Impact of Inherent Magnesium in Biochar for Phosphate Removal from Reclaimed Water Streams

Abstract

In water treatment processes, the use of biochars has risen due to their relative abundance, sustainability, and low cost compared with conventional technologies. The objective of this work was to investigate the physical and chemical properties of six chars that promote phosphate removal. The chars were further engineered with magnesium chloride (engineering biochars) prior to heat treatment to increase removal of phosphate. The impregnated magnesium was converted to magnesium oxide (MgO) under inert conditions at temperatures greater than 450°C. The solutions were analyzed using an ultraviolet visible spectrophotometer at 810 nm for phosphate concentration. To simulate real-world municipal water reuse applications, reclaimed wastewater was used as the source water for the experiments. Thermally treated sugar beet pellets were converted to biochar with inherent magnesium content, and demonstrated removal of more than 85% when pyrolyzed at greater than or equal to 550°C for 15 min. Dosing of MgO increased the solution pH, thus having phosphate species as HPO42− and H2PO4−, and bound the hydrated Mg(OH)2 as MgHPO4 and Mg2(H2PO4). The porosity of biochar is not solely responsible for phosphate removal but is required after impregnation of magnesium for increasing accessibility. Contact pH is a good indicator of biochar performance. MgO was shown to be an excellent sorbent for phosphate removal in reclaimed water when impregnated as MgCl2 and dosed separately with biochar.