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

Recycling steel slag as fertiliser proxy in agriculture is good circular economy but disrupts plant microbial symbioses in the soil

Modern agriculture depends on synthetic fertilisers to ensure food security but their manufacture and use accounts for ~5 % of the global greenhouse gas emissions. Achieving climate change targets therefore requires alternatives, that while maintaining crop productivity, reduce emissions across the lifecycle of fertiliser utilisation. Steel slag, a nutrient-rich by-product of steel manufacture, offers a viable alternative. Being substantially cheaper than fertilisers, it is economically attractive for farmers, particularly in low-middle income countries of the Global South. However, slag application in agriculture poses risk of pollutant transfer to the human food chain and disruption of key plant-microbe symbioses like the arbuscular mycorrhizal fungi (AMF). Here, using barley as a model crop, we tested the suitability of slag as a fertiliser proxy. Mycorrhizal and non-mycorrhizal barley were grown in soils ameliorated with slag in concentrations of 0, 2, 5 and 10 t ha-1. We analysed slag-mycorrhiza interaction and their combined effects on crop yield and risks to human nourishment. Slag increased grain yield by respective 32 and 21 % in mycorrhizal and non-mycorrhizal barley. Grain concentration of metal pollutants in mycorrhizal and non-mycorrhizal barley fertilised with slag were within the WHO recommended limits. But slag reduced mycorrhizal colonisation in barley roots and extraradical hyphal spread in the soil. The consequent decline in symbiont function lowered AMF-mediated plant nutrient uptake and increased mineral losses in leachates. AMF are keystone species of the soil microbiome. Loss of AMF function presents long-term ecological consequences for agriculture and necessitates a careful evaluation of slag application to soil.

Efficiency, mechanism and application prospect of ammonium adsorption and desorption over a sodium-acetate-modified synthetic zeolite

Adsorption is an effective approach for remediating ammonium pollution, and zeolite has exceptional efficacy for the adsorption of ammonium. The investigation of ammonium adsorption using coal-fly-ash-based zeolite has gained remarkable attention in contemporary research. In this work, a sodium-acetate-modified synthetic zeolite (MSZ) was used to absorb ammonium in simulated wastewater. The MSZ had an adsorption capacity for ammonium of 27.46 mg g-1, and the adsorption process followed the Langmuir isotherm model and pseudo-second-order kinetics model. The adsorption and desorption of ammonium were controlled by ion exchange, pore diffusion, and electrostatic attraction processes. Ion exchange was responsible for 77.90% of the adsorption process and 80.16% of the desorption process. The MSZ was capable of continuously removing large amounts of ammonium from wastewater through fixed bed adsorption. After 5 regeneration cycles, MSZ still maintained 75% adsorption characteristics for ammonium. Using MSZ adsorbed with ammonium as a soil amendment increased the germination rate of mung beans by 10%. Furthermore, it also increased the stem length, root length, and fresh weight by 20-30%. These findings suggest that MSZ provides a promising application prospect to mitigate ammonium pollution and recycle ammonium resources.

Lanthanum-modified tobermorite synthesized from fly ash for efficient phosphate removal

Phosphate removal from water by lanthanum-modified tobermorite synthesized from fly ash (LTFA) with different lanthanum concentrations was studied. LTFA samples were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and Brunauer‒Emmett‒Teller specific surface area analysis. The results showed that the LTFA samples were mainly composed of mesoporous tobermorite-11 Å, and LTFA1 with a lanthanum concentration of 0.15 M had a high specific surface area (83.82 m2/g) and pore volume (0.6778 cm3/g). The phosphate adsorption capacities of LTFA samples were highest at pH 3 and gradually decreased with increasing pH. The phosphate adsorption kinetics data on LTFA samples were most accurately described by the Elovich model. The adsorption isotherms were in the strongest agreement with the Temkin model, and LTFA1 showed the highest phosphate adsorption capacity (282.51 mg P/g), which was higher than that of most other lanthanum-modified adsorbents. LTFA1 presented highly selective adsorption of phosphate with other coexisting ions (HCO3-, Cl-, SO42-, and NO3-). In addition, phosphate was adsorbed onto LTFA samples by forming inner-sphere phosphate complexes and amorphous lanthanum phosphate. This study provides technical support for development of efficient fly ash-based phosphate adsorbents.

Novel process for high value utilization of high-alumina fly ash: valuable metals recovery and mesoporous silica in situ preparation

Extraction of valuable metals besides silica from high-alumina fly ash is one of the most important high-value utilization pathways. However, it is difficult to realize high-efficiency extraction due to the stable structure e.g. of quartz and mullite. In this paper, mineral phase transformation for valuable metal recovery and mesoporous silica in situ preparation from fly ash by a selective acid leaching method was proposed. The mineral phase transformation, dissolution behavior of each metal, and pore structure of fly ash derived mesoporous silica were systematically investigated. The results show that the co-activation of fly ash by Na2CO3-K2CO3 formed the phases of kalsilite and (Na, K)AlSiO4. During the acid leaching process, Al, Li, and Ga could be leached with the efficiencies of 86.17%, 89%, and 80% in the FK system. In the FN system, the efficiencies of Al, Li, and Ga are 92.38%, 95%, and 83%, respectively. The crystal plane (002) was destroyed for kaliophilite while all the crystal planes were destroyed for nepheline. With the increase of HCl solution concentration, the porous silica exhibited the same change order of pore shape. The pore structure of as-prepared porous silica was type IV and the hysteresis loop was type H3, and the specific surface areas could be 565.54, 448.02, and 746.76 m2 g-1, respectively. Finally, the leaching liquors can be used to produce crystal aluminum chloride, lithium carbonate and gallium. This paper might provide technical support for full recycling of high-value resources from fly ash.

Bi-functional biochar-g-C3N4-MgO composites for simultaneously minimizing pollution：Photocatalytic degradation of pesticide and phosphorus recovery as slow-release fertilizer

Significant progress has been made in the development of phosphorus recovery adsorbents and photocatalysts for degradation of pesticides. However, the bifunctional materials for phosphorus recovery and photocatalytic degradation of pesticides have not been designed, and the mechanism of the interaction between photocatalysis and P adsorption remains unexplored. Herein, we develop biochar-g-C3N4-MgO composites (BC-g-C3N4-MgO) with bi-function application to minimize water toxicity and eutrophication. The results show phosphorus adsorption capacity of the BC-g-C3N4-MgO composite reaches 111.0 mg·g-1, and its degradation ratio of dinotefuran reaches 80.1% within 260 min. The mechanism studies show that MgO can play variety roles in BC-g-C3N4-MgO composite, in which can improve the adsorption capacity of phosphorus, enhance the utilization efficiency of visible light and the separation efficiency of photoinduced electron-hole pairs. The biochar existed in BC-g-C3N4-MgO serves as charge transporter with a good conductivity, which promotes the fluent transfer of photo-generated charge carriers. The ESR indicates that both •O2- and •OH generated from BC-g-C3N4-MgO are responsible for dinotefuran degradation. Finally, pot experiments reveal that P laden BC-g-C3N4-MgO promotes the growth of pepper seedlings with high P utilization efficiency of 49.27%.

Preparation, Features, and Efficiency of Nanocomposite Fertilisers Based on Glauconite and Ammonium Dihydrogen Phosphate

This paper studies the chemical and mechanochemical preparation of glauconite with ammonium dihydrogen phosphate (ADP) nanocomposites with a ratio of 9:1 in the vol.% and wt.%, respectively. The methods include X-ray diffraction analysis, scanning electron microscope with energy-dispersive X-ray spectroscopy, transmission electron microscopy, infrared spectroscopy, and differential thermal analysis with a quadruple mass spectrometer. The manufactured nanocomposites keep the flaky glauconite structure. Some glauconite unit structures have been thickened due to minimal nitrogen (ammonium) intercalation into the interlayer space. The globular, granular, or pellet mineral particles of nanocomposites can be preserved via chemical techniques. Globular and micro-aggregate particles in nanocomposites comprise a thin film of adsorbed ADP. The two-step mechanochemical method makes it possible to slightly increase the proportion of adsorbed (up to 3.2%) and intercalated (up to 6.0%) nutrients versus chemical ways. Nanocomposites prepared via chemical methods consist of glauconite (90%), adsorbed (1.8-3.6%), and intercalated (3.0-3.7%) substances of ADP. Through the use of a potassium-containing clay mineral as an inhibitor, nitrogen, phosphorus, and potassium (NPK), nanocomposite fertilisers of controlled action were obtained. Targeted and controlled release of nutrients such as phosphate, ammonium, and potassium are expected due to various forms of nutrients on the surface, in the micropores, and in the interlayer space of glauconite. This is confirmed via the stepwise dynamics of the release of ammonium, nitrate, potassium, and phosphate from their created nanocomposites. These features of nanocomposites contribute to the stimulation of plant growth and development when fertilisers are applied to the soil.

Zeolite Composite Materials from Fly Ash: An Assessment of Physicochemical and Adsorption Properties

Waste fly ash, with both low (with the addition of vermiculite) and high contents of unburned coal, were subjected to hydrothermal syntheses aiming to obtain zeolite composite materials-zeolite + vermiculite (NaX-Ver) and zeolite + unburned carbon (NaX-C). The composites were compared with parent zeolite obtained from waste fly ash with a low content of unburned carbon (NaX-FA). In this study, the physicochemical characteristics of the obtained materials were evaluated. The potential application of the investigated zeolites for the adsorption of ammonium ions from aqueous solutions was determined. Composite NaX-Ver and parent zeolite NaX-FA were characterized by comparable adsorption capacities toward ammonium ions of 38.46 and 40.00 mg (NH₄⁺) g^-1, respectively. The nearly 2-fold lower adsorption capacity of composite NaX-C (21.05 mg (NH₄⁺) g^-1) was probably a result of the lower availability of ion exchange sites within the material. Adsorbents were also regenerated using 1 M NaCl solution at a pH of 10 and subjected to 3 cycles of adsorption-desorption experiments, which proved only a small reduction in adsorption properties. This study follows the current trend of waste utilization (fly ash) and the removal of pollutants from aqueous solutions with respect to their reuse, which remains in line with the goals of the circular economy.

Synthesis of zeolite A from fly ash and its application in the slow release of urea

The study focused on the transformation of coal fly ash to zeolite A (ZA) as a potential carrier for the slow release of urea. After being treated with HCl aqueous solution and NaOH successively, SiO₂ and Al₂O₃ were converted into sodium silicoaluminate. The obtained silicoaluminate was then heated with NaAlO₂ in an aqueous NaOH solution at 70-110 °C for 3-18 h and zeolite A was successfully prepared according to the X-ray diffraction measurements. By changing the hydrothermal temperature and time, ZA could reach 237.3 mmol/100 g in maximum cation exchange capacity. ZA impregnated with urea (ZA-U) at a mass ratio of more than 5:1 exhibited slow release of urea and the kinetics release mechanism of ZA-U was proposed. The plant growth test proved that the slow release of urea from ZA-U can promote the growth of maize seedling.

Macro-nutrients recovery from liquid waste as a sustainable resource for production of recovered mineral fertilizer: Uncovering alternative options to sustain global food security cost-effectively

Global food security, which has emerged as one of the sustainability challenges, impacts every country. As food cannot be generated without involving nutrients, research has intensified recently to recover unused nutrients from waste streams. As a finite resource, phosphorus (P) is largely wasted. This work critically reviews the technical applicability of various water technologies to recover macro-nutrients such as P, N, and K from wastewater. Struvite precipitation, adsorption, ion exchange, and membrane filtration are applied for nutrient recovery. Technological strengths and drawbacks in their applications are evaluated and compared. Their operational conditions such as pH, dose required, initial nutrient concentration, and treatment performance are presented. Cost-effectiveness of the technologies for P or N recovery is also elaborated. It is evident from a literature survey of 310 published studies (1985-2022) that no single technique can effectively and universally recover target macro-nutrients from liquid waste. Struvite precipitation is commonly used to recover over 95 % of P from sludge digestate with its concentration ranging from 200 to 4000 mg/L. The recovered precipitate can be reused as a fertilizer due to its high content of P and N. Phosphate removal of higher than 80 % can be achieved by struvite precipitation when the molar ratio of Mg²⁺/PO₄^3- ranges between 1.1 and 1.3. The applications of artificial intelligence (AI) to collect data on critical parameters control optimization, improve treatment effectiveness, and facilitate water utilities to upscale water treatment plants. Such infrastructure in the plants could enable the recovered materials to be reused to sustain food security. As nutrient recovery is crucial in wastewater treatment, water treatment plant operators need to consider (1) the costs of nutrient recovery techniques; (2) their applicability; (3) their benefits and implications. It is essential to note that the treatment cost of P and/or N-laden wastewater depends on the process applied and local conditions.

Combing phosphorus-modified hydrochar and zeolite prepared from coal gangue for highly effective immobilization of heavy metals in coal-mining contaminated soil

Considering the adverse effects of heavy metals (HMs) on agriculture soil, in-situ immobilization has been paid great attention worldwide. P-modified biochar/hydrochar along with synthetic zeolite for efficient HMs immobilization in contaminated soil becomes a promising choice. In this study, H₃PO₄-modified hydrochar (BPH) derived from banana peels, and Na-X zeolite (ZL) prepared from coal gangue was applied individually and synergistically (1%BPH, 2%BPH 1%ZL, 2%ZL, and 1%BPH+1%ZL) to remediate a farmland soil polluted by Cd, Cu, and Pb near the coal-mining area. Compared with the mono-application of these two amendments, their combination significantly improved the soil organic carbon (SOC), electric conductivity (EC), and dehydrogenase activity. Besides, the addition of 1%CLH+1%ZL remarkably reduced the Cd, Cu, and Pb bioavailability by 67.01%, 57.01%, and 78.72%, respectively, in the soil after 100 d incubation by transforming these metals to more stable forms. The order of the HMs immobilization capacity for these two amendments was as follows: Pb > Cu > Cd. Moreover, the dominated immobilization mechanism of their synergistic application was that BPH could immobilize HMs by precipitation, complexation, and π-π electron-donor-acceptor interaction. The precipitation and complexation blocked the surface pores of BPH. The sustained release of phosphorus groups and radicals was prevented. This obstacle was possibly alleviated by adding ZL. Besides, the formation of cationic bridging, the enhancement of soil properties, and the physical adsorption of these amendments were also conducive to HMs immobilization in soil. This work indicated that co-application of BPH and ZL possibly was an excellent choice for immobilizing HMs in soil.

Simultaneous adsorption of ammonia and phosphate using ferric sulfate modified carbon/zeolite composite from coal gasification slag

Removal of nutrients in water is crucial to control eutrophication. Fly ash has been increasingly used to synthesize zeolite to remove nutrients, but it is still poorly understood about the removal capacity of zeolite synthesized from coal gasification slag (CGS), which has not been well recycled in many countries. In this study, the CGS was acid leached, alkali dissolved, and synthesized to carbon/zeolite composite (C/ZC) under induction by medical stone. After being modified by ferric sulfate, the composite was analyzed for the adsorption of NH₄⁺ and PO₄^3-. Results showed that the maximum adsorption capacity by C/ZC is 5.17 mg/g, but C/ZC has no adsorption capacity of PO₄^3-. The ferric sulfate was used to modify C/ZC to obtain carbon/zeolite composite modified by iron (M-C/ZC). M-C/ZC has a higher specific surface area (348.3 m²/g), and the negatively charge of M-C/ZC can adsorb NH₄⁺ and form Fe-O-P between PO₄^3- and Fe-OH bonds. The maximum adsorption capacity of NH₄⁺ and PO₄^3- by M-C/ZC are 7.44 mg/g and 6.94 mg/g, respectively. The removal efficiency of NH₄⁺ and PO₄^3- are up to 88% and 99% under initial NH₄⁺ (5 mg/L) and PO₄^3- (10 mg/L) concentration. The regeneration capacity of M-C/ZC of NH₄⁺ was stronger than that of PO₄^3-. After three cycles, the regeneration rate of M-C/ZC of NH₄⁺ was still up to 76.96%. Our findings suggest the good application potential of M-C/ZC for removing NH₄⁺ and PO₄^3- from wastewater.

Phosphorus and nitrogen recovery from wastewater by ceramsite: Adsorption mechanism, plant cultivation and sustainability analysis

Recovery of the nitrogen (N) and phosphorus (P) in wastewater would help to minimize eutrophication and their reuse would lead to a more sustainable society. Sewage sludge and fly ash were used to fabricate ceramsite in the laboratory. After modified with alkali or lanthanum it was shown in benchtop experiments to effectively recover N and P from real wastewater treatment plant effluent. The N&P-adsorbed ceramsite was then applied as an eco-friendly, slow-release fertilizer to promote the germination, growth and blooming of Impatiens commelinoides, realizing the recycling of N and P from wastewater. Emergy analysis shows that such recycling is more sustainable than the current two approaches (i.e., landfill and incineration) for sludge disposal. This work thus demonstrates a sustainable solution combining the reuse of solid waste, effective wastewater purification and recovery of N and P nutrients. Applying the technologies demonstrated would help to minimize the environmental impact of wastewater and solid waste.

Efficient reclamation phosphate by alginate-g-BMOF using poly(N-isopropyl acrylamide-co-acrylamide) as coating for temperature-responsive slow-release P-fertilizer

Poly(N-isopropyl acrylamide) and its derived copolymer, as a temperature-responsive material, are widely used in the field of anticancer drug carrier. And it also plays an important role as carrier in slow-release fertilizer in recent years. In this paper, a smart poly(N-isopropyl acrylamide-co-acrylamide)-coated Alg-BMOF (PABMOF) was fabricated in ionic liquids microemulsion ([Bmim]PF₆/TX-100/water) as nano-reactor. The structure and morphology of PABMOF were characterized by FT-IR, XRD, XPS, SEM, TG and BET. The resultant PABMOF was used as a adsorbent for H₂PO₄^- adsorption. The adsorption kinetics, isotherms and mechanism of H₂PO₄^- onto the resultant PABMOF were studied. The adsorption kinetic data was well suitable for pseudo-second-order kinetic model, and adsorption isotherm results demonstrated that the equilibrium data was fitted for Freundlich model. The water-holding and water-retention capacity of soil with TRSRFs addition of 2 wt% were74.3% and 52.13% at 30th day, respectively. Moreover, the release behavior of TRSRFs in water show that the cumulative release rate (C_r%) were 81.4% at 45 °C and 97.6% at 25 °C within 172 h, which displayed the excellent temperature-responsive property. The effect of TRSRFs on the growth of Chinese cabbage was investigated, which was indexed with the germination rate, plant height and root length of the crop.

Recovery of phosphate and ammonia from wastewater via struvite precipitation using spent refractory brick gravel from steel industry

Spent refractory brick (SRB) generated from the steel industry has a high magnesium content. In this study, a procedure was developed to utilize SRB gravels for efficient recovery of phosphate and ammonia from high strength wastewater via struvite (MgNH₄PO₄∙6H₂O(s)) precipitation. Mg²⁺ and Ca²⁺ were first leached from SRB gravels using nitric acid solution. Ca²⁺ in the solution could inhibit struvite precipitation and was sequestered by dosing SO₃^2- to form calcium sulfite (CaSO₃(s)). The resulting Mg²⁺-rich solution was then employed to initiate struvite precipitation for phosphate and ammonia recovery. The optimal precipitation was achieved with a molar ratio of [Mg²⁺]:[NH₃-N]:[PO₄³-P] = 2:1:2 at pH 9.5. The residual phosphate in the solution can be further removed via the precipitation of calcium phosphate minerals. Overall, 99.6% phosphate and 98.2% ammonia could be recovered and the treated wastewater could meet the discharging standards of ammonia and phosphate. The resulting solids, including calcium sulfite, struvite and calcium phosphate can be potentially used in the cement industry and agriculture sector to achieve sustainable recycle of spent materials.

Utilization of coal fly ash waste for effective recapture of phosphorus from waters

Reutilization of the waste by-products from industrial and agricultural activities is crucially important towards attainment of environmental sustainability and the 'circular economy'. In this study, we have developed and evaluated a sustainably-sourced adsorbent from coal fly ash, which was modified by a small amount of lanthanum (La-FA), for the recapture of phosphorous (P) from both synthetic and real natural waters. The prepared La-FA adsorbent possessed typical characteristic diffraction peaks similar to zeolite type Na-P1, and the BET surface area of La-FA was measured to be 10.9 times higher than that of the original FA. Investigation of P adsorption capability indicated that the maximum adsorption (10.8 mg P g^-1) was 6.14 times higher than that (1.8 mg P g^-1) of the original fly ash material. The ζ potentials measurement and P K-edge X-ray Absorption Near Edge Structure (XANES) spectra demonstrated that P was bonded on La-FA surfaces via an adsorption mechanism. After applying the proposed adsorbent to real lake water with La/P molar ratios in the range from 0.5:1 to 3:1, the La-FA adsorbent showed the highest phosphate removal ability with a La/P molar ratio 1:1, and the P adsorption was similar to that performance with the synthetic solution. Moreover, the La-FA absorbent produced a negligible effect on the concentrations of total dissolved nitrogen (TDN), NH₄⁺-N and NO₃^--N in water. This study thus provides a potential material for effective P recapture and details of its operation.

Electro-oxidation to convert dissolved organic nitrogen and soluble non-reactive phosphorus to more readily removable and recoverable forms

Conventional wastewater treatment processes cannot effectively remove dissolved organic nitrogen (DON) and soluble non-reactive phosphorus (sNRP), which can pose regulatory compliance challenges for total nitrogen and total phosphorus discharges. Moreover, DON and sNRP are not easily recoverable for beneficial reuse as part of the waste to resource paradigm. Conversion of DON and sNRP to more readily removable dissolved inorganic nitrogen (DIN) and soluble reactive phosphorus (sRP), respectively, will help meet stringent nutrient limits and facilitate nutrient recovery. In this study, electro-oxidation (EO) was evaluated for conversion of four DON compounds to DIN and five sNRP compounds to sRP. EO was more efficient and provided higher extents of conversion of the recalcitrant nutrient fractions compared to a more traditional advanced oxidation process, UV/H₂O₂. Direct electron transfer was likely the dominant oxidation mechanism for EO-based DON and sNRP conversion, with DON being more recalcitrant. Among the DON compounds tested, greater availability of primary amine (C-N bonds) yielded greater conversion compared to compounds with fewer primary amine or those with secondary amine (C-N-C bond). Among the sNRP compounds tested, those with P-O-C bonds (organic sNRP) converted more readily than those with P-O-P bonds (inorganic sNRP), presumably because cleavage of the latter bond requires greater energy. Using 30 min of EO treatment, the highest DON and sNRP compound conversion was 11.7 ± 0.09% for urea and 31.1 ± 0.75% for beta-glycerol phosphate. A similar extent of EO-based conversion of DON (6.41 ± 1.5%) and sNRP (32.7 ± 3.3%) was observed in real wastewater.

Improvement of Phosphate Adsorption Kinetics onto Ferric Hydroxide by Size Reduction

Ball milling and ultra-sonication size reduction procedures were applied to granular ferric hydroxide (GFH) to obtain two micro-sized adsorbents. These two adsorbents and GFH were investigated to improve the removal of phosphates from water. The size reduction procedures, using the milling method, allowed a reduction of size from 0.5–2 mm to 0.1–2 µm and total disaggregation of the GFH structure. Using an ultra-sonication method yielded a final size of 1.9–50.3 µm with partial disaggregation. The Langmuir model correlated well with the isotherms obtained in batch equilibrium tests for the three adsorbents. The maximum adsorption capacity (qmax) for the milled adsorbent was lower than GFH, but using ultra-sonication was not different from GFH. The equilibrium adsorption of two wastewater samples with phosphate and other anions onto the GFH corresponded well with the expected removal, showing that potential interferences in the isotherms were not important. Batch kinetics tests indicated that the pseudo second-order model fitted the data. Long-term adsorption capacity in kinetics (qe) showed the same trend described for qmax. The application of milling and ultra-sonication methods showed 3.5- and 5.6-fold increases of the kinetic constant (k2) versus the GFH value, respectively. These results showed that ultra-sonication is a very good procedure to increase the adsorption rate of phosphate, maintaining qe and increasing k2.