Magnesium or bentonite modified almond kernel biochar for phosphate adsorption from contaminated water solutions

The potential of almond kernel, physically activated by nitrogen/steam, or chemically activated by Mg2+ or bentonite and then by nitrogen/steam, to adsorb phosphate from wastewaters was evaluated. The performance of biochar was studied as a function of initial phosphate concentration, adsorbent dose, contact time and solution pH. The mechanism of adsorption was investigated by carrying out structural, chemical and mineralogical analyses of the material before and after phosphate adsorption, as well as by applying two isotherm models, simulating the experimental data. The results showed that the specific surface area increased about 260 times after activation. Adsorption of phosphate was fast within 30 min, while reached equilibrium after 12 h. For Mg2+ modified material a higher ionic strength facilitated the uptake of phosphate by the biochar up to 59%, in contrast to the bentonite modified material, where adsorption of phosphate was complete (100%) at an initial concentration of 50 mg/L, declining thereafter. Mg2+ modified material was best fitted by the Freundlich model, while bentonite modified material was best fitted by the Langmuir model. For an adsorbent dose of 2 g/L maximum phosphate adsorption capacity after Mg2+ or bentonite modification was 82 mg/g and 54 mg/g, respectively. In the case of Mg2+ modification the potential mechanisms of adsorption were surface precipitation, electrostatic attraction, chemical complexation or electron coordination between the phosphate ion and the adsorbent. For bentonite modification, chemical complexation and π-π electron coordination were the potential adsorption mechanisms. •Almond kernel was physically activated by steam, or chemically activated by Mg2+ or bentonite.•Effects of phosphate concentration, adsorbent dose, contact time, solution pH were examined.•Maximum phosphate adsorption capacity after Mg2+ or bentonite modification was 82 mg/g and 54 mg/g.•Mechanisms for Mg2+-biochar were precipitation, electrostatic attraction, complexation, electron coordination.•Mechanisms for bentonite-biochar were complexation and π-π electron coordination.