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
Journal of Water and Environment Technology, Vol 7, No 1, 2009 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 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 is found in Taiwan, Xinjiang, Inner Mongolia, Shanxi, Ningxia, Jilin, Qinghai, Received November 21, 2008, Accepted January 27, 2009 - 29 - Journal of Water and Environment Technology, Vol 7, No 1, 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 highlycontaminated 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 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 - 30 - Journal of Water and Environment Technology, Vol 7, No 1, 2009 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 Stirring Raw Water Pump 150 cm Raw water (As-Fe) Treated water Gravel Pea-gravel Sand filter Treated water Sand Filter Sand Filter Sand filter 300 mm Treated water Strainer 30 cm Community-type AIRU Community- Dia, Φ = 25 mm Household-type AIRU Household- Lab-scale AIRU Lab- Figure 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 Table Summary of the procedures and methods for sampling and analyses of selected water quality parameters in this study Water Quality Parameters Sampling Procedure Container Storage time / Temperature Method Trace Metals: Fe, As -Acidifying with HNO3 (pH