Biochar produced from oak sawdust by Lanthanum (La)-involved pyrolysis for adsorption of ammonium (NH4+), nitrate (NO3−), and phosphate (PO43−)

A series of biochars were prepared by pyrolyzing oak sawdust with/without LaCl3 involvement at temperature of 300–600°C, and approximate and ultimate analyses were carried out to check their basic characteristics. Meanwhile, the releases of readily soluble NH4+, NO3− and PO43− from biochars and the adsorption of NH4+, NO3− and PO43− by biochars were investigated. Results indicated that the involvement of LaCl3 in pyrolysis could advance the temperature of maximum mass loss by 10°C compared with oak sawdust (CK), and potentially promoted biochar yield. Overall, the releases of readily soluble NH4+, NO3− and PO43− from biochars were negatively related to pyrolysis temperature, and the releases were greatly weakened by La-biochars. Additionally, the adsorption to NH4+ can be promoted by the biochars produced at low temperature. On the contrary, the NO3− adsorption can be improved by increasing pyrolysis temperature. The highest PO43− adsorption was achieved by the biochars produced at 500°C. According to the results of adsorption isotherms, the maximum adsorption capacity of NH4+, NO3− and PO43− can be significantly promoted by 1.9, 11.2, and 4.5 folds using La-biochars. Based on the observations of FT-IR, SEM–EDS, and surface functional groups, the improvement of NH4+ adsorption was potentially associated with the existing acidic function groups (phenolic-OH and carboxyl CO). The increased basic functional groups on La-biochars were beneficial to improve NO3− and PO43− adsorption. Besides, PO43− adsorption was also potentially related to the formed La2O3.