Detrimental effects of magnesium (II) on hydroxyapatite precipitation from synthetic industrial brines

The influence of Mg(II) on phosphorous recovery as hydroxyapatite (Hap) from alkaline phosphate concentrates using desalinated industrial brines as the calcium source in a batch reactor was evaluated. Two synthetic brines with Mg/Ca molar ratios of 2.2 and 3.3 were continuously fed to reach a Ca/P molar ratio of ~1.67 to promote Hap formation under different constant pH values (8, 9.5, 10.5, 11.5 and 12). For both brines, inhibition of Hap precipitation and formation of the amorphous mineral phases of Ca-, Mg- and Ca/Mg-phosphates were observed at pH > 9.5. Mg(II) severely inhibited phosphate precipitation, allowing the formation of amorphous calcium phosphate from meta-stable clusters due to Mg(II) incorporation into Ca-phosphate. For the Mg/Ca (3.3) brine, a more soluble Mg-phosphate mineral (cattiite) was formed at pH 11.5. Thermal treatment of the amorphous solids to increase crystallinity confirmed the presence of Hap and chlorapatite as Ca-phosphate, stanfieldite as Ca–Mg-phosphate and farringtonite as Mg-phosphate. In the experiments at pH 8, the formation of stable nanometer-sized pre-nucleation clusters promoted nucleation inhibition, even in supersaturated solutions, and no solids were recovered after filtration. Although sulfate was involved in some of the precipitation reactions, its role in the inhibition of Hap formation is not clearly elucidatedPostprint (author's final draft

Phosphate recovery from aqueous solution by K-zeolite synthesized from fly ash for subsequent valorisation as slow release fertilizer

The sorption of phosphate by K-zeolites synthesized from fly ash (FA) by hydrothermal conversion is investigated in this study. The aim is the synthesis of Ca bearing K-zeolites to recover phosphate from urban and industrial wastewater effluents. The loaded zeolites are considered as a by-products rich in essential nutrients such K and P (KP1) with a potential use as slow release fertilizer. A number of synthesis conditions (temperature, KOH-solution/FA ratio, KOH concentration, and activation time) were applied on two FA samples (FA-TE and FA-LB) with similar glass content but different content of crystalline phases, to optimize the synthesis of a zeolitic sorbent suitable for the subsequent phosphate uptake. Merlinoite and W rich zeolitic products synthesized from FA-LB and FA-TE were found to have sorption properties for phosphate removal. A maximum phosphate sorption capacity of 250 mgP-PO4/g and 142 mgP-PO4/g for the zeolitic products selected (KP1-LB and KP1-TE, respectively) was achieved. The dominant phosphate sorption mechanism, in the pH range (6–9) of treated wastewater effluents, indicated that sorption proceeds via a diffusion-controlled process involving phosphate ions coupled with calcium supply dissolution from K-zeolitic products and subsequent formation of brushite (CaHPO4 2H2O(s)). The phosphate loaded sorbent containing a relatively soluble phosphate mineral is appropriate for its use as a synthetic slow release fertilizer. The simultaneous valorisation of fly ash waste and the P recovery from treated wastewaters effluents, (a nutrient with scarce natural resources and low supply) by obtaining a product with high potential for land restoration and agriculture will contribute to develop one example of circularityPeer ReviewedPostprint (author's final draft

Simultaneous ammonium and phosphate recovery and stabilization from urban sewage sludge anaerobic digestates using reactive sorbents

The use of low-cost inorganic sorbents as a new sustainable strategy to enhance the valorization of nutrients (N-P-K), from the urban water cycle (e.g., side streams from sewage sludge anaerobic digestion), in agriculture applications is presented. The simultaneous recovery and stabilization of ammonium and phosphate by using a mixture of two reactive sorbents (Na and K zeolites and magnesium oxide) was evaluated. The nutrients stabilization process, favoured at alkaline pH values, is carried out by a) the precipitation of phosphate ions with magnesium and/or ammonium ions and b) the sorption of ammonium by Na- and K-zeolites. MgO(s) promoted the stabilization of phosphate as bobierrite (Mg3(PO4)2(s)) or struvite (MgNH4PO4(s)) depending on the applied dose. Doses with the stoichiometric molar ratio of Mg/P promote the formation of bobierrite, while molar ratios higher than 3 favour the formation of struvite. Na zeolites (NaP1-NA, NaP1-IQE) demonstrated efficiency on ammonium stabilization between 60 ± 2 (for 15 gZ/L) to 90 ± 3% (for 50 gZ/L). The ammonium recovery efficiency is limited by the zeolite sorption capacity. If the target of the fertilizing criteria should include K, then the use of a K-zeolite (e.g., 5AH-IQE) provides a good solution. The optimum pH for the precipitation of struvite and bobierrite is 9.5 and the optimum pH for ammonium removal is between 4 and 8.5. N is present in higher concentrations (up 0.7–1 g NH4+/L) when pH is ranged between 8.2 and 8.6. The ammonium recovery ratios were better than those previously reported using only magnesium oxide or even a more expensive reagent as newberrite (MgHPO4(s)). The recovery mechanisms described generate low-solubility stabilized nutrients forms that potentially can be applied as slow-release fertilizers in agriculture. Thus, the use in agriculture of blends of digested sludge with low-solubility stabilized nutrients forms will improve soils quality properties in terms of organic matter and nutrients availability.Peer ReviewedPostprint (author's final draft

Simultaneous ammonium and phosphate recovery and stabilization from urban sewage sludge anaerobic digestates using reactive sorbents

The use of low-cost inorganic sorbents as a new sustainable strategy to enhance the valorization of nutrients (N-P-K), from the urban water cycle (e.g., side streams from sewage sludge anaerobic digestion), in agriculture applications is presented. The simultaneous recovery and stabilization of ammonium and phosphate by using a mixture of two reactive sorbents (Na and K zeolites and magnesium oxide) was evaluated. The nutrients stabilization process, favoured at alkaline pH values, is carried out by a) the precipitation of phosphate ions with magnesium and/or ammonium ions and b) the sorption of ammonium by Na- and K-zeolites. MgO(s) promoted the stabilization of phosphate as bobierrite (Mg3(PO4)2(s)) or struvite (MgNH4PO4(s)) depending on the applied dose. Doses with the stoichiometric molar ratio of Mg/P promote the formation of bobierrite, while molar ratios higher than 3 favour the formation of struvite. Na zeolites (NaP1-NA, NaP1-IQE) demonstrated efficiency on ammonium stabilization between 60±2 (for 15gZ/L) to 90±3% (for 50gZ/L). The ammonium recovery efficiency is limited by the zeolite sorption capacity. If the target of the fertilizing criteria should include K, then the use of a K-zeolite (e.g., 5AH-IQE) provides a good solution. The optimum pH for the precipitation of struvite and bobierrite is 9.5 and the optimum pH for ammonium removal is between 4 and 8.5. N is present in higher concentrations (up 0.7-1gNH4+/L) when pH is ranged between 8.2 and 8.6. The ammonium recovery ratios were better than those previously reported using only magnesium oxide or even a more expensive reagent as newberrite (MgHPO4(s)). The recovery mechanisms described generate low-solubility stabilized nutrients forms that potentially can be applied as slow-release fertilizers in agriculture. Thus, the use in agriculture of blends of digested sludge with low-solubility stabilized nutrients forms will improve soils quality properties in terms of organic matter and nutrients availability

Fly ash as reactive sorbent for phosphate removal from treated waste water as a potential slow release fertilizer

There is interest in recovering phosphate (P(V)) from secondary sources, such as waste water streams for potential use as fertilizers reducing the environmental impacts of P(V) discharges and providing alternative phosphorus sources. The goal of this work was to provide an understanding of P(V) removal by fly ash (FA) from coal power plants. Phosphate removal using Ca(II) rich FA was evaluated in terms of i) sorption equilibrium, ii) sorption kinetics under the expected pH values and P(V) concentrations in wastewaters effluents, and iii) P(V) availability of the FAs in agricultural applications. At the pH values (6–9) expected for wastewater effluents, P(V) removal proceeds as a combination of CaO(s) dissolution and brushite (CaHPO4(s)) formation on the FA particles. This process avoids the formation of relatively insoluble Ca–phosphates, such as, hydroxyapatite (Hap) with limited fertilizing properties. High P-loadings were achieved (up to 50 mgP-PO4/g FA (5% P(V) by weight)) at a pH of 8. The removal kinetics data were well described as a diffusion-based process of phosphate ions (H2PO4- and HPO42-) on FA particles, and the CaO(s) dissolution process was discarded as the rate controlling step. The P(V) availability from loaded samples was determined via an agronomical assay with NaHCO3 solutions with P(V) release ratios of 10–30 mgP-PO4/g in FA, confirming the appropriateness of this material as a potential fertilizer, even in calcareous soils.Peer ReviewedPostprint (author's final draft

Recovery of nutrients (N-P-K) from potassium-rich sludge anaerobic digestion side-streams by integration of a hybrid sorption-membrane ultrafiltration process: Use of powder reactive sorbents as nutrient carriers

Here, an alternative nutrient (N-P-K) recovery route from potassium-rich sludge anaerobic digestion side-streams using powder reactive sorbents (PRSs) is presented. In the first step, the optimum PRS system was determined in batch experiments with mixtures of: a) a sodium zeolite (NaP1) to facilitate the NH4+ and K+ sorption; b) a Ca-zeolite (CaP1) to facilitate the removal of P by formation of Ca-phosphates (e.g., CaHPO4(s)), and c) caustic magnesia containing mixtures of MgO to facilitate the formation of Mg/NH4/PO4 minerals (e.g., struvite and magnesium phosphates). Evaluation of the continuous and simultaneous N-P-K removal with mixtures of PRSs was carried out using a hybrid sorption/filtration system with ultrafiltration (UF) hollow-fibre membranes. The dosing ratios of the PRS mixtures were optimised on the basis of the equilibrium and kinetic sorption data, and a PRS dose (< 2–5 g PRS/L) was selected to ensure the hydraulic performance of the system. Under such conditions, and with synthetic anaerobic side-stream removal capacities (qt) of 220 ± 10 mg N-NH4/g, 35 ± 5 mg P-PO4/g, and 8 ± 2 mg K/g, removal efficiencies of 32 ± 3, 78 ± 5, and 26 ± 3% for ammonium, phosphate, and potassium, respectively, were obtained for the binary mixtures of NaP1/CaP1 zeolites. Contrary to the batch results, the use of tertiary mixtures of NaP1/CaP1/MgO only improved the K removal capacity and efficiency to 18 ± 2 mg K/g and 55 ± 4%, respectively, while the phosphate removal capacity and efficiency remained unchanged (ca. 35 ± 3 mg P-PO4/g; 80 ± 5%) and the ammonium capacity and efficiency were reduced to 185 ± 12 mg N-NH4/g and 20 ± 2%, respectively, due to the competing Mg2 + ion effect. Nutrient removal trials with real anaerobic side-streams using binary mixtures of Na/Ca zeolites showed a reduction of both the hydraulic performance and the nutrient removal ratios due to the presence of dissolved organic matter. However, constant removal ratios of N, P, and K were recorded throughout the filtration experiments. The loaded PRSs exhibited suitable nutrient release rates and bioavailability as co-substrates for soil quality improvement. Chemical analyses detected the formation of Ca/P/O and Mg/N/P/O neo-minerals; however, the mineralogical data revealed only the formation of struvite, even when no magnesium oxide was used.Preprin

Detrimental effects of magnesium (II) on hydroxyapatite precipitation from synthetic industrial brines

The influence of Mg(II) on phosphorous recovery as hydroxyapatite (Hap) from alkaline phosphate concentrates using desalinated industrial brines as the calcium source in a batch reactor was evaluated. Two synthetic brines with Mg/Ca molar ratios of 2.2 and 3.3 were continuously fed to reach a Ca/P molar ratio of ~1.67 to promote Hap formation under different constant pH values (8, 9.5, 10.5, 11.5 and 12). For both brines, inhibition of Hap precipitation and formation of the amorphous mineral phases of Ca-, Mg- and Ca/Mg-phosphates were observed at pH > 9.5. Mg(II) severely inhibited phosphate precipitation, allowing the formation of amorphous calcium phosphate from meta-stable clusters due to Mg(II) incorporation into Ca-phosphate. For the Mg/Ca (3.3) brine, a more soluble Mg-phosphate mineral (cattiite) was formed at pH 11.5. Thermal treatment of the amorphous solids to increase crystallinity confirmed the presence of Hap and chlorapatite as Ca-phosphate, stanfieldite as Ca–Mg-phosphate and farringtonite as Mg-phosphate. In the experiments at pH 8, the formation of stable nanometer-sized pre-nucleation clusters promoted nucleation inhibition, even in supersaturated solutions, and no solids were recovered after filtration. Although sulfate was involved in some of the precipitation reactions, its role in the inhibition of Hap formation is not clearly elucidate

Future patterns of retailing in Scotland

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Fly ash as reactive sorbent for phosphate removal from treated waste water as a potential slow release fertilizer

There is interest in recovering phosphate (P(V)) from secondary sources, such as waste water streams for potential use as fertilizers reducing the environmental impacts of P(V) discharges and providing alternative phosphorus sources. The goal of this work was to provide an understanding of P(V) removal by fly ash (FA) from coal power plants. Phosphate removal using Ca(II) rich FA was evaluated in terms of i) sorption equilibrium, ii) sorption kinetics under the expected pH values and P(V) concentrations in wastewaters effluents, and iii) P(V) availability of the FAs in agricultural applications. At the pH values (6–9) expected for wastewater effluents, P(V) removal proceeds as a combination of CaO(s) dissolution and brushite (CaHPO4(s)) formation on the FA particles. This process avoids the formation of relatively insoluble Ca–phosphates, such as, hydroxyapatite (Hap) with limited fertilizing properties. High P-loadings were achieved (up to 50 mgP-PO4/g FA (5% P(V) by weight)) at a pH of 8. The removal kinetics data were well described as a diffusion-based process of phosphate ions (H2PO4- and HPO42-) on FA particles, and the CaO(s) dissolution process was discarded as the rate controlling step. The P(V) availability from loaded samples was determined via an agronomical assay with NaHCO3 solutions with P(V) release ratios of 10–30 mgP-PO4/g in FA, confirming the appropriateness of this material as a potential fertilizer, even in calcareous soils.Peer Reviewe