Recovery of Phosphorus From Swine Manure by Ultrasound/H2O2 Digestion, Struvite Crystallization, and Ferric Oxide Hydrate/Biochar Adsorption

Exploring biochar and fishpond sediments potential to change soil phosphorus fractions and availability

Phosphorus (P) availability in soil is paradoxical, with a significant portion of applied P accumulating in the soil, potentially affecting plant production. The impact of biochar (BR) and fishpond sediments (FPS) as fertilizers on P fixation remains unclear. This study aimed to determine the optimal ratio of BR, modified biochar (MBR), and FPS as fertilizer replacements. A pot experiment with maize evaluated the transformation of P into inorganic (Pi) and organic (Po) fractions and their contribution to P uptake. Different percentages of FPS, BR, and MBR were applied as treatments (T1-T7), T1 [(0.0)], T2 [FPS (25.0%)], T3 [FPS (25.0%) + BR (1%)], T [FPS (25%) +MBR (3%)], T5 [FPS (35%)], T6 [FPS (35%) +BR (1%)], and T7 [FPS (35%) + MBR (1%)]. Using the modified Hedley method and the Tiessen and Moir fractionation scheme, P fractions were determined. Results showed that various rates of MBR, BR, and FPS significantly increased labile and moderately labile P fractions (NaHCO3-Pi, NaHCO3-Po, HClD-Pi, and HClC-Pi) and residual P fractions compared with the control (T1). Positive correlations were observed between P uptake, phosphatase enzyme activity, and NaHCO3-Pi. Maximum P uptake and phosphatase activity were observed in T6 and T7 treatments. The addition of BR, MBR, and FPS increased Po fractions. Unlike the decline in NaOH-Po fraction, NaHCO3-Po and HClc-Po fractions increased. All Pi fractions, particularly apatite (HClD-Pi), increased across the T1-T7 treatments. HClD-Pi was the largest contributor to total P (40.7%) and can convert into accessible P over time. The T5 treatment showed a 0.88% rise in residual P. HClD-Pi and residual P fractions positively correlated with P uptake, phosphatase activity, NaOH-Pi, and NaOH-Po moderately available fractions. Regression analysis revealed that higher concentrations of metals such as Ca, Zn, and Cr significantly decreased labile organic and inorganic P fractions (NaHCO3-Pi, R 2 = 0.13, 0.36, 0.09) and their availability (NaHCO3-Po, R 2 = 0.01, 0.03, 0.25). Excessive solo BR amendments did not consistently increase P availability, but optimal simple and MBR increased residual P contents in moderately labile and labile forms (including NaOH-Pi, NaHCO3-Pi, and HClD-Pi). Overall, our findings suggest that the co-addition of BR and FPS can enhance soil P availability via increasing the activity of phosphatase enzyme, thereby enhancing plant P uptake and use efficiency, which eventually maintains the provision of ecosystem functions and services.

Mining phosphorus from waste streams at wastewater treatment plants: a review of enrichment, extraction, and crystallization methods

Two interrelated problems exist: the non-renewability of phosphate rock as a resource and the excess phosphate in the water system lead to eutrophication. Removal and recovery of phosphorus (P) from waste streams at wastewater treatment plants (WWTPs) is one of the promising solutions. This paper reviews strategies for P recovery from waste streams in WWTPs are reviewed, and the main P recovery processes were broken down into three parts: enrichment, extraction, and crystallization. On this basis, the present P recovery technology was summarized and compared. The choice of P recovery technology depends on the process of sewage treatment and sludge treatment. Most P recovery processes can meet the financial requirements since the recent surge in phosphate rock prices. The safety requirements of P recovery products add a high cost to toxic substance removal, so it is necessary to control the discharge of toxic substances such as heavy metals and persistent organic pollutants from the source.

Optimization and kinetic modeling of phosphate recovery as struvite by electrocoagulation from source-separated urine

Phosphorus recovery is indispensable due to the rapid depletion of its natural reserves and excessive utility in agriculture. Though human urine has high nutrient content including phosphate, nitrogen and potassium; direct use as a fertilizer is restricted due to hygienic, environmental, social and ethical issues. To overcome these limitations, the nutrients are precipitated by the external addition of magnesium (Mg) to form a slow-releasing fertilizer called struvite. The present study aims to maximize phosphate recovery through optimizing struvite production by an emerging electrocoagulation technique. A maximum of 95% phosphate recovery was achieved using inter-electrode distance of 0.5 cm, 2 A current from undiluted urine using Mg-Mg electrodes in a reaction time of 30 min. Further, kinetic modeling of phosphate recovery through electrocoagulation was conducted to comprehend the intended mechanism through the order of kinetics. The results revealed that the data best correlated with first-order kinetics with a correlation coefficient of 0.95. Electrocoagulation improved the supernatant quality by reducing the ion concentrations other than phosphate (30-50%), salinity (40-45%), and microbial population (99%). Qualitative assessment of the precipitate through sophisticated analysis further confirmed the presence of struvite crystals.

Kinetics and morphological characteristics of struvite (MgNH4PO4.6H2O) under the influence of maleic acid

This work reports a stirred-batch lab crystallization to examine the influence of maleic acid (HO₂CCHCHCO₂H), and temperatures (30 and 40 °C) on crystallization kinetics and morphology of struvite. The crystallization was followed by measuring the pH change up to 70 min. The pH decreased drastically for the first 5 min of the run, then started to tail off. It was found that the crystallization rate constants range from 1.608 to 6.534 per hour, which agrees with the most published value. Higher maleic acid concentrations resulted in greater growth retardation; the highest retardation was 74.21%, which was achieved for 30 °C with 20.00 ppm maleic acid. SEM imaging of the obtained precipitates showed irregular prismatic morphology, and the associated EDX confirmed that the precipitates were struvite (MgNH₄PO₄⋅6H₂O). As checked through XRD, the crystalline nature of the struvite was further confirmed, and that co-precipitation of struvite with struvite-K was observed. The co-precipitation was the result of K⁺ adsorption onto the crystal surface. Temperatures had less influence on struvite crystallization. At 40^oC and 20.00 ppm the rate constant was 1.332 per hour; whereas at 30^oC and 0.00 ppm) the corresponding was 1.776 per hour, indicating the retardation of about 25%. Thus, the temperature effect is only 1/3 of the maleic acid effect. The current findings suggest that the presence of maleic acid can be used to elucidate the mechanism of crystallization as well as the crystalline phase transformation of struvite. In practical terms, maleic acid could be potential as a scale inhibitor.

Phosphorus recovered from digestate by hydrothermal processes with struvite crystallization and its potential as a fertilizer

Phosphorus (P) recovery from digestate has attracted considerable interest. In this study, hydrothermal processes in combination with struvite crystallization were performed to promote P solubilization and capture from digestate; its potential as a phosphate-based fertilizer was also investigated. Hydrothermal treatment with HCl and H₂O₂ showed good results for the solubilization of organic and slightly soluble P, and achieved the lowest input energy need (768 kWhkg^-1_P). Struvite crystallization reached 99.3% (Mg²⁺:PO₄^3-1.84:1, pH 9.98). X-ray diffractometry and energy dispersive X-ray spectrometer mapping demonstrated the main precipitate component was struvite. For the fertilization of maize, P utilization from struvite was 19.0%. Light microscope analysis revealed that appropriate amounts of struvite may have an influence on the growth of the primary root. Overall, 16.6% of total P was recovered after P was solubilized, captured and made available.