Effect of Different Modification Methods on the Adsorption of Manganese by Biochar from Rice Straw, Coconut Shell, and Bamboo

The adsorption capacity of pristine biochar without modification is usually low. In this experiment, we comprehensively evaluated the adsorption of Mn(II) by biochar with different modification methods from different biomass. The biochar from rice straw, coconut shell, and bamboo was produced by pyrolysis at 600 °C under nitrogen and then modified with HNO3, NaOH, and Na2S, respectively. The results showed that the adsorption capacities of these modified biochar samples were in the order Biochar-NaOH > Biochar-Na2S > Biochar-HNO3. Among the three modification methods, biochar modified with NaOH is the optimum for the adsorption of Mn(II). However, the same method of modification has different effects on different biomass feedstocks. Rice straw: R-C > R-NaOH-C > R-Na2S-C > R-HNO3-C; coconut shell: C-NaOH-C > C-Na2S-C > C-HNO3-C > C-C; bamboo: B-NaOH-C > B-Na2S-C > B-C > B-HNO3-C. At the pH of 5 and 30 °C, R-C, C-NaOH-C, and B-NaOH-C showed the highest maximum adsorption capacity for Mn(II). Equilibrium data were evaluated by Langmuir, Freundlich, and Temkin isotherm models, and the results suggested that the Langmuir model is the most suitable to expound the adsorption behavior of Mn(II) on R-C, C-NaOH-C, and B-NaOH-C. Overall, the results from this work suggest that the key for preparing biochar adsorbents with high capacity is to choose the appropriate biomass feedstock and modification method.