| dc.description.abstract | Phosphorus is an essential resource, yet global phosphorous reserves are limited, and increasing quantity of high-grade ore is being mined and turned into fertilizers to meet the increasing demand of food. However, since currently the phosphorus cycle is not closed loop, considerable amounts of excessive phosphorus are discharged from agricultural activities to environment causing eutrophication and wasting the resource. Hence, recovering and reusing phosphorus are key to building a sustainable phosphorus cycle. Materials derived from agricultural wastes such as biochar showed great potential of serving as a matrix for sorbents. Therefore, this research focused on sorbents prepared from agricultural wastes for recovering phosphorus from animal wastewater and reuse as fertilizer. Batch isotherm sorption phosphorus recovery studies and continuous flow column phosphorus release studies were conducted, as well as the life cycle assessment and cost analysis of producing the sorbents.
Biochars with magnesium amendments (referred as Mg-chars hereafter) from MgCl2 were tested in model animal wastewater at different pH conditions. Results showed precipitation as struvite was responsible for phosphorus recovery in model wastewater containing phosphate and ammonium, as confirmed through X-ray Diffraction (XRD). Additionally, since the solubility of struvite decreases with increasing pH, pH 8.0 and 9.0 were more beneficial for struvite formation. Furthermore, alkalinity in wastewater was found to compete with dissolved phosphorus on the sorbent.
Magnesium amendments such as using MgCl2 would often account for the major portion of the total cost producing the sorbents. To find cheaper solutions with similar phosphorus recovery performance, this research tested low-price magnesium sources including natural minerals and industrial by-products. Correspondingly, mineral magnesium hydroxide and bittern, a residue from sea salt evaporation, were identified as two alternatives for commercial MgCl2 salt. In concentrated animal wastewater at pH 8.0, Mg-char (Mg(OH)2) and Mg-char (bittern) both obtained phosphate recovery capacities over 220 mg P/g through struvite formation.
Additional to biochar, this research also tested materials synthesized from crop waste ashes. First, magnesium silicate minerals were prepared by extracting silicate from rice straw and wheat straw ash and then precipitated using magnesium salt. Even though magnesium silicate worked only slightly poorer than Mg-chars in terms of phosphorus recovery, the interference from dissolved silicate in concentrated wastewater on phosphate analysis was too high. Thus, magnesium silicate minerals were not tested further. Next, calcium silicate hydrate (CSH) was prepared using rice husk ash and calcium hydroxide. Unlike Mg-chars’ precipitation dominant phosphorus recovery mechanism, CSH fixed dissolved phosphate through a Langmuir type adsorption with maximum adsorption capacity of 55 mg P/g.
Next, post-phosphorus-exposure Mg-char (Mg(OH)2), Mg-char (bittern), and CSH were selected for phosphorus release test in continuous flow column studies considering the effects of pH and soil minerals goethite and kaolinite. Post-phosphorus-exposure sorbents, or spent sorbents, were collected from phosphorus recovery studies, then added to columns. Spent Mg-char (Mg(OH)2) and Mg-char (bittern) had phosphorus content of 182 and 198 mg P/g, respectively, and spent CSH had 46 mg P/g. Results showed that, first, both spent Mg-chars effectively released over 80 % of the recovered phosphorus within five pore volumes at pH 5.5, 7.0 and 8.5, but CSH needed significantly more pore volumes (time). Second, pH 5.5 and 7.0 yielded similar release characteristics while pH 8.5 was notably slower, which could be attributed to the lower solubility of struvite at higher pH. Third, soil minerals affect dissolved phosphate concentration greatly and goethite had more impact on phosphate availability than kaolinite. Further, at lower pH conditions, both minerals retained phosphate more than higher pH conditions, which was due to the fact that the further pH was lower than point of zero charge, the more affinity the mineral surface has for attracting phosphate ions.
Finally, this research evaluated the environmental impact of using Mg-char (Mg(OH)2), Mg-char (bittern), and CSH for phosphorus recovery and reuse comparing to commercial fertilizer monoammonium phosphate (MAP) through life cycle assessment (LCA), as well as cost analysis. While results suggested that both Mg-chars had lower environmental impact than MAP, CSH had the greatest environmental impact due to its high chemical inputs and biomass required. Further, Mg-char (bittern) had less impact than Mg-char (Mg(OH)2), illustrating the advantage of using bittern. Moreover, since Mg-chars achieved higher phosphorus release efficiency at lower pH soil, or when greater quantities of phosphorus release were needed for farmland with the same transportation distance, using Mg-chars for phosphorus management and fertilization could be even more advantageous than MAP. Finally, the two Mg-chars can be less expensive than MAP when the transportation cost was minimized or greater amount of phosphorus was needed.
Overall, this research provided knowledge on the use of several sorbents including magnesium amended biochars and silicate minerals that were derived from agricultural wastes for phosphorus recovery and reuse. These sorbents showed high potential as keys to a closed phosphorus cycle, and as substitutions to commercial fertilizers with less environmental impact and lower cost. | en_US |
