To enhance the removal of mercury (Hg(II)) and cadmium (Cd(II)) from aqueous solutions, rice bran (RB) was reacted with epichlorohydrin and then modified with ethylenediamine and sodium chloroacetate to bear iminodiacetate functional groups. The modified rice bran (MRB) was characterized by Fourier transform-infrared spectroscopy (FT-IR), thermogravimetric analysis (TG), energy dispersive spectroscopy (EDS), back titration, and X-ray photoelectron spectroscopy (XPS). The adsorption properties

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... MRB for Hg(II)/Cd(II) ions were also evaluated in batch experiments. The sorption kinetic experimental data were best described by the pseudo-second-order model. The maximum adsorption capacity (163.9 mg/g for Hg(II) at pH 5.0 and 106.4 mg/g for Cd(II) at pH 6.0) was observed at 298 K, and the isotherm adsorption equilibrium of MRB was followed by Langmuir isotherm equation. The major adsorption mechanisms should be predominantly controlled by the formation of complexes between the functional groups of MRB and Hg(II)/Cd(II) ions as well as ion-exchange. The regeneration experiments showed that the MRB could be successfully reused for six cycles when 0.1 M HCl eluent was used. Rice bran metal sorbent," BioResources 11 (3) , 7145-7161. 7146 et al. 2012; Tran et al. 2015), zeolites (Fardmousavi and Faghihian 2014; Nguyen et al. 2015), and biomaterials (Kushwaha et al. 2015; Yu et al. 2015). Moreover, bio-sorbents like chitosan (Kandile et al. 2015; Karthik and Meenakshi 2015), algae (Carro et al. 2013), bacteria (Huang and Liu 2013), and agricultural by-products (Liu et al. 2010; Anagnostopoulos et al. 2012; Huang and Lin 2015) have excellent adsorption capacity for these toxic ions. Rice bran (RB) is an agricultural by-product resulting from rice processing and represents approximately 10% of the entire rice grain weight. A large amount of this byproduct is used as animal feed or discarded as waste. RB contains vitamins, carbohydrates, nitrogen, and phosphorus compounds, which have carboxyl groups, hydroxyl groups, and imino groups (Montanher et al. 2005) . Due to its granular structure, chemical stability, insolubility in water, and local availability, RB is a potential absorbent for removing toxic metals from wastewater. RB can remove Cd(II), Cu(II), Pb(II), Zn(II), Cr(III), Cr(VI), and Ni(II) from water (Wang et al. 2008; Chen et al. 2012) . However, the adsorption capability of RB for these heavy metal ions is relatively poor due to its low content of active functional groups. Improving the adsorption properties of RB via carbonization (Zhang et al. 2014) or chemical modification (Fatima et al. 2013) is thus important. It has been reported that the presence of -COOH or -NH2 groups can greatly improve the adsorption capacity of a sorbent on Hg(II)/ Cd(II) (Cui et al. 2015) . The purpose of this study was to enhance the Hg(II) and Cd(II) adsorption capacities of RB by chemical modification using bearing iminodiacetate function on RB as chelating ligands. The characteristics of the resulting adsorbent were analyzed. The adsorption properties for Hg(II) and Cd(II) on the modified rice bran (MRB) from aqueous solution were evaluated at different pHs, contact times, temperatures, heavy metal ion concentrations, and ionic strength. The regeneration of the MRB was also investigated in its application. EXPERIMENTAL Materials Rice bran was obtained from a local farmer's market (Mianyang, China), thoroughly washed with distilled water, dried, crushed, and screened by a 100-mesh sieve. 2-(5-Bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP) (98%) and sodium chloroacetate were procured from Aladdin Chemical Ltd. (Shanghai, China). Mercury (II) chloride, cadmium nitrate, epichlorohydrin, ethylenediamine, sodium hydroxide, and other chemicals were all analytical grade and purchased from Kelong Chemicals Company (Chengdu, China). Preparation of MRB Preparation of the MRB adsorption material included the epoxidation of RB, ammoniation of the epoxidized RB, and carboxymethylation of aminated RB. For the epoxidation reaction, 5 g of RB was dispersed into 100 mL of sodium hydroxide (4%) with vigorous stirring. The reaction vessel was immersed into a water bath at 70 °C, and 30 mL of epichlorohydrin was added. After 3 h at 70 °C, the mixture was cooled to 25 °C and adjusted to pH 6 with HCl. The product was precipitated with acetone, filtered thoroughly, washed with ethanol (95%), and dried overnight at 90 °C. For the ammoniation reaction, 5 g of epoxidized RB, 25 mL of ethylenediamine, and 0.1 g of sodium carbonate were added to a round-bottomed flask and refluxed with magnetic stirring in a water bath at 60 °C for PEER-REVIEWED ARTICLE bioresources.com Zhou et al. (2016). "Rice bran metal sorbent," BioResources 11(3), 7145-7161. 7147