Phosphorus recovery from waste streams using adsorbents

Phosphorus (P) is an important macro-nutrient required for the growth and development of all living organisms. Globally, livestock production generates a large quantity of manure and effluents which are rich in plant nutrients especially nitrogen (N) and P. These waste resources are land applied as an organic fertilizer or soil amendments. However, excess application of manure and effluents beyond the carrying capacity of the soil results in leaching of important nutrients into aquatic environments. This can lead to nutrient enrichment thereby causing various ecological problems such as eutrophication of waterways and resulting in algal blooms. One alternate way of minimising the impact of P is to recover from waste source and reuse as a fertilizer. The main objective of this thesis was to investigate the application of various adsorbents for recovering P from various waste streams. Based on the surface characterization, three adsorbents (allophane, iron oxide and biochar) were selected for the subsequent P adsorption studies. In waste streams, speciation of P in various forms is a significant factor necessary to understand its distribution and transformation in terrestrial and aquatic ecosystems. It was observed that most of the P in manures and biosolids were present as inorganic orthophosphate and orthophosphate monoesters. The sorption kinetics studies showed that among the three adsorbents, biochar showed excellent adsorption characteristics for P recovery which fitted well into the adsorption isotherms, and the maximum adsorption capacity recorded for the adsorbents were 43.29, 35.71 and 90.90 mg g-1 for clay, iron oxide and biochar, respectively. The pseudo-second order kinetics model revealed that the concentrations of both adsorbate as well as adsorbent are the controlling factors for rate of adsorption while the intra-particle diffusion model suggested the prevalence of film diffusion and pore diffusion in the adsorption process. Evaluation of the thermodynamic parameters asserted the adsorption process being spontaneous, favourable and mainly through physisorption. The mechanisms of P recovery were investigated with the help of XPS and XANES techniques, and from the results it is confirmed that various functional groups and minerals such as calcium and magnesium present in the biochar were responsible for adsorption and precipitation of P from waste streams.