Efficiency and Mechanism in Preparation and Heavy Metal Cation/Anion Adsorption of Amphoteric Adsorbents Modified from Various Plant Straws

Abstract

Cellulose can be modified for the loading of functional groups such as amino groups, sulfydryl groups, and carboxyl groups. Cellulose-modified adsorbents generally have specific adsorption capacities for either heavy metal anions or cations, and possess the advantages of wide raw material source, high modification efficiency, high adsorbent recyclability, and great convenience in recovery of the adsorbed heavy metals. However, the difference in efficiency of preparing the same heavy metal adsorbent by modification of various plant straw materials and mechanism for the difference have not been fully investigated. In this study, three plant straws, including Eichhornia crassipes (EC), sugarcane bagasse (SB) and metasequoia sawdust (MS), were sequentially modified by tetraethylene pentamine (TEPA) and biscarboxymethyl trithiocarbonate (BCTTC) to obtain amphoteric cellulosic adsorbents (EC-TB, SB-TB and MS-TB, respectively), which can simultaneously adsorb heavy metal cations and anions. The heavy metal adsorption properties and mechanism before and after modification were compared. Pb(II) and Cr(VI) removal rates by the three adsorbents were 2.2–4.3 folds and 3.0–13.0 folds of those before modification, respectively, following the order of MS-TB > EC-TB > SB-TB. In the five-cycle adsorption-regeneration test, the Pb(II) and Cr(VI) removal rate by MS-TB decreased by 58.1% and 21.5%, respectively. Among the three plant straws, MS contained the most abundant hydroxyl groups and the largest specific surface area (SSA), and accordingly MS-TB had the highest load of adsorption functional groups [(C-)N-H, (S-)C=S and (HO-)C=O] and the largest SSA among the three adsorbents, which accounts for its highest modification and adsorption efficiency.

Keywords: lignocellulose, modification, Tetraethylene pentamine, Heavy metals, Amphoteric adsorbents, Functional groups