Published: January 18, 2011 r2011 American Chemical Society 2220 dx.doi.org/10.1021/ie101220a | Ind. Eng. Chem. Res. 2011, 50, 2220–2226 ARTICLE pubs.acs.org/IECR Adsorption Isotherms of Arsenic on Conditioned Layered Double Hydroxides in the Presence of Various Competing Ions Megha Dadwhal, Muhammad Sahimi, and Theodore T. Tsotsis* Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, University Park, Los Angeles, California 90089-1211, United States ABSTRACT: Analysis of eﬄuents from various power plants indicates that, in addition to arsenic (As), these eﬄuents also contain such ions as ﬂuorides, nitrates, chlorides, carbonates, sulfates, and phosphates. Adsorption is considered the method of choice for the treatment of waste streams containing As. Of concern, therefore, is the impact of such competing anions on the capacity for As adsorption of the various materials utilized. In this work, we systematically investigated the eﬀects of such ions on the As removal capacity of layered double hydroxides (LDHs), a class of materials investigated by our group and others as eﬀective adsorbents for As and other heavy metals. The adsorption of As in the presence of each of these competing ions on LDH follows the extended Sips isotherm. It is concluded from our study that, of all of these ions, ﬂuorides and nitrates have the least eﬀect on As adsorption, followed by chlorides, carbonates, sulfates, and phosphates. Phosphates, in particular, have a very strong impact. 1. INTRODUCTION The accumulation of toxic heavy metals in the environment due to discharges from mining operations and various other industries is presently a serious concern. Among such metals, arsenic (As) is a key priority pollutant, naturally occurring in the environment as a result of the weathering of rocks, but also occurring in the discharge of eﬄuents from various industries and power plants, as well as agricultural eﬄuents because of the use of arsenic-containing pesticides. Arsenic is a metalloid chemical element and is widely known for its high chronic toxicity to humans. 1 The maximum concentration limit (MCL) for As in drinking water was recently revised in various countries including the United States, Europe, and Japan 2 from 50 to 10 μg/L. In countries such as Bangladesh, more than 60% of the groundwater contains naturally occurring As with a concentration that often signiﬁcantly exceeds the World Health Organization (WHO) guidelines of 10 ppb. Given how widespread As contamination is and the health threat that it poses, it is important to develop eﬀective techniques for its cleanup. Methods that have been investigated include precipitation-ﬁltration, ion exchange, membrane separation, and adsorption; the latter technique, in particular, has been found to be superior for water reuse applications, in terms of initial capital costs, simplicity in design, and ease of operation. So far, common adsorbents, such as various clays and naturally occurring minerals, 3,4 activated 5-7 and mesoporous alumina, 8 ferric hydroxide, 9 and ferrihydrite, 10 have all been investigated. Layered double hydroxides (LDHs), which oﬀer a large inter- layer surface to host diverse anionic species 11-14 with the additional advantage of being potentially easily recyclable, have also received considerable attention in recent years by various groups, 15-19 including our own. 12-14 LDHs have been shown to be eﬀective adsorbents for As (as well as for other heavy metals such as Se); nevertheless, the performance of these materials in practical situations still needs further investigation. In the present study, the focus was on the use of an LDH for the removal of As from real power-plant eﬄuents. Such eﬄuents contain, in addition to As, various others ions, so the ability of the LDH to remove As from these eﬄuents still needs to be tested, because these ions can, in principle, strongly compete with As for the available adsorption sites on the LDH. In the past, our group carried out preliminary studies on the eﬀects of some of these competing ions on the As and Se adsorption capacities of uncalcined and calcined LDHs. 12 In those experiments, solutions were treated containing 20 ppb of As (or Se) and one additional competing ion (such ions included NO 3 - , CO 3 2- , SO 4 2- , and HPO 4 2- ) with concentrations ranging from 40 ppb to 1000 ppm. It was found that the eﬀect of competing anions on the adsorption of As(V) decreased in the order HPO 4 2- > CO 3 2- > SO 4 2- > NO 3 - and for Se(IV) in the order HPO 4 2- > SO 4 2- > CO 3 2- > NO 3 - . The structural characteristics of the LDH and their eﬀect on As adsorption were also investigated in these studies, as were the eﬀects of other process parameters, such as adsorbent dosage. 12-14 Other groups have also studied the eﬀects of competing ions on the adsorption of As (and Se) on LDHs. Their eﬀorts were limited, however, and involved experiments with solutions con- taining a single competing ion and As (or Se), both at ﬁxed concentrations. Based on such experiments, for example, You et al. 20 concluded that competing ions such as phosphates, sulfates, carbonates, ﬂuorides, chlorides, bromides, iodides, and nitrates strongly aﬀect As adsorption on LDHs in the order HPO 4 2- > SO 4 2- > CO 3 2- >F - > Cl - > Br - ≈ I - > NO 3 - . Wang et al., 21 who also studied the eﬀects of such competing ions on As adsorption on LDHs, reported similar eﬀects, with ions aﬀecting Received: June 3, 2010 Accepted: December 15, 2010 Revised: December 8, 2010