRUs and the treatment performance A precise and large-scale spatial survey was required to represent the real field situation of the above aspect Considering the requirements of resources and time related to the precise field survey and the potential human health-risk related to the organic contamination in AIRUs, laboratory experiments were carried out using artificially contaminated groundwater in the simulated AIRU

Performance of the laboratory AIRU

The removal efficiency of arsenic and iron in the developed AIRU at the laboratory was monitored in controlled condition where organic contamination was totally avoided Arsenic and iron concentrations in effluent water never exceeded 15μg/l and 0.1mg/l, respectively from their influent concentrations of 500μg/l and 5mg/l, respectively indicating removal

efficiencies over 97% for arsenic and 98% for iron (Figure 2)

Figure 2 Arsenic and iron removal performances in laboratory-scale AIRU (control) which

was free from organic contamination Arsenic and iron were spiked in influent water to concentrations of 500 μg/L and 5 mg/L, respectively Column values represent the average of triplicate samples, and error bars show the range of standard deviation

Such a high arsenic removal performance in the laboratory-scale AIRU, in comparison to the field-AIRUs data shown in Table 2 (V and VI), was achieved due to using arsenate for the

0 5 10 15 20

Effluent bed volume (BV)

0.00 0.05 0.10 0.15 0.20

Iron concentration Arsenic concentration

1

0 5 10 15 20

Effluent bed volume (BV)

0.00 0.05 0.10 0.15 0.20

Iron concentration Arsenic concentration

1

preparation of synthetically contaminated feed water This is because, the adsorption affinity

of arsenate onto iron hydroxide solid phase is much higher than that of arsenite, which is also

an associated contaminant in most arsenic-affected groundwater Moreover, other anions (phosphate, sulfate, silicate, bicarbonate, nitrate etc.), influencing the sorption process of arsenic in the AIRU treatment process, were not present in the synthetic feed water In the laboratory-scale AIRU, the treatment performances for both the arsenic and iron were found

to be almost stable throughout the whole observation period of 100 bed volumes of effluent water (standard deviation < 0.6% of removal efficiency) Slightly, higher removal performance with increased bed volume of treated water was due to deposition of iron hydroxides within the interstices of the filter bed media which provided increased adsorption surfaces and mechanical straining as well

Organic matter causes chemical leaching of arsenic in AIRU

The leaching of arsenic and iron in the presence of organic matter in AIRU feed water was investigated under dosage-response observations in multiple sets of laboratory reactors In case of simulated wastewater organic contaminations of 15 mg/L and 30 mg/L as TOC in the influent water of the AIRU, arsenic concentrations of 55 μg/L and 70 μg/L, respectively were observed in the effluent water (Figure 3) In the absence of organic contamination, however, effluent arsenic concentration never exceeded 15 μg/L (Figure 2) The TOC removal efficiency was found to be less than 8% in the laboratory-scale AIRU (Table 3) Thus, two major phenomena would be related to the decrease in arsenic removal efficiency in response

to organic matter inclusion in the feed water of the AIRU Firstly, some portion of arsenic could not form surface complexes with the iron hydroxide solid phase due to the competitive adsorption by organic matters and finally released in the effluent water Secondly, some part

of the dissolved arsenic anions combined with the aqueous organic-metal complexes and eventually came out with the effluent water

0 25 50 75 100

Influent organic concentration in AIRU (mg/L TOC)

Humic acid Simulated wastewater

Sampling in 10 BV

of effluent water

Figure 3 Effect of influent organic matter (humic acid and simulated wastewater)

contamination on the removal performance of arsenic in the laboratory-scale AIRU Arsenic and iron were spiked in influent water to concentrations of 500 μg/L and 5 mg/L, respectively Sampling was done at the 10th bed volume (BV) of effluent water

Table 3 Concentration of organic matters in the influent and effluent waters of the laboratory-scale AIRU

HA: Humic Acid; SWW: Simulated Waste Water.

Typically, the organic contents in groundwater are mostly contributed by humic substances Thus, the impact of humic acid contamination on the arsenic removal performance in AIRU was also studied The concentration of arsenic with effluent water in this case was found to be less in comparison to previously used simulated wastewater, possibly due to having fewer adsorption sites in higher molecular weight humic acids The functional groups of the organic matter molecules deprotonate at different pH conditions At low pH values, they are protonated and uncharged with a tightly coiled and cross-linked conformation, but at high pH values, they are dissociated and become negatively charged with a more open conformation

At the neutral pH value, some functional groups were reported to become negatively charged (Stevenson, 1982) The occurrence of negative charges and open conformation enables organic matter to be adsorbed onto positively charged reactive sites at the surface of metal hydroxides Regardless of the concentration and type of the organic matter inclusion in the AIRU feed water, the arsenic removal efficiency over 85% was achieved in the laboratory study (Figure 3), possibly due to the partially charged functional groups of the organic matter molecules On the other hand, iron concentration with effluent water was found to be higher

in the case of humic acid contamination (Figure 4), possibly due to the delay in the precipitation process of iron hydroxides

0.00 0.20 0.40 0.60 0.80 1.00

Influent organic concentration in AIRU (mg/L TOC)

Humic acid Simulated wastewater

Sampling in 10 BV

of effluent water

Figure 4 Effect of influent organic matter (humic acid and simulated wastewater)

contaminationon the removal performance of iron in the laboratory-scale AIRU Arsenic and iron were spiked in influent water to concentrations of 500 μg/L and 5 mg/L, respectively Sampling was done at the 10th bed volume (BV) of effluent water

A physiochemical explanation for the decrease in hydration reaction has been proposed by Ong and Bisque (1968) on the basis of the Fuoss-effect Mutual repulsion of the negatively charged functional groups of humic acid (carboxyls and hydroxyls) causes the polyelectrolyte

to adopt a stretched configuration Cations like ferric ions attaching themselves to the negatively charged groups cause a reduction in the intermolecular repulsion in the polymer chain favoring coiling Coiling expels a portion of the water of hydration that surrounds the molecule, converting it from a hydrophilic to a hydrophobic colloid Thus, the soluble complexes of iron-humic substances were released in effluent water Autoclaved experiment for the inclusion of organic matter in the AIRU feed water, showed similar performance in the removal of arsenic and iron Moreover, in the continuous-flow mode of the AIRU (Figure 2-4), the dissolved oxygen concentration within the sand-filter bed was always found to be greater than 6 mg/L Thus, it was evident that the above leaching phenomena were chemical rather than biological in nature

Organic matter causes bioleaching of arsenic in AIRU

In the real field situation and especially for the household-type AIRU, sometimes it is kept inoperative for few days when the family members go out for a trip The bioleaching of arsenic from the accumulated sludge in the filter bed was studied under the inoperative condition of the laboratory-scale AIRU Considering the favorable growth of the microorganism, the simulated wastewater organic contamination was used The initial decreasing pattern of the effluent arsenic concentration (Figure 5) might be due to less stable iron-arsenic complexes, which were driven out at the first stage In addition to this, biological iron oxidation (Dimitrakos et al 1992) in aerobic condition would contribute to the greater retention of iron-arsenic sludge

0 50 100 150 200

Inoperative time span of AIRU (days)

Control (Simulated wastewater + Antibiotic) Bioleaching (Simulated wastewater)

Figure 5 Effect of organic matter (simulated wastewater of 30 mg/L as TOC) contamination

on the bioleaching of arsenic from the accumulated sludge in filter bed under the inoperative condition of the laboratory-scale AIRU

However, significantly high concentration of arsenic, 93 μg/L on the 7th day of observation,

in the AIRU effluent water was noticed afterwards in the case of simulated wastewater inclusion in feed water In the course of the biodegradation of organic matter, aerobic oxidation was expected to precede the other reactions because oxygen reducers would derive

more energy (i.e., higher Gibbs free energy) from the substrate than the iron and arsenic reductions Thus, the depletion of dissolved oxygen due to microbial action led to an anaerobic reducing environment within the accumulated sludge in the filter bed and hence caused the bioleaching of arsenic by anaerobic bacteria The ratio of [As (III)] / [As (V)] concentrations in the effluent water was around 1.10 (data not shown.), indicating the microbial reduction and dissolution of ferric hydroxide sorbing phase as well as the dissimilatory arsenate reduction

On the contrary, the effluent iron concentration in the bioleaching experiment was not worth mentioning and it was alwaysfound to be less than 0.22 mg/L (data not shown) Thus, it may

be considered that the arsenic-iron sludge, retained mostly at the top portion of the sand filter bed, caused the bioleaching of arsenate, arsenite and ferrous iron under anaerobic conditions While collecting the effluents, the aqueous ferrous iron, reduced in the bioleaching process, re-oxidized to insoluble ferric form due to the intrusion of supernatant aerobic water into the sand-filter bed and was trapped by the mechanical straining mechanism, whereas a significant portion of arsenate and arsenite escaped due to inadequate iron hydroxide sorption sites at the bottom part of the sand-filter bed, and they were eventually released in the effluent water It was also evident from other studies that arsenic release from contaminated soils and sediments proceeds considerably faster under conditions favoring dissimilatory reduction of ferric iron leading to the dissolution of sorbing phases (Langner and Inskeep, 2000) and the reduction of arsenate plays a relatively minor role in the solubilization of arsenic sorbed to iron hydroxides In the case of the controlled microbial activity, using antibiotic in addition to the simulated wastewater, the effluent arsenic concentrations were found to decrease gradually The sorption equilibrium, established in the arsenic-iron sludge, might be interrupted due to the competing anions in the organic matter (Redman et al 2002) and consequently the desorbed arsenic would be released in the effluent water in the initial stage

In the bioleaching experiment, the dissolved oxygen (DO) concentration was checked through microelectrode studies (Figure 6) to verify the anaerobic condition within the accumulated sludge in the filter bed of the AIRU

0 2 4 6 8 10

Air-s urate d Liqui

d phs

e

Depth in sand filter bed (mm)

Figure 6 Dissolved oxygen (DO) concentration profile in AIRU in the bioleaching

observation time on the 7th day