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.

The CS/ZnO–Fe₃O₄ nanoparticles synthesized by chemical ultrasound with large surface area can effectively decompose RhB and MO dyes under visible light, with degradation rates of 99.49% and 90.73%, respectively, and can be reused many times.

In this study, Fe₃O₄ nanoparticle-loaded biochar derived from the pomelo peel (FO-PPB) was synthesized and applied as an affordable material for the adsorption of Reactive Red 21 (RR21) in an aqueous solution. The characteristics of FO-PPB were evaluated by scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX), X-ray diffraction (XRD), Raman spectra, Fourier transform infrared spectra (FTIR), and Brunauer–Emmett–Teller (BET) surface area. The adsorption process of FO-PPB with RR21 was evaluated through batch experiments to examine various parameters including solution pH, contact time, adsorbent dose, initial RR21 concentration, and solution temperature. Results show that FO-PPB produced by the impregnation ratio between iron (Fe) and pomelo peel biochar (PPB) of 5 : 1 (w/w) had the best adsorption performance. The adsorption capacities of PPB and FO-PPB at optimum experimental conditions (solution pH 3, contact time of 60 min, solution temperature of 40°C, initial RR21 concentration of 300 mg/L, and adsorbent dose of 2 g/L) were 18.59 and 26.25 mg/g, respectively. The adsorption isotherms of RR21 on PPB and FO5-PPB were described well by Langmuir and Sips models with high R² values of 0.9826 and 0.9854 for FO5-PPB and 0.9701 and 0.9903 for PPB, respectively. The obtained data also well matched the pseudo-first-order and pseudo-second-order models with R² values ≥ 0.96. Chemisorption through sharing or electronic exchange was determined as the main adsorption mechanism.