Recycling rice husk for removal of phosphate and nitrate from synthetic and swine wastewater: Adsorption study and nutrient analysis of modified rice husk

The objective of this study was to determine the adsorbent potential of rice husk and its modified form for phosphate and nitrate removal from synthetic and swine-farm wastewater. The mechanism of adsorption as well as the potential of phosphate-/nitrate- adsorbed rice husk as nutrient rich residue was also investigated. Two-step modification of RH (using base-washing (BW) and chemical modification (CM) was conducted to compare the phosphate and nitrate removal. The effects of several factors (pH, sorbent dosage, contact time, initial concentration, and coexistence of both ions) were investigated to gain insight into the adsorption rate, behavior, and mechanism of the modified RH regarding phosphate and nitrate removal. The results of Fourier-transform infrared spectroscopy showed that the modification was successful by crosslinking with the amine group of the chemical agent. Fitting the adsorption kinetic data of phosphate showed physical adsorption, intraparticle diffusion, and chemisorption, whereas for nitrate, the data indicated mainly chemisorption. Fitting the adsorption isotherm data of phosphate and nitrate together showed adsorption on a monolayer coating of anions on the homogeneous sorbent’s surface. The maximum phosphate and nitrate adsorption capacities were 6.94 and 2.46 mg/g, respectively, for a single adsorbate and 11.14 and 1.76 mg/g, respectively, for the binary solution. In real swine wastewater, removal efficiencies of phosphate, nitrite, nitrate, sulfate, and ammonia were 83.8%, 65.0%, >45.0%, 36.6%, and 2.6%, respectively, indicating that the modified RH would be effective for phosphate and nitrate removal from real wastewater. Finally, nutrient analysis of the phosphate- and nitrate-sorbed RH showed increases in nitrogen and phosphorus, which would be beneficial for further use of the RH as nutrient or fertilizer after adsorption.