Chemical Engineering Journal 183 (2012) 365–371 Contents lists available at SciVerse ScienceDirect Chemical Engineering Journal j ourna l ho mepage: www.elsevier.com/locate/cej Long-root Eichhornia crassipes as a biodegradable adsorbent for aqueous As(III) and As(V) Sen Lin a , Guoxing Wang a , Zhongyuan Na b , Diannan Lu a,∗∗ , Zheng Liu a,∗ a Dept. of Chemical Engineering, Tsinghua University, Beijing 100084, China b Yunnan Research Institute of Ecological Agriculture, Yunnan 610203, China a r t i c l e i n f o Article history: Received 25 October 2011 Received in revised form 31 December 2011 Accepted 3 January 2012 Keywords: Arsenic adsorption Eichhornia crassipes Pseudo-second-order dynamic model Langmuir model a b s t r a c t A biodegradable adsorbent prepared from long-root Eichhornia crassipes has been tested for aqueous adsorption of As(III) and As(V). The surface properties and morphology of the root powder have been characterized by means of SEM, zeta-sizer, potentiometric titrimeter, FTIR, and XPS. Chemical composi- tion analysis confirmed that the hydroxyl, amino, and carboxyl groups on the surface of the root powder facilitated the adsorption of As(III) and As(V) by either electron sharing or electron transfer. The optimal pH values for the adsorption of As(III) and As(V) were determined as 7.5 and 3.0, respectively. The effects of co-ions and counter-ions on the adsorption of arsenite and arsenate have also been examined. It was shown that the adsorption efficiencies of As(III) and As(V) remained unchanged in the presence of Cl - , SO 4 2- , and NO 3 - . The presence of PO 4 3- , however, which has a similar stereochemical structure to that of the arsenic species, reduced the adsorption capacity substantially. The adsorption of As(III) or As(V) could be well described by the pseudo-second-order dynamic model at different initial concentrations and the Langmuir model at different temperatures. © 2012 Elsevier B.V. All rights reserved. 1. Introduction As a metalloid, arsenic [1] is harmful to humans and animals due to its toxicity, such as teratogenicity, carcinogenicity, and muta- genicity. Arsenite and arsenate [2] are the major forms of As in water. Since arsenite is more toxic and difficult to handle, oxida- tion of arsenite to arsenate is often the first step in waste treatments in practice [3]. The treatments of arsenic-contaminated water that have been applied to date can be grouped into six categories [4], namely chemical precipitation, ion-exchange adsorption, mem- brane separation, electrolysis, biosorption, and adsorption. Among these methods, adsorption has been extensively investigated [5], since it is more economically efficient. Active metals and their oxides or hydroxides, metal alloys [6], metal-loaded polymers [7], rare-earth materials [8], pure substances with numerous func- tional groups [5], minerals [9], and biodegradable waste [10] have been studied as adsorbents for arsenic in water. The adsorption mechanisms of metal oxides or hydroxides [11] and some organic substances (e.g., chitosan [12], chitin [13], macrocyclic materials [14]) have been delineated. Naturally occurring and biodegrad- ∗ Corresponding author. Tel.: +86 10 6277 9876. ∗∗ Corresponding author. Tel.: +86 10 6278 3153. E-mail addresses: ludiannan@mail.tsinghua.edu.cn (D. Lu), liuzheng@mail.tsinghua.edu.cn (Z. Liu). able adsorbents, such as spent grain [15], orange juice residue [16], shelled Moringa oleifera Lamarck seed powder [17], and so on [10], have been extensively studied in recent years because of their high abundances, low cost [18], and convenience of subsequent pro- cessing. While noteworthy efforts have been made onto improving the adsorption capacity [19,20] and understanding the underlying mechanisms [21,22], the research into biodegradable adsorbent is far from sufficient. In the present work, a new kind of biodegradable adsorbent, root powder of long-root Eichhornia crassipes, has been tested for the adsorption of arsenic. Long-root Eichhornia crassipes is distin- guished by its long root, which accounts for more than 80% of its biomass. Excellent availability and biodegradability make this root powder an ideal adsorbent for arsenic compounds. The objective of the present study has been to provide a detailed appraisal of the adsorption of arsenic compounds, with particular focus on the adsorption mechanism. The present work started with a character- ization of the structure of the root powder of long-root Eichhornia crassipes. FTIR and XPS were then applied to probe the interac- tions between the adsorbent and As(III) and As(V), respectively. The effects of pH, temperature, as well as the presence of other ions were examined. The adsorption data were also subjected to different adsorption models. Based on the results obtained, the adsorption mechanism was established, which is essential for the subsequent application of long-root Eichhornia crassipes as a biodegradable adsorbent. 1385-8947/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2012.01.013