Heavy metal adsorption of brewers spent grain in aqueous solution: Impact of mechanochemical esterification

Brewer’s spent grain (BSG) has shown promise as a biosorbent to remove heavy metals from water. However, the adsorptive performance of native BSG is limited and modifications to enhance said performance suffer from extensive use of organic solvents. A potential solution is mechanochemical esterification, in which solid-phase reactivity is enhanced through high-energy milling. In this study, the adsorptive performance of BSG, which has been mechanochemically esterified with citric acid, was investigated (BSGE). The maximum adsorption capacities q_max for nickel Ni(II), cadmium Cd(II), and lead Pb(II) ions were evaluated following bottle-point isotherm procedure in a buffered solution of sodium acetate, with a pH value of 4.5. The impact of the heavy metal initial concentration C₀ 1 – 250 mg/L on the equilibrium capacity q_e was studied and the data was fitted to the Langmuir and Freundlich models. The Langmuir model was best to describe data variance, as such q_max was used to describe adsorptive performance: 65.83 mg/g for Pb(II), 24.72 mg/g for Cd(II), and 15.11 mg/g for Ni(II), with Langmuir constants K_L of 0.149 L/mg, 0.031 L/mg and 0.023 L/mg, respectively. These findings indicate that mechanochemical esterification yielded a 232.2 % Pb(II), 576.3 % Ni(II) and 164.4 % Cd(II) gain in adsorptive performance over native BSG. Based on molar comparisons, adsorptive affinities were determined as follows: BSGE (Pb > Ni ≈ Cd) and BSG (Pb > Cd > Ni).