Journal of Colloid and Interface Science 315 (2007) 47–53 www.elsevier.com/locate/jcis Adsorption of arsenic(III) and arsenic(V) from groundwater using natural siderite as the adsorbent Huaming Guo a,∗ , Doris Stüben b , Zsolt Berner b a School of Water Resources and Environment, China University of Geosciences, Beijing 100083, People’s Republic of China b Institute for Mineralogy and Geochemistry, University Karlsruhe (TH), Karlsruhe 76131, Germany Received 4 April 2007; accepted 19 June 2007 Available online 26 July 2007 Abstract Batch and column tests were performed utilizing natural siderite to remove As(V) and As(III) from water. One hundred milligrams of siderite was reacted at room temperature for up to 8 days with 50 mL of 1000 μg/L As(V) or As(III) in 0.01 M NaCl. Arsenic concentration decreased exponentially with time, and pseudoequilibrium was attained in 3 days. The estimated adsorption capacities were 520 and 1040 μg/g for As(V) and As(III), respectively. Column studies show that effluent As was below 1.0 μg/L after a throughput of 26,000 pore volumes of 500 μg/L As water, corresponding to about 2000 μg/g of As load in the filter. Results of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) reveal that high As retention capacity of the filter arose from coprecipitation of Fe oxides with As and subsequently adsorption of As on the fresh Fe oxides/hydroxides. Arsenic adsorption in the filter from As-spiked tap water was relatively lower than that from artificial As solution because high HCO − 3 concentration restrained siderite dissolution and thus suppressed production of the fresh Fe oxides on the siderite grains. The TCLP (toxicity characteristic leaching procedure) results suggest that these spent adsorbents were inert and could be landfilled.  2007 Elsevier Inc. All rights reserved. Keywords: Arsenate; Arsenite; Drinking water; Removal; Retention 1. Introduction Groundwater enriched with As species such as arsenate (As(V)), arsenite (As(III)), and organic arsenic has become one of the most serious problems in water environment, espe- cially in the southeast of Asia, including West Bengal, India, Bangladesh, and China [1,2]. It is particularly worse when the groundwater is utilized as drinking water [3,4]. Although organic As species can be presented as a result of in situ biomethylation, inorganic As as As(III) and As(V) are generally considered to be the dominant species in natural water. The oxidation state of As depends primarily on pH and redox conditions, with As(V) being the most stable form under aerobic conditions as the pH-dependent deprotonated oxyan- ions of arsenic acid (H 2 AsO − 4 and HAsO 2− 4 ) and As(III) the chemically dominant forms in reducing environment as a neu- * Corresponding author. Fax: +86 10 8232 1081. E-mail address: hm_guo@hotmail.com (H. Guo). tral species (i.e., pK a1 = 9.2) at natural pH. Therefore, the As(III) is more difficult to remove from water at neutral pH by means of adsorption and coprecipitation due to the lack of elec- trostatic attraction [5,6]. However, most As-enriched ground- water is generally dominated by As(III), up to 96% of total As [2,7]. Furthermore, As(III) is about 60 times more toxic than As(V) [8,9]. Thus, the removal of As(III) from drinking water has received significant attention, as well as As(V) [10–14], and it is of major concern to many water utilities and governmental agencies. Among a variety of adsorbents for As removal, natural geo- materials are promising because they are relatively cheap, read- ily available in different particle sizes, and therefore may easily be used as column fillings in small-scale treatment plants. Ar- senic adsorption onto natural zeolite and volcanic stone [15], natural iron ores [16], oxisol [17], red mud [18], and fer- ruginous manganese ore [19] has been examined intensively. Contrastingly, few studies on As removal by natural siderite have been reported [20,21], where the combination of natural hematite and natural siderite has been utilized for As reten- 0021-9797/$ – see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.06.035