Crystallization of struvite-K from pumpkin wastes.

BACKGROUND: Use of slow-release fertilizers derived from biological sources is important in sustainable agricultural development. Struvite-K (KMgPO4 ·6H2 O) is magnesium potassium phosphate mineral that has high potential for use as fertilizer in agriculture. Struvite-K is particularly suitable for slow-release fertilizer systems since struvite-K crystals are sparingly soluble in water. Seeds of pumpkin Cucurbita pepo L. are recovered and consumed as food, but the remaining pulp has no economic value. RESULTS: The present study evaluated the feasibility of struvite-K crystals recovery from pyrolysis products of pumpkin wastes. In the study C. pepo pulp was decomposed at high temperatures and potassium was extracted from the residue and then crystalized from the solution by addition of NaH2 PO4 ·2H2 O and MgCl2 ·6H2 O salts. Struvite-K was characterized by Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy-energy-dispersive X-ray (SEM-EDX) spectroscopy and X-ray diffraction (XRD) analysis. CONCLUSIONS: The study revealed pumpkin wastes can be evaluated as source of potassium and 80% of potassium could be recovered as struvite-K crystals, which have a potential use as a slow-release mineral fertilizer for sustainable agriculture operations. © 2021 Society of Chemical Industry.

A new approach to removing and recovering phosphorus from livestock wastewater using dolomite.

Recovering phosphorus from livestock wastewater could partly mitigate the global phosphorus resource crisis. Crystallization is a promising method for removing phosphorus from wastewater, but the costs of calcium- and magnesium-containing reagents are increasing. Cheap, available, efficient materials are required to replace conventional calcium and magnesium reagents. Here, we describe a new approach to removing and recovering phosphorus from livestock wastewater of a large pig farm, containing a high phosphorus concentration. The effects of the pH, stirring speed, stirring time, and extract dose (containing calcium and magnesium) on phosphorus removal from livestock wastewater were investigated. The product was characterized by X-ray diffractometry, Fourier-transform infrared spectroscopy, and scanning electron microscopy. Under optimized conditions (pH 9.0, stirring speed 200 r/m, stirring time 600 s, Ca 207.62 mg/L, Mg 122.86 mg/L), 92% of the phosphorus was removed from livestock wastewater. The product was mainly the hydroxyapatite (Ca5(PO4)3OH) precursor amorphous calcium phosphate but also contained 1.65% (by mass) magnesium ammonium phosphate (MgNH4PO4·6H2O) crystals. The cost of dolomite to treat 1 m3 of high-phosphorus wastewater was 0.20 yuan (45.9%, 25.9%, and 75.9% lower than for pure MgCl2, MgSO4, and CaCl2, respectively) in 2019. Using dolomite to provide calcium and magnesium effectively decreases the crystallization process cost and should encourage the use of crystallization to remove phosphorus from wastewater.

Enhanced phosphorus removal using acid-treated magnesium slag particles.

Magnesium-enriched magnesium slag particles (MSPs) can be used as an adsorption substrate as well as the magnesium source for struvite precipitation. In this study, an HCl treatment was used to enhance MSPs for phosphorus removal. After soaking in 1 mol/L HCl, an 11.27% decrease in median diameter (D50) and a 6.73% increase in specific surface area were observed when compared with the original MSPs. The improvement of the MSP surface properties resulted in 188.96 mg/kg increase in the PO43- adsorption capacity. Irrespective of HCl treatment, the phosphorus adsorption process followed the Dubinin-Radushkevich (D-R) model much more accurately than the Langmuir and Freundlich equations with correlation coefficients higher than 0.94. The adsorption free energy obtained through the D-R model revealed a 9.75% decrease after HCl treatment. Sequential fraction extraction results indicated that 96% of the Mg2+ released from the HCl-treated MSPs came from acid-soluble magnesium (exchangeable and carbonate-bound). Mg2+ obtained from HCl-treated solutions provided a reliable magnesium source for struvite precipitation. The PO43- removal rate can reach 53.63% with the optimal pH value of 10.0 and molar ratio of NH4+ to PO43- of 1:1. Struvite precipitation and adsorption can simultaneously occur in HCl-treated MSP solution. It contributed 63.19% to the overall PO43- removal and is a major contributor compared with adsorption. Thus, HCl treatment greatly enhanced the potential of MSPs for phosphorus removal due to an improved adsorption capacity and is a reliable Mg2+ source for struvite precipitation.

Recovery of nutrients from digested sludge as struvite with a combination process of acid hydrolysis and Donnan dialysis.

In this study, the effects of the combination of acid hydrolysis and Donnan dialysis on the high-quality struvite production from digested sewage sludge were examined. The Box-Behnken design was applied in the hydrolysis using oxalic acid for the optimization of the conditions that affect the nutrients and metal release from digested sludge. An optimal condition was obtained at oxalic acid 0.5 M, acid/sludge ratio (mL/g) 10/1, and reaction time 60 min. The separation of the metals from hydrolyzed sludge liquid was carried out with the Donnan dialysis using a Nafion 117 cation exchange membrane. At the end of the 4 hours of operating time, the recovery values for Zn, K, Na, Mg, Fe, and Al were obtained as 67.9%, 62.1%, 57.6%, 39.4%, 5.3%, and 2.5%, respectively. The produced struvite meets the legal limits for fertilizer use in terms of Cd, Cu, Ni, Pb, Zn, Hg, and Cr content specified by Turkish regulations. A high Ca/Mg molar ratio and the presence of K, Na, and Al affected the efficiency of struvite crystallization. The use of oxalic acid resulted in low Ca release, and the metal ions that affect struvite purity were eliminated using Donnan dialysis.