Chemically activated cocoa leaves biochar (CLB) was successfully prepared from fallen cocoa leaves (CLs) via ZnCl₂-activation and pyrolysis at 700 °C for sequestration of toxic crystal violet (CV) dye from aqueous solution. CLs and CLB were characterized using elemental analysis (CHN/O), Brunauer-Emmett-Teller method (BET), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR), X-ray diffraction (XRD) spectroscopy and scanning electron microscopy (SEM). The optimum conditions for effective removal of CV dye from aqueous solution (75.67% for CLs and 99.87% for CLB) were pH 9, initial CV dye concentration 100 mg/L, adsorbent (CLs/CLB) dose 0.4 g/L, contact time 160 min and temperature 300 K. Modified Ritchie second order best described kinetic and Liu model described equilibrium adsorption. CLs and CLB with maximum adsorption capacities 190.70 and 253.3 mg/g respectively, compete favorably with adsorbents used for removal of CV dye from wastewater in the literature. The high BET surface area (957.02 m²/g) and mean pore diameter (7.21 nm) were indicators of better adsorption efficiency of CLB. CLs showed adsorption to proceed towards endothermic process, while it was exothermic process for CLB. This study established the suitability of cocoa leaves as sustainable and environmental friendly precursor for preparation of adsorbent for the treatment of dye-containing wastewater.

Iron Oxide-commercial activated carbon nanocomposite (CAC-IO) was prepared from commercial activated carbon (CAC) by the co-precipitation method, and the resulting nanocomposite was used as an adsorbent to remove hexavalent chromium (Cr⁶⁺) ions and Mordant Violet 40 (MV40) dye from wastewater. The produced materials (CAC, CAC after oxidation, and CAC-IO) were comparatively characterized using FTIR, BET, SEM, EDX TEM, VSM, and XRD techniques. The adsorption mechanism of Cr⁶⁺ ions and MV40 dye on CAC-IO was examined using Langmuir and Freundlich isotherm models.. Different models were applied to know the adsorption mechanism and it was obtained that Pseudo-second order fits the experimental data better. This means that the adsorption of the adsorbate on the nanocomposite was chemisorption. The maximum removal percent of Cr⁶⁺ ions by CAC-IO nanocomposite was 98.6% determined as 2 g L^–1 adsorbent concentration, 100 mg L^–1 initial pollutant concentration, solution pH = 1.6, the contact time was 3 h and the temperature was room temperature. The maximum removal percentage of Mordant Violet 40 dye (C.I. 14,745) from its solutions by CAC-IO nanocomposite was 99.92% in 100 mg L^–1 of initial dye concentrations, 1.0 g L^–1 of adsorbent concentration, solution pH = 2.07, the contact time was 3 h. The MV40 dye adsorption on CAC-IO was the most fitted to the Freundlich isotherm model. The maximum adsorption capacity was calculated according to the Langmuir model as 833.3 mg g^–1 at 2 g L^–1 of adsorbent concentration and 400 mg L^–1 of initial MV40 dye concentration. The Cr⁶⁺ ions adsorption on CAC-IO was more fitted to the Freundlich model with Q_max, equal to 312.50 mg g^–1 at 1 g L^–1 adsorbent concentration and 400 mg L^–1 of Cr⁶⁺ ions initial concentrations.

Chromium (Cr) ion is regarded as a particularly hazardous contaminant due to its high toxicity and potential carcinogens. The Cr⁶⁺ ions were removed in the current work using a new fabricated low-cost adsorbent called Pea pods biochar-NH₂ (PBN). PBN was characterized using BET, BJH, SEM, FTIR, TGA, DSC, XRD, XPS and EDX. The surface area and the pore size of PBN were decreased due to the amination process on the pea pods’ biochar-H₂O₂ (PBO). Optimized condition of different parameters of Cr⁶⁺ ions adsorption by PBN was observed at the pH 1.5, contact time (180 min), starting concentration (100 mg/L), and 0.1 g of PBN. The maximal adsorption capacity (Q_m) of PBN is 384.62 mg/g. Different error models were used to test the isotherm models’ results. The mechanism of the adsorption was proposed based on the XPS analysis. The adsorption process had an R² value of 0.999 as the best fit with the Freundlich isotherm (FIM) and pseudo-second-order kinetic (PSOM) models. These obtained data explored that the generated PBN can be a novel, cost-effective material for the adsorption of Cr⁶⁺ ions from an aqueous solution.

A new biochar was produced from pea peel residues by the dehydration process. The effect of the obtained new biochar on the ability to remove Cr⁶⁺ ions from the aqueous solution was investigated. Biochar-SO was obtained from pea peel by dehydration of biochar with 50% sulfuric acid. The obtained biochars were characterized by Fourier transform infrared (FT-IR); Brunauer, Emmett and Teller (BET); Barrett-Joyner-Halenda (BJH); thermogravimetric analysis (TGA); differential scanning calorimetry (DSC); scanning electron microscope (SEM); and energy-dispersive X-ray (EDAX) analyses. The optimum pH value for Cr⁶⁺ ion removal was determined as 1.48. The maximum removal percentage of Cr⁶⁺ ions was 90.74% for Biochar-SO of 100 mg·L⁻¹ Cr⁶⁺ ions initial concentration and 1.0 g L⁻¹ adsorbent dosage. The maximum adsorption capacity (Q_m) of biochar-SO was 158.73 mg·g⁻¹. The data obtained were analyzed with Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) isotherm models. In addition, the data obtained from these isotherm models were tested using different error functions (hybrid error function (HYBRID), average percent errors (APE), the sum of the absolute errors (EABS), chi-square error (X²), and Marquardt’s percent standard deviation (MPSD and the root mean square errors (RMS)) equations. It was the Freundlich isotherm model that best fits the experimental data of biochar-SO. Kinetic data were evaluated by pseudo–first-order (PFO), pseudo–second-order (PSO), Elovich, and intraparticle diffusion models. The adsorption rate was primarily controlled by the PSO rate model with a good correlation (R² = 1). The adsorption mechanism of biochar-SO to remove Cr⁶⁺ ions can be based on electrostatic interaction and ion exchange with exchangeable cations in biochar such as aluminum, silicon, and calcium ions for chromium. The results indicate that biochar-SO is a promising adsorbent for the adsorption of Cr⁶⁺ ions that can be employed repeatedly without substantial loss of adsorption effectiveness.