Investigation of chromate and nitrate removal by adsorption at the surface of an amine-modified cocoa shell adsorbent

Nitrate (NO3-) and chromium (Cr(VI)) ions from contaminated aqueous solutions were successfully adsorbed onto functionalized cocoa shell (ECAB, an agricultural waste material) via chemical grafting of 3-aminopropyltriethoxysilane (APTES). The resulting adsorbent - ECAB-APTES-HCl - was characterized by SEM, zeta potential, calorimetry, FTIR spectroscopy and point of zero charge. The adsorption parameters were investigated to optimize the removal of chromium and nitrate ions. The maximum removal efficiency stabilized at around 91-99 % for chromium and 58 % for nitrate, for an initial concentration of 50 mg.L-1 of the ions. The adsorption process fitted well with both Freundlich and Langmuir isotherms, and with the pseudo-second order model. Accordingly, adsorption onto ECAB-APTES-HCl involved both physisorption and chemisorption. Desorption studies demonstrated that 82 % of nitrate and 79.8 % of chromium were recovered in less than 2 h. These results open onto the possible removal of heavy metals from aqueous solutions in the absence of energy constraints, using low-cost biomass relevantly functionalized for high adsorption efficiency.