Comparative investigation of characteristics and phosphate removal by engineered biochars with different loadings of magnesium, aluminum, or iron

Abstract Engineered biochars (EBCs) loaded with metal oxides/hydroxides have been used as sorbents to remove and recycle phosphate (P) from wastewater. However, P removal by EBCs made with different types and loading of metals have rarely been compared in a single study. Thus, in this study, EBCs were synthesized through pyrolysis of bamboo or hickory wood chips (25 g) pretreated with four amounts (25, 50, 75, and 100 mmol) of magnesium (Mg), aluminum (Al), or iron (Fe) salt solutions (Mg-EBC, Al-EBC, and Fe-EBC, respectively). The resulting EBCs were loaded with metal oxides/hydroxides that served as P adsorption sites. Al-EBCs showed the highest aqueous stability with little metal dissolution, which can be attributed to the low level of residual (unconverted) metal salt as well as the extremely low solubility of loaded Al metal oxyhydroxide. After the leaching/washing, the metal loading efficiencies of the Al- and Mg-EBCs were similar (50–60%) and stable metal loadings increased with pretreatment salt amounts, indicating that the amount of the two metal oxides/hydroxides in the EBCs can be controlled during pretreatment. However, stable iron oxide on the Fe-EBCs remained almost the same for all the four levels of pretreatment, reflecting saturation of the biochar surface. All the EBCs showed increasing P adsorption with increasing metal loading. At low initial P concentrations of 31 mg/L, Fe- and Al-EBCs removed up to 68% and 94% of P, likely through an electrostatic interaction mechanism. At high P concentrations, Mg-EBC had the largest P adsorption capacity (119.6 mg P/g), mainly through the combination of surface precipitation and electrostatic interaction mechanisms. This study demonstrates that metal oxide/hydroxide-loaded EBCs are promising sorbents that can be designed to meet specific needs for the removal of aqueous P in various applications.