Radiation-Induced Carboxyl-Functionalized Sweet Sorghum Stalk for Adsorption of Cu(II), Zn(II), Pb(II) and Cr(VI)

Heavy metal pollution caused by industrial activities and improper waste disposal poses severe threats to ecosystems and human health, as heavy metal can accumulate in the food chain. Conventional remediation methods are limited by high costs, secondary pollution and poor selectivity for low-concentration heavy metal ions, while agricultural waste-based biosorbents suffer from insufficient active sites and low adsorption capacity. Carboxyl-functionalized sweet sorghum stalk fermentation residue biosorbent FSSR-g-AAm-EDA-SA (FSSR-AES) was synthesized via a two-step modification: Co-60 γ-ray irradiation grafting of acrylamide followed by reaction with ethylenediamine and succinic anhydride. FSSR-AES was characterized by FTIR, SEM-EDS and titration, with carboxyl content reaching 2.55 mmol/g. Static adsorption experiments were carried out under the following conditions: initial metal ion concentrations ranged from 25 to 250 mg/L, prepared with CuCl2, ZnCl2, PbCl2, and K2Cr2O7, respectively. The initial pH was adjusted to 6.0 for the adsorption of Cu(II), Zn(II), and Pb(II), and to 2.0 for Cr(VI). The adsorbent dosage was 0.1 g for Cu(II), Zn(II), and Cr(VI) adsorption, while 0.05 g of adsorbent was used for Pb(II) adsorption. Isothermal model fitting showed FSSR-AES achieved maximum adsorption capacities of 21.7, 29.5, 43.1 and 34.9 mg/g for Cu(II), Zn(II), Pb(II) and Cr(VI) at 313.15 K, respectively. Adsorption conformed to the Langmuir model (monolayer chemisorption), with Cu(II) following Freundlich at low temperatures. The higher adsorption of FSSR-AES compared to unmodified FSSR (qm of 1.9, 6.9 and 23 mg/g for Cu(II), Pb(II) and Cr(VI) under the same operational conditions) is attributed to its high carboxyl group content, which provides abundant active sites for heavy metal cations binding. This study provides a potential green modification method for agricultural waste valorization for heavy metal removal and environmental remediation.