The adsorption of arsenate from wastewaters was investigated by applying Mg-, Zn- or Ca-modified nut residue biochar activated by nitrogen/steam. The parameters studied were the contact time, adsorbent dose, initial arsenate concentration and solution pH. The adsorption mechanism was investigated. Various analyses of the material before and after arsenate adsorption were carried out, and experimental data were simulated by applying two isotherm models. The results indicated that the maximum removal efficiency of arsenate was 29.4% at an initial concentration of 10 mg/L. The modification of biochar by Mg, Zn or Ca oxides increased the removal rate significantly, from 49.4% at 100 mg/L As⁵⁺ up to 8%, 97% and 97%, respectively. Zn-modified biochar presented an excellent performance for both low and high As⁵⁺ concentrations. All experimental data were accurately fitted by the Freundlich isotherm model (R² = 0.94–0.97), confirming a multilayer adsorption mechanism. For a biochar dose of 2 g/L, the maximum capacity of adsorption was enhanced after Mg-, Zn- or Ca-modification from 12.4 mg/g to 35 mg/g, 50 mg/g and 49 mg/g, respectively. The potential mechanisms of adsorption were ligand exchange, chemical complexation, surface precipitation and electron coordination.

The potential of a nut residue, activated only by physical means, to simultaneously adsorb Sr²⁺, Mn²⁺, Pb²⁺ and As⁵⁺ from contaminated waters was investigated. Mono-metal and multi-metal adsorption behavior and adsorption capacities by the solid material were compared for a range of initial metals concentrations, adsorbent dose, contact time and solution pH. The mechanisms of adsorption were examined by conducting structural, mineralogical and chemical analyses and applying two isotherm models to simulate experimental data. In the case of mono-metal adsorption, the maximum uptake of Sr²⁺ by the biochar was 12.3, of Mn²⁺ 23.8, of Pb²⁺ 24.7 and of As⁵⁺ 12.4 mg/g. In the case of multi-metal adsorption, the uptake of each metal was enhanced as the total ion flux density increased, revealing that there was no competition between these ions for biochar’s sorption sites. For an adsorbent dose of 2 g/L, the maximum adsorption capacity of biochar for Sr²⁺, Mn²⁺, Pb²⁺ and As⁵⁺ could be raised up to 47.6, 49.9, 25 and 48.7 mg/g, respectively. Potential adsorption mechanisms were chemical complexation or electron coordination between the metals and the biochar matrix, as well as mineral precipitation on the adsorbent surface.

This study examined the removal of toxic heavy metals Pb²⁺ and As⁵⁺ from wastewaters by applying a waste nut biomass for adsorption. The waste material was steam-activated and further modified with zinc nitrate tetrahydrate to improve its affinity for arsenate. The adsorption performance was evaluated across a range of contact times, sorbent dosages, initial metal concentrations, and pH levels, for both single-metal and mixed-metal systems. When the raw material was modified by Zn, the maximum uptake of As⁵⁺ reached a value of 51 mg/g, at a sorbent dosage of 2 g/L. In binary metal solutions, the biochar exhibited maximum capacities of 25.5 mg/g for Pb²⁺ and 48.5 mg/g for As⁵⁺, indicating minimal competition between the two ions for adsorption sites on the biochar surface.