Nitric acid surface pre-modification of novel Lasia spinosa biochar for enhanced methylene blue remediation

Abstract

Abstract Mineral acid post treatment is a common value-addition method for biochar (BC). However, it can cause digestion of the product, thus reducing the yield. Acid treatment on raw feedstock is anticipated to enhance adsorption properties of produced BC while avoiding such digestive effects. Hence, the presented study has focused on pre-modification of Lasia spinosa (Kohila) BC by using nitric acid , to improve its sorption capacity towards removal of methylene blue (MB). Raw and pre-modified BC (abbreviated as RBC and PMBC respectively) were produced at three different temperatures (300, 500 and 700\xa0°C). Highest capacities were obtained for 300\xa0°C produced BC (abbreviated as R300 and PM300 respectively). The experimental data was best fitted to Sips isotherm model, with a maximum Sips capacity of 9.58\xa0mg/g and 81.35\xa0mg/g at 30\xa0°C for R300 and PM300 respectively. The enhanced capacity of PM300 could be attributed to the increased surface functionality and the reduced pore size which resulted in a feasible adsorptive removal of MB. XPS results showed a slight increase in nitrogen containing groups upon modification. The adsorption kinetics accurately described the pseudo-first order (PFO) behavior of PM300 and pseudo-second order (PSO) behavior of R300. The MB uptake of PMBC was governed by a pore filling mechanism whereas electrostatic attractions were predominant in the sorption process of RBC. Both BCs showed a spontaneous endothermic adsorption and the overall rate of the adsorption process was a combination of liquid film diffusion and intra particle diffusion . After four cycles of regeneration, the dynamic capacity of PMBC remained more than 50% of its initial value, which was approximately twice of RBC. Pre-modification caused an eight-fold increase in MB adsorption capacity, qualifying it to be a simple, economically feasible and energetically effective technique for the value-addition of BC.