(424b) Adsorptive Removal of Phosphorus from Wastewater Using Raw and Engineered Biochars

Releasing high concentration of nutrients (i.e., nitrogen, phosphorus) to environments has caused various environmental problems such as occurrence of algal bloom, eutrophication and lethal level of nitrate in surface and ground water leading to deteriorating water quality and threating human health. Adsorption of nutrients onto various carbon materials among various methods has been actively studied for eliminating nutrients from agricultural and municipal wastewater. Among various adsorbents biochar is a “charcoal-like” carbon material made from gasification or pyrolysis of various solid wastes including biosolids, sludges, agricultural residues and animal manure. Since it has rich nutrients, various functional groups and chemically inert to environments, its application has been extended to soil amendment, water treatment, gas separation, supports for catalysts and fuel sources.

This study focused on recovery of phosphorus from synthetic and dairy wastewater using the dairy manure-derived biochar including raw biochar, CaO-immobilized biochar (CaO-BC) and MgO-immobilized biochar (MgO-BC). The MgO-BC and CaO-BC were prepared by reacting CaCl₂ or MgCl₂ solution to the raw biochar at ambient temperature under vigorous stirring for 24 h followed by drying at 80 ^oC and calcination at 120 ^oC for 3 h. The adsorption experiments using a synthetic wastewater containing 30 ppm phosphate showed that the CaO-BC removed 89-100% of initial phosphate which was higher than those by the raw biochar and the MgO-BC by a factor of 1.1 to 1.7. When the dairy wastewater (lagoon effluent) was treated by the raw biochar, MgO-BC and Ca-BC, these biochars showed high removal of phosphate (79-90%) and low removal of ammonium.

For better understanding of adsorption characteristics, the effects of pH on adsorption of phosphate were investigated using the CaO-BC and the synthetic wastewater with 30 ppm phosphate. Interestingly the adsorption of phosphate onto CaO-BC was highest at higher pH (7-8) and lower at acidic conditions which was different from others’ studies. While the mechanisms associated with adsorption of phosphate will need to be investigated, multiple interactions including ligand complexation, hydrogen bonding and π-π interaction were thought to act on the adsorption of phosphate onto the CaO-BC. The results from the adsorption isotherm experiments supported that Langmuir isotherm was the better-fitted model for adsorption of phosphorus onto the CaO-BC indicating monolayer adsorption of phosphorus onto the biochar. The adsorption capacities of phosphorus onto the CaO-BC were found to be 1 – 6 mg phosphorus per g CaO-BC at the selected conditions. The adsorption capacities were similar and higher than those by the biochars from different feedstock and preparation methods. The adsorption kinetics of phosphorus on the CaO-BC revealed that the pseudo-second order kinetic model was the one to better fit to the experimental results that indicated the chemical interaction-driven adsorption.

Further studies will include the detailed mechanisms of adsorption, thermodynamic analysis associated with adsorption and the column studies for practical application in wastewater treatment.