Mathematical model predicting arsenic bioleaching in groundwater treatment Khondoker Mahbub Hassan, Kensuke Fukushi, Fumiyuki Nakajima, Kazuo Yamamoto and Quazi Hamidul Bari ABSTRACT In the Southeast Asia region, arsenic contamination of groundwater has been reported in several countries and a large number of arsenic treatment units were installed in many regions. Microbial activity can greatly affect the mobilization of arsenic under anaerobic conditions when coupled with the oxidation of organic matter. In this study, a mathematical model was developed to predict the bioleaching of arsenic in response to organic matter contamination in the treatment unit. Calibration of the model was performed using the data of batch experiments, and finally the results obtained from the laboratory experiments were used to verify the developed model. The microbial decomposition of organic matter led to an anaerobic condition within the accumulated sludge in the sand filter and hence caused the bioleaching of arsenic. Under the inoperative condition of the treatment unit for 5, 10, and 15 days along with organic matter contamination of 15 mg/L as biochemical oxygen demand, the concentrations of bioleached arsenic were found to be 59, 184, and 275 μg/L, respectively. This study revealed the bioleaching potential of arsenic due to organic matter inclusion in the treatment process, which might contribute in designing safe options for drinking water. Khondoker Mahbub Hassan (corresponding author) Quazi Hamidul Bari Department of Civil Engineering, Khulna University of Engineering & Technology (KUET), Khulna-9203, Bangladesh E-mail: khmhassan@yahoo.com Kensuke Fukushi Integrated Research System for Sustainability Science (IR3S), The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8654, Japan Fumiyuki Nakajima Kazuo Yamamoto Environmental Science Center, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Key words | arsenic removal, bioleaching, groundwater, mathematical model, organic matter INTRODUCTION Arsenic contamination of groundwater in several parts of the world has become a high-profile problem in recent years due to the use of tubewells for water supply, causing serious arsenic poisoning to large numbers of people. With newer-affected sites discovered during the last decade, a sig- nificant change has been observed in the global scenario of arsenic contamination, especially in the Southeast Asia region (Mukherjee et al. ). Microbial activity can greatly affect the mobilization of arsenic under anaerobic con- ditions by either an indirect or a direct mechanism (Zobrist et al. ). The former is the reductive dissolution of iron hydroxide minerals, leading to the release of associ- ated arsenic into solution and the latter is the direct reduction of As(V) associated with a solid phase to the less adsorptive As(III). Dissimilatory arsenate-reducing bac- terium (DARB), which achieve growth by the respiratory reduction of As(V) to As(III), are able to mobilize arsenic via reduction of As(V) contained in minerals or contami- nated sediments. Ormland & Stolz () reported that the reaction is energetically favorable when coupled with the oxidation of organic matter because the As(V)/As(III) oxi- dation/reduction potential is þ135 mV. Bioleaching of arsenic is an oxidation–reduction process, where organic matter is oxidized to supply the required electrons and ferric iron or As(V) is reduced by acting as an electron acceptor (Figure 1). This process is incorporated within a sequential term- inal electron acceptor reaction framework, including aerobic, ferric iron reducing, and As(V) reducing pro- cesses (Lim et al. ). According to another study, the reduction of arsenate plays a relatively minor role in the solubilization of arsenic sorbed to iron hydroxides and 427 © IWA Publishing 2012 Journal of Water Supply: Research and Technology—AQUA | 61.7 | 2012 doi: 10.2166/aqua.2012.041