Characteristics of Cd2+ sorption/desorption of modified oilrape straw biochar

Biochar has recently received much attention as an adsorbent for removing pollutants. It is meaningful to develop innovative biochar with high adsorption capacity through physical, chemical and even biological processes. Based on our previous study, steam explosion was an ideal method to pretreat oil rape straw, because it enhanced the surface area of biochar by approximately 16 times, which was helpful for the adsorption as well as the further modification. In this study, steam exploded oil rape straw was soaked in hydroxyapatiteslurry and KMnO4 solution prior to biochar production. In addition, the pristine biochar was also modified by chitosan and NaOH solution. Nitrogen adsorption/desorption isotherms, scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), and Fourier transform infrared spectroscopy (FTIR), etc. were used to determine and compare the characteristics of different modified biochars, and then explore the Cd2+ sorption/desorption mechanisms. The results showed that the elemental composition of biochar was significantly affected by the nature of the modification materials. All the modified biochar had higher H/C and O/C ratios, therefore, it had higher polarity and hydrophilicity. The surface area of biochar derived from hydroxyapatite impregnated feedstocks was 225.68 m2/g. Chitosan, KMnO4 and NaOH modification, however, led to the decrease of surface area and pore volume by 76%-96% and 43%-84%, respectively. All these modification treatments induced in the loading of relevant materials on the biochar surface. For example, hydroxyapatite treatment resulted in the increase of phosphates, while chitosan, KMnO4 and NaOH modification introduced the acidic oxygen-containing groups such as hydroxyl (-OH), carboxyl (-COOH), and -NH2 as well. Although both positive and negative charges were reduced due to the modification treatment, the Cd2+ adsorption capacity increased by 13%-315%, and NaOH modified biochar provided the highest value (54.12 mg/g). Batch experiment displayed that the sorption of Cd2+ by biochar was well fitted to Langmuir isotherm (R2>0.93) and pseudo-second kinetics (R2>0.90), which suggested that the Cd2+ was immobilized as monolayer on biochar by physicochemical reactions. The initial adsorption constant (K2qe2) of the modified biochar was 65%-379% higher than that of pristine biochar, showing that the modified biochars could immobilize Cd2+ more quickly. The Cd2+ was mainly adsorbed by the modified biochar through specific coordination rather than columbic force. Therefore, the adsorbed Cd2+ was much more stable on the modified biochar with a 17%-91% lower desorption rate, compared to the pristine biochar. In conclusion, hydroxyapatite modified biochar showed the potential to quickly capture Cd2+ from aqueous solution via physical adsorption, cation exchange, and precipitation, which was, however, at a risk of desorption. Chitosan, KMnO4 and NaOH modified biochars immobilized the Cd2+ by coordination and complexation, which could be more stable and effective. By comparing the influence of different modification methods on the adsorption/desorption characteristics of Cd2+ by biochar, our results demonstrate that it was valuable for further using these biochars in environment. However, we recommend to choose the modification method carefully based on the purpose and in case of secondary contamination.