This study presents the removal of phosphate from aqueous solution using a new silver nanoparticles-loaded tea activated carbon (AgNPs-TAC) material. In order to reduce costs, the tea activated carbon was produced from tea residue. Batch adsorption experiments were conducted to evaluate the effects of impregnation ratio of AgNPs and TAC, pH solution, contact time, initial phosphate concentration and dose of AgNPs-AC on removing phosphate from aqueous solution. Results show that the best conditions for phosphate adsorption occurred at the impregnation ratio AgNPs/TAC of 3% w/w, pH 3, and contact time lasting 150 min. The maximum adsorption capacity of phosphate on AgNPs-TAC determined by the Langmuir model was 13.62 mg/g at an initial phosphate concentration of 30 mg/L. The adsorption isotherm of phosphate on AgNPs-TAC fits well with both the Langmuir and Sips models. The adsorption kinetics data were also described well by the pseudo-first-order and pseudo-second-order models with high correlation coefficients of 0.978 and 0.966, respectively. The adsorption process was controlled by chemisorption through complexes and ligand exchange mechanisms. This study suggests that AgNPs-TAC is a promising, low cost adsorbent for phosphate removal from aqueous solution.

This study presents the modification of cassava root husk-derived biochar (CRHB) with ZnO nanoparticles (ZnO-NPs) for the simultaneous adsorption of As(III), Cd(II), Pb(II) and Cr(VI). By conducting batch-mode experiments, it was concluded that 3% w/w was the best impregnation ratio for the modification of CRHB using ZnO-NPs, and was denoted as CRHB-ZnO3 in this study. The optimal conditions for heavy metal adsorption were obtained at a pH of 6–7, contact time of 60 min, and initial metal concentration of 80 mg L⁻¹. The heavy metal adsorption capacities onto CRHB-ZnO3 showed the following tendency: Pb(II) > Cd(II) > As(III) > Cr(VI). The total optimal adsorption capacity achieved in the adsorption of the 4 abovementioned metals reached 115.11 and 154.21 mg g⁻¹ for CRHB and CRHB-ZnO3, respectively. For each Pb(II), Cd(II), As(III), and Cr(VI) metal, the maximum adsorption capacities of CRHB-ZnO3 were 44.27, 42.05, 39.52, and 28.37 mg g⁻¹, respectively, and those of CRHB were 34.47, 32.33, 26.42 and 21.89 mg g⁻¹, respectively. In terms of kinetics, both the pseudo-first-order and the pseudo-second-order fit well with metal adsorption onto biochars with a high correlation coefficient of R², while the best isothermal description followed the Langmuir model. As a result, the adsorption process of heavy metals onto biochars was chemisorption on homogeneous monolayers, which was mainly controlled by cation exchange and surface precipitation mechanisms due to enriched oxygen-containing surface groups with ZnO-NP modification of biochar. The FTIR and EDS analysis data confirmed the important role of oxygen-containing surface groups, which significantly contributed to removal of heavy metals with extremely high adsorption capacities, comparable with other studies. In conclusion, due to very high adsorption capacities for metal cations, the cassava root husk-derived biochar modified with ZnO-NPs can be applied as the alternative, inexpensive, non-toxic and highly effective adsorbent in the removal of various toxic cations.

Magnetic modified-corncob biochar with an impregnation ratio of iron at 20% (w/w) was used for removal of Cr(VI) from aqueous solution. Batch adsorption experiments were conducted to investigate the adsorption isotherm, kinetics and mechanism of Cr(VI) onto MCB20. The results indicated that the maximum adsorption capacity achieved was 25.94 mg g⁻¹. The adsorption kinetic data were found to fit best to the pseudo-second order model with a high correlation coefficient (R² = 0.992). The adsorption mechanisms of Cr(VI) onto MCB20 were electrostatic attraction, anion exchange and adsorption coupled-reduction. The adsorption mechanisms occurring between Cr(VI) anions and MCB20 were due to mainly the contribution of Fe₃O₄ present in the corncob biochar structure after magnetization by FeCl₃. Among the aforementioned mechanisms, adsorption coupled-reduction plays a vital role in removal of Cr(VI) by enhancement of the reduction of Cr(VI) to Cr(III) through generation of electron-donor groups (hydroxyl groups) on the MCB20 surface. Then, Cr(VI) interacted with the electron-donor groups and Cr(VI) is reduced to Cr(III). Besides, Fe²⁺ ions in the MCB20 structure were simultaneously oxidized to Fe³⁺ ions and enhanced the transformation of Cr(VI) into Cr(III). The reduced Cr(III) cations, finally, were adsorbed by MCB20 through substitution of Fe³⁺ with Cr³⁺ under acidic conditions, complexation with surface functional groups of MCB20 and formation of Cr(OH)₃. This study both developed a new way to produce a low cost adsorbent for removal of Cr(VI) from aqueous solution and solved waste by waste.