The adsorption behavior and mechanism for arsenate by lanthanum-loaded biochar with different modification methods

Lanthanum (La)-loaded biochar (BC) was considered as an efficient adsorbent for phosphate removal in wastewater. Arsenate has a similar chemical structure to phosphate, however, performance and mechanisms of arsenate adsorption onto La-doped biochar are less studied and remain unclear. In this study, La-modified biochar (BC) was prepared using two dinstinct methods: pyrolysis of LaNO 3-doped rice husk (pre-pyrolysis modification, referred to as La-BC) and pyrolysis of LaNO 3-doped rice husk biochar (post-pyrolysis modification, referred to as BC-La). Isotherm experiments showed that the maximum arsenate adsorption content onto La-BC and BC-La was similar, respectively of 21.3 mg g−1 and 16.5 mg g−1 at pH = 5. Combined results of SEM-EDS, FTIR, and XPS analyzing revealed that La oxides and hydroxides loaded on BC played a vital role in arsenate adsorption. Electrostatic attraction and ligand exchange were considered as key mechanisms for arsenate adsorption. Compared to La-BC, the results revealed that La oxides/hydroxides exhibited more crystalline structures on BC-La. Moreover, the BC-La displayed more efficient adsorption capabilities over a wider pH range (pH 4–10) than La-BC. The XPS O1s spectra analysis confirmed the formation of different inner-sphere complexes on La-BC and BC-La. Results of this study revealed mechanisms of arsenate adsorption onto La-modified biochar, and highlighted that the adsorption behavior of arsenate onto La-modified biochar could be significantly influenced by modification methods.