We assessed the applicability of rice husk (RH) to remove cationic dyes, i.e., methylene blue (MB) and crystal violet (CV), from water. RH thermally treated at 75 °C showed a higher adsorption capacity than that at high temperatures (300–700 °C). For a suitable CV-adsorption model, a pseudo-first-order model for MB adsorption was followed by the kinetics adsorption process; however, a pseudo-second-order model was then suggested. In the qt versus t1/2 plot, the MB line passed through the origin, but that of CV did not. The Langmuir isotherm model was better than the Freundlich model for both dye adsorptions; furthermore, the adsorption capacity for MB and CV was 24.48 mg/g and 25.46 mg/g, respectively. Thermodynamically, the adsorption of both MB and CV onto the RH was found to be spontaneous and endothermic. This adsorption increased insignificantly on increasing the solution pH from 4 to 10. With an increasing dosage of the RH, there was an increase in the removal percentages of MB and CV; however, adsorption capacity per unit mass of the RH was observed to decrease. Therefore, we conclude that utilizing RH as an available and affordable adsorbent is feasible to remove MB and CV from wastewater.

In this study, we explored the adsorption potential of biochar derived from palm kernel shell (BC-PKS) as an affordable adsorbent for the removal of crystal violet from wastewater. Kinetics, equilibrium, and thermodynamics studies were carried out to evaluate the adsorption of crystal violet onto BC-PKS. The kinetics adsorption process followed the pseudo-second-order model, indicating that the rate of adsorption is principally controlled by chemisorption. The adsorption equilibrium data were better fitted by the Langmuir isotherm model with a determination coefficient of 0.954 and a maximum adsorption of 24.45 mg/g. Thermodynamics studies found the adsorption of crystal violet by BC-PKS to be endothermic with increasing randomness at the BC-PKS/crystal violet interface. The percentage removal and adsorption capacity increased with the pH of the solution, as the negative charges on the biochar surface at high pH enhance the electrostatic attraction between crystal violet molecules and BC-PKS. Increasing the BC-PKS dosage from 0.1 to 1.0 g increased percent removal and decreased the adsorption capacity of crystal violet onto BC-PKS. Therefore, biochar from agricultural by-products, i.e., palm kernel shell, can be cost-effective adsorbents for the removal of crystal violet from textile wastewater.

We investigated the applicability of oyster (OS) and mussel shells (MS) as capping materials to inhibit the movement of nitrogen (N) and phosphorus (P) from river sediments. OS and MS are difficult to dispose of as waste, and have been used environmentally to clean up sediments contaminated with nitrogen and phosphorus. OS and MS increased the nutrient adsorption efficiency through simple heat treatment. The effectiveness of OS and MS capping with sand armor (SA) was evaluated in laboratory incubation experiments for 63 d. The sediments were capped with an active capping material (OS or MS) and then with either 1 cm or 3 cm SA. The pH and EC values were remarkably high under MS capping conditions because Ca2+ and Mg2+ were eluted from the MS material. The elution of Ca2+ and Mg2+ negatively affected the inhibition of NH4-N release by MS capping. OS capping demonstrated better performance for blocking the release of NH4-N and T-N than that of MS capping; the efficiency was enhanced by adding a 3 cm SA layer on top of the MS layer. In contrast, the PO4-P and T-P releases from the river sediments were effectively impeded by MS capping. OS capping with a 3 cm SA layer (OS/SA3) was recommended as the best capping strategy for inhibiting N and P releases from river sediments. The capping efficiencies of OS/SA3 for NH4-N, T-N, PO4-P, and T-P were 92.2%, 51.4%, 101.3%, and 93.3%, respectively.