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The abundantly available agricultural waste product walnut shells were treated with alkali for enhancement in adsorptive capacity for Cd(II) ions. The raw walnut shells (RWS) and alkali treated walnut shells (AWS) were characterized for functional groups and morphological changes using FTIR and SEM analysis. The adsorption increased drastically between pH 2 and 6. The Langmuir isotherm model provided a better fit with the experimental data than Freundlich by giving high correlation coefficients. The biosorption capacity of RWS and AWS for Cd(II) ions was found to be 4.20 and 14.29 mg g−1, respectively. Dubinin–Radushkevich (D‐R) isotherm model was used to find out the mean adsorption energy. The adsorption followed pseudo‐second order kinetics, suggesting monolayer coverage and a chemisorption process. The thermodynamic parameters including change in free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) were derived. Negative value of ΔG confirmed spontaneity of biosorption. Desorption efficiency showed no significant decrease till third cycle. © 2015 American Institute of Chemical Engineers Environ Prog, 34: 1613–1619, 2015

Alkali treated Citrus limetta peels were used for recovery of Ga(III) from its aqueous solution. The raw and alkali treated peels were characterized for functional groups. The efficiency of adsorption increased from 47.62 mg/g for raw peels to 83.33 mg/g for alkali treated peels. Between pH 1 and 3, the adsorption increased and thereafter decreased drastically. The adsorption followed pseudosecond order kinetics and Langmuir isotherm gave the best fit for the experimental data. Desorption studies showed 95.28% desorption after 3 cycles for raw peels while it was 89.51% for alkali treated peels. Simulated Bayer liquor showed 39.57% adsorption for gallium ions on raw peels which was enhanced to 41.13% for alkali treated peels.