Specific Phosphate Sorption Mechanisms of Unaltered and Altered Biochar

Biochar has been shown to act as an effective sorbent for many organic and inorganic contaminants (including phosphate) and can help to improve the quality of our fresh water resources by preventing eutrophication. Most of the high efficiency biochar phosphate-adsorbent feedstocks are modified with chemical pretreatment, phytoremediation or anaerobic digestion to accumulate desired elements. The main objectives of this project were to first engineer magnesium (Mg) and calcium (Ca) altered biochar by chemical pretreatment followed by pyrolysis at either 350 or 550°C and evaluate their phosphate adsorption rate and potential as compared to their unaltered counterparts. Determination of surface physiochemical characteristics of the unaltered (U350 and U550), Mg-altered (Mg350 and Mg550), and Ca-altered (Ca350 and Ca550) hardwood biochars was also completed. The unaltered biochars did not adsorb any phosphate which was likely due to a lack of Mg or Ca and crystalline structures as confirmed by ICP-AES and XRD. Adsorption kinetics revealed that both the Ca-altered and Mg-altered biochars took about 10 days to reach equilibrium. The Langmuir isotherm model was the best fitting model for all altered biochars and Mg550 adsorbed the most phosphate with a Langmuir maximum capacity (Q) of 135.8 mg/g at pH 9. XRD spectra confirmed crystallinity for both the Ca-altered and Mg-altered biochars for calcite (CaCO3) on the Ca-altered biochar and both periclase (MgO) and brucite (Mg(OH)2)on the Mg-altered biochars. SEM images of Mg550 followed by EDS confirmed the presence of nano-sized flakes on the biochar surface and identified them as the primary phosphate adsorption sites. SEM images of Ca550 identified globular, rather than flakey structures on the biochar surface and EDS offered further confirmation that these were calcite minerals. Although Ca350 and Ca550 adsorbed phosphate from solution, Mg350 and Mg550 exhibited a much greater phosphate sorption potential with Q at 18.91, 16.34, 39.59 and 135.8 mg/g (pH 9), respectively. This may be attributed to the morphological differences between the highly crystalline flake-like structures of the Mg altered biochars and the globular structures on the surface of the Ca altered biochars. The mechanisms revealed for phosphate sorption include electrostatic interactions, precipitation, and ligand exchange