Black liquor-derived calcium-activated biochar for recovery of phosphate from aqueous solutions

Black liquor-derived calcium-activated biochars (Ca-biochar) were synthesized by treating rice straw with Ca(OH)2 to create an adsorbent that was effective for removing phosphate from aqueous waste streams. The Ca(OH)2 acts to separate lignin from the biomass, create pores in the biochar solids and form active adsorption sites. The Ca-biochar adsorbent was efficient for the removal of phosphate from aqueous solutions (pH 1.0 to pH 13.0) with a highest phosphate adsorption capacity of 197 mg/g. Phosphate adsorption was correlated with pseudo-second-order kinetics and the Langmuir model with primary mechanisms being attributed to chemical precipitation and ligand exchange. Application of the Ca-biochar (0.2 g/L) to actual wastewater from a cattle farm (phosphorus content 3.78 mg/L) reduced the phosphorus content to 0.021 mg/L. This work utilizes waste black liquor to prepare functionalized biochar materials, providing a promising approach for black liquor reuse and phosphate removal and recovery from phosphorus-rich waste streams.