Submitted to: International Journal of Food, Agriculture, and the Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/18/2003
Publication Date: 11/7/2003
Citation: Dao, T.H. 2003. Competitive anion sorption effects on dairy wastewater dissolved phosphorus extraction with zeolite-based sorbents. International Journal of Food, Agriculture, and the Environment. 1(3):263-269. Interpretive Summary: Mineral dietary phosphorus contributed to high levels of dissolved phosphorus (DP) in animal manure. There is increased interest on manure treatments to chemically bind or remove DP before manure is applied to agricultural fields. Following solid-liquid mechanical separation, dairy manure solids can be composted and directly land-applied. The liquid phase is used for irrigation or treatment and clarification can be made, allowing the water to be reused in animal production. Soluble and colloidal P forms must be removed from the liquid phase to avoid further loading and P buildup in high-P production fields. Combinations of organic coagulants and mineral P immobilization chemicals were shown to be effective in solid-liquid separation and DP removal from solution. Chemical precipitation, however, defers the phosphorus removal from a diluted liquid to a more concentrated solid fraction. Anion exchangers present an alternate approach to P recovery instead of P immobilization. Materials possessing such properties include natural and synthetic zeolites and coal-combustion ash. While natural zeolites do not bind phosphate, modified and synthetic zeolites, and fly ash have significant capacity to bind and remove phosphate from dairy wastewater. In mixed solutions of anions and in dairy wastewater, nitrate and sulfate can compete for sorption sites and reduce DP retention by the zeolites. The sorbed phosphorus can be re-extracted by water and differences in P release exist between the natural and synthetic zeolites and fly ash. The mineral sorbents all demonstrated their usefulness as temporary phosphorus storage sinks to treat animal wastewaters. These capabilities make the sorbents, fly ash in particular, very versatile P sorbent for low cost reclamation and treatment processes. Retention of inorganic anionic contaminants is reversible and suggests that the spent sorbents can be further used in agricultural production.
Technical Abstract: Dairy wastewater is often used to irrigate field crops. Soluble and colloidal P must be removed from the supernatant liquid phase to avoid further phosphorus (P) loading and buildup in high-P fields. Information is needed on natural and synthetic zeolites and fly ash behavior and sorption capacity for PO4-P in a complex wastewater and on how the spent products release the sorbed P. Sorption isotherms were determined in single and multi-ion PO4-P standard solutions and dairy wastewater to quantify the sorption capacity of modified zeolites and fly ash and increase our understanding of underlying mechanisms of oxyanion retention. Solution-phase anion concentrations were determined by high-performance ion chromatography. The results show that natural zeolites have negligible affinity for NO3- or PO43- anions. Surfactant-modified (SMZ) and synthetic zeolites (SZBP) and fly ash exhibit significant capacities to bind PO4-P. Phosphate sorption on SMZ and SZBP was described by the Langmuir equation, with sorption maxima, Smax, averaging 0.71, and 0.31 mmol g-1, respectively. Class C fly ash strongly sorbs and removes PO4-P from solution. Sorption maximum increases by 4-fold and the Langmuir K constant indicates a higher bonding energy than those of SMZ and SZBP. Competitive sorption is evident in PO4-P sorption from mixed solutions of SO4-S, NO3-N, and PO4-P. All three sorbents retain the capacity to sorb PO4-P in dairy wastewater. The order of efficacy is fly ash > SZBP > SMZ. Although differences in affinity and desorption exist, the zeolite-based sorbents prove valuable as temporary sinks for the treatment of P-laden wastewater and offer promise in the development of reversible P recovery approaches.