Sorption and desorption of phosphorus by shale: batch and column studies

Constructed wetland systems have gained attention as attractive solutions for wastewater treatment. Wetlands are not efficient to treat wastewater with high concentrations of phosphorus (P). In order to remove high soluble P loads by wetland, sorbent beds can be added prior to the discharge of wastewater into wetlands. Sorption by sorbent materials is identified as a method for trapping excess P in wastewaters. In the present investigation, shale has been identified as a sorbent material for removal of phosphate (PO(4)-P) due to the cost effectiveness, stability and possibility of regeneration. The study focuses on the removal of PO(4)-P from wastewater using shale and the feasibility of using the P-sorbed material as slow-release fertilizer. Phosphorus sorption experiments were conducted by using shale (2 mm and 2-4.7 mm). Results indicate that Shale I (particle size = 2 mm) showed the highest sorption of PO(4)-P (500 +/- 44 mg kg(-1)). Breakthrough point was reached within 10 h in columns with flow rates of 2 and 3 ml min(-1). Lower flow rate of 1 ml min(-1) showed an average residence time of about 2 h while columns with a higher flow rate of 3 ml min(-1) showed a residence time of about 40 minutes. Variation in flow rate did not influence the desorption process. Since very low concentrations of PO(4)-P are released, Shale saturated with PO(4)-P may be used as a slow nutrient release source of P or as a soil amendment. The sorbent can also be regenerated by removing the sorbed PO(4)-P by using 0.1 N HCl.