Removal of phosphate from wastewater using alkaline residue

Sludge-derived alginate-like extracellular polymers (ALE) for preparation of Fe-ALE and FeCaMg-ALE: Application to the adsorption of phosphate

Excess phosphorus (P) in wastewater has potential risk of causing harmful algal bloom and eutrophication in receiving wastewater. In this study, alginate-like extracellular polymers (ALE) derived from conventional activated sludge were modified with ionic cross-linking agents (Fe3+, Ca2+, and Mg2+) to develop Fe-ALE and FeCaMg-ALE for the adsorption of phosphate from wastewater. The adsorption process of phosphate by Fe-ALE and FeCaMg-ALE can be well described by pseudo-second-order kinetics and Freundlich isotherm model with a high level of accuracy, indicating that the adsorption processes were chemical, multi-layer adsorption process. The maximum adsorption capacity of dry Fe-ALE and FeCaMg-ALE concerning phosphate were 15.06 and 20.10 mg/g, respectively at 298 K. The adsorption capacity remained relatively consistent across a pH range of 2.0-11.0. FT-IR, XRD, SEM coupled with XPS analysis demonstrated the ALE had been successfully compounded with Fe3+ or Fe3+/Ca2+/Mg2+. Based on the experimental results and characteristic analysis, the main mechanism of phosphate by Fe-ALE and FeCaMg-ALE are physical filling, electrostatic attraction, ligand exchange and precipitation reaction. This work provides a new perspective for preparing ALE-based adsorbent using conventional activated sludge as raw material, realizing the treatment of waste with waste and effectively recovering phosphate from wastewater.

Phosphate adsorption on dried alum sludge: Modeling and application to treatment of dairy effluents

This study evaluates Alum sludge from drinking water treatment plants for the efficient and cost-effective removal of phosphates from aqueous solutions. Extensive characterization and batch experiments have established that optimal phosphate removal was achieved with a sludge dosage of 20 g L-1 (at an initial phosphate concentration of 100 mg L-1), a pH of 5, a temperature of 23 °C, and a stirring speed of 200 rpm. These conditions significantly reduced phosphate levels, ensuring compliance with legal discharge limits. The Langmuir isotherm, pseudo-second-order kinetic and intraparticle diffusion models best described the adsorption process, highlighting the spontaneous and endothermic nature of the phenomenon. The sludge effectively reduced phosphate concentrations to acceptable levels when applied to dairy effluents. This study underscores the potential of Alum sludge as a viable solution for phosphate management in environmental cleanup efforts.

Research status of soda residue in the field of environmental pollution control

High-quality soda ash (Na2CO3) is mainly produced using the ammonia-alkaline method, generating a significant amount of industrial waste called soda residue. In China, the annual production of soda residue exceeds 10 million tons. The large-scale open-air storage of soda residue not only occupies land but also causes severe pollution to the surrounding environment. Soda residue displays characteristics such as strong alkalinity, high reactivity, and a well-developed pore structure, making it a valuable raw material for producing environmentally functional materials. This article provided an overview and summary of soda residue, including its sources and hazards, basic properties, applications in environmental management (wastewater treatment, flue gas desulfurization, and soil remediation), and associated risks. The limitations of using soda residue in "waste to waste" technologies were also analyzed. Based on this analysis, the article suggests focusing on simultaneous removal of heavy metal ions using soda residue, safely disposing of and acquiring resources from metal-laden sludge, efficiently dechlorinating soda residue, using soda residue for contaminated soil solidification, stabilization, and assisted remediation, controlling pollution via green and circular utilization approaches, and assessing long-term risk.

Solidification/stabilization of soil heavy metals by alkaline industrial wastes: A critical review

Solidification/stabilization technology is one of the most desirable technologies for the remediation of heavy metal contaminated soils due to its convenience and effectiveness. The annual production of alkaline industrial wastes in China is in the hundreds of millions of tons. Alkaline industrial wastes have the potential to replace conventional stabilizers because of their cost effectiveness and performance in stabilizing heavy metals in soils. This paper systematically summarizes the use of four alkaline industrial wastes (soda residue, steel slag, carbide slag, and red mud) for the solidification/stabilization of heavy metal contaminated soils and provides a comprehensive analysis of the three mechanisms of action (hydration, precipitation, and adsorption) and factors that influence the process. In addition, the environmental risks associated with the use of alkaline industrial wastes are highlighted. We found that soda residues, steel slag and carbide slag are appropriate for solidification/stabilization of Pb, Cd, Zn and Cu, while red mud is a potential passivation agent for the stabilization of As in soils. However, implementation of remediation methods using alkaline industrial wastes has been limited because the long-term effectiveness, synergistic effects, and usage in soils containing multiple heavy metals have not been thoroughly studied. This review provides the latest knowledge on the mechanisms, risks, and challenges of using alkaline industrial wastes for solidification/stabilization of heavy metal contaminated soils.

Utilization of Calcium Carbide Residue as Solid Alkali for Preparing Fly Ash-Based Geopolymers: Dependence of Compressive Strength and Microstructure on Calcium Carbide Residue, Water Content and Curing Temperature

Calcium carbide residue (CCR) is a solid waste resulting from acetylene gas production. In this study, CCR was used as an alkali activator to prepare fly ash (FA)-based geopolymers without any alkali supplementation. We studied the factors (FA/CCR ratio, curing temperature, and water/binder ratio) influencing the mechanical property of FA/CCR-based geopolymers. The compressive strength results showed that, by optimizing these three factors, the FA/CCR mixture has great potential for use as a cementitious material and geopolymer with a dense microstructure having a maximal compressive strength of 17.5 MPa. The geopolymers' chemical structure, microstructure, and chemical composition were characterized and determined by a combination of techniques. All these results revealed that amorphous C-(A)-S-H (calcium (aluminate) silicate hydrate) gels mainly formed after geopolymerization resulting from the reaction of FA and CCR. In addition, some crystallines, such as ettringite and monosulfate, were also formed. Further, geopolymers prepared with a suitable FA/CCR ratio (1:1 or 1:2) possessed a compact microstructure because of their sufficient reactive SiO₂ and Al₂O₃ and high-enough alkalinity, responsible for higher content of C-(A)-S-H formation and better mechanical property. Too high curing temperature or water content induced the formation of a loosely bound geopolymer matrix that strongly weakens its mechanical property.

Environmental fungi and bacteria facilitate lecithin decomposition and the transformation of phosphorus to apatite

Organophosphorus compounds (OP) are stable P source in nature, and can increase eutrophication risk in waterbodies. Lecithin was the most difficult OP to be broken down. In this study, two typical phosphate-solubilizing microorganisms, Aspergillus niger and Acinetobacter sp., were applied to evaluate their ability to decompose both inorganic phosphates and lecithin. A. niger and Acinetobacter sp. could solubilize calcium phosphates by secreting various organic acids, e.g., oxalic and formic acids. The fungus, A. niger, shows significantly higher ability of solubilizing these inorganic phosphates than Acinetobacter sp., primarily due to its secretion of abundant oxalic acid. However, the bacterium, Acinetobacter sp., could secrete more acid phosphatase than A. niger for lecithin decomposition, i.e., 9300 vs. 8500 μmol L-1 h-1. Moreover, after addition of CaCl2, the released P from lecithin was transformed to stable chlorapatite in the medium. To the contrast, Ca cations inclined to form calcium oxalate (rather than stable phosphate mineral) after the incubation of A. niger, as it induced relatively acidic environment after breaking down lecithin. Therefore, this work sheds light on the bright future of applying bacteria and Ca cations in OP pollutant management.

Recovery of phosphate from aqueous solutions using calcined eggshell as an eco-friendly adsorbent

Phosphorus scarcity has become a significant issue in the European Union (EU) during 21st Century, due to its relevance as an irreplaceable macronutrient for life, and because of the total dependency of EU regarding imports. This work aims to evaluate the phosphorus recovery by adsorption in batch and fixed-bed column, using a thermally modified eggshell as an adsorbent. The screening phase revealed that calcined eggshell at 700 °C (CES700) is the most suitable material compared with the other thermally modified eggshells tested. Thus, CES700 was characterized regarding the specific surface area, pore volume, zero-point charge pH, total dissolved solids and organic matter. The influence of pH and adsorbent dosage was investigated in batch conditions. Langmuir-Freundlich model described the equilibrium data and the maximum adsorption capacity was about 39 mg P-PO₄/g. The kinetics follows a pseudo-first order model, with constants between 0.063 and 0.224 min^-1. Fixed-bed studies indicated that increasing fluid superficial velocity and feed concentration led to an early saturation of the adsorbent. Yoon-Nelson, Thomas and Bohard-Adams empirical models properly adjusted the breakthrough curves with R² ≥ 0.98. Germination tests using CES700 loaded with phosphate revealed a germination index of 120 and 124% to 48 and 72 h, respectively. CES700 is statically better than the other tested materials, which opens the possibility of its use as fertilizer. This study showed that the developed material, CES700, can be applied in batch or fixed-bed processes to recover phosphate ions from liquid effluents, and the loaded adsorbent has potential to be further used as fertilizer.

Investigation on mechanism of phosphate removal on carbonized sludge adsorbent

For the removal of phosphate (PO₄^3-) from water, an adsorbent was prepared via carbonization of sewage sludge from a wastewater treatment plant: carbonized sludge adsorbent (CSA). The mechanism of phosphate removal was determined after studying the structure and chemical properties of the CSA and its influence on phosphate removal. The results demonstrate that phosphate adsorption by the CSA can be fitted with the pseudo second-order kinetics and Langmuir isotherm models, indicating that the adsorption is single molecular layer adsorption dominated by chemical reaction. The active sites binding phosphate on the surface are composed of mineral particles containing Si/Ca/Al/Fe. The mineral containing Ca, calcite, is the main factor responsible for phosphate removal. The phosphate removal mechanism is a complex process including crystallization via the interaction between Ca²⁺ and PO₄^3-; formation of precipitates of Ca²⁺, Al³⁺, and PO₄^3-; and adsorption of PO₄^3- on some recalcitrant oxides composed of Si/Al/Fe.

Utilization of phosphorus loaded alkaline residue to immobilize lead in a shooting range soil

The alkaline residue generated from the production of soda ash using the ammonia-soda method has been successfully used in removing phosphorus (P) from aqueous solution. But the accumulation of P-containing solid after P removal is an undesirable menace to the environment. To achieve the goal of recycling, this study explored the feasibility of reusing the P loaded alkaline residue as an amendment for immobilization of lead (Pb) in a shooting range soil. The main crystalline phase and micromorphology of amendments were determined using X-ray diffraction (XRD) and scanning electron microscopy-electron dispersion spectroscopy (SEM-EDS) methods. The toxicity characteristic leaching procedure (TCLP), sequential extraction procedure, and physiologically based extraction test (PBET) were employed to evaluate the effectiveness of Pb immobilization in soil after 45 d incubation. Treatment with P loaded alkaline residue was significantly effective in reducing the TCLP and PBET extractable Pb concentrations in contrast to the untreated soil. Moreover, a positive change in the distribution of Pb fractions was observed in the treated soil, i.e., more than 60% of soil-Pb was transformed to the residual fraction compared to the original soil. On the other hand, P loaded amendments also resulted in a drastic reduction in phytoavailable Pb to the winter wheat and a mild release of P as a nutrient in treated soil, which also confirmed the improvement of soil quality.

Remediation of aquaculture water in the estuarine wetlands using coal cinder-zeolite balls/reed wetland combination strategy

In this paper, the modified coal cinders and zeolite powders in proportion of 2:1 were mixed with modified polyvinyl alcohol (PVA) with a ratio of 20:1 (w/v) to make a new sorbent and biological carrier-the coal cinder-zeolite balls (CCZBs). The maximum absorption capacities of ammonia nitrogen and Chemical Oxygen Demand (CODCr) on CCZBs, adsorption process were evaluated in batch experiments. And then they were combined with reed wetland for bioremediation of micro-polluted aquaculture water in estuarine wetlands. The results showed that the removal efficiencies of ammonia nitrogen and CODCr improved with the decrease in water inflow and increase in inflow concentrations. Efficiencies of 67.3% and 71.3% for ammonia nitrogen and CODCr under water flow of 10 L/h were obtained when their inflow concentrations were 1.77 and 56.0 mg/L respectively. This strategy can be served as a model system for bioremediation in situ of aquaculture water and other organic polluted or eutrophic water.

Phosphate recovery from anaerobic digester effluents using CaMg(OH)4

Dolomite lime (DL) (CaMg(OH)4) was used as an economical source of Mg(2+) for the removal and recovery of phosphate from an anaerobic digester effluent of a municipal wastewater treatment plant (MWWTP) wastewater. Batch precipitation results determined that phosphate was effectively reduced from 87 to less than 4mg-P/L when the effluent water was mixed with 0.3g/L of DL. The competitive precipitation mechanisms of different solids in the treatment system consisting of Ca(2+)-Mg(2+)-NH4(+)-PO4(3-)CO3(2-) were determined by comparing model predictions with experimental results. Thermodynamic model calculations indicated that hydroxyapatite (Ca10(PO4)6(OH)2), Ca4H(PO4)3∙3H2O, Ca3(PO4)2(beta), and Ca3(PO4)2(am2) were more stable than struvite (MgNH4PO3∙6H2O) and calcite (CaCO3). However, X-ray diffraction (XRD) analysis determined the formation of struvite and calcite minerals in the treated effluent. Kinetic experimental results showed that most of the phosphate was removed from synthetic effluent containing NH4(+) within 2hr, while only 20% of the PO4(3-) was removed in the absence of NH4(+) after 24hr of treatment. The formation of struvite in the DL-treated effluent was due to the rapid precipitation rate of the mineral. The final pH of the DL-treated effluent significantly influenced the mass ratio of struvite to calcite in the precipitates. Because more calcite was formed when the pH increased from 8.4 to 9.6, a pH range of 8.0-8.5 should be used to produce solid with high PO4(3-) content. This study demonstrated that DL could be used for effective removal of phosphate from the effluent and that resultant precipitates contained high content of phosphate and ammonium.

Adsorption of phosphate on surface of magnetic reed: characteristics, kinetic, isotherm, desorption, competitive and mechanistic studies

A magnetic biocomposite was prepared by Fe₃O₄in situ co-precipitation and amine functionalization processes by using virgin reed as starting material.

A fixed-bed column for phosphate removal from aqueous solutions using an andosol-bagasse mixture

It is difficult to eliminate phosphate from large volumes of water in batch mode using an adsorbent such as andosol. In a fixed-bed column, andosol has a very low permeability. In this study, andosol was mixed with bagasse to increase permeability. The mixture was then applied for the adsorption of phosphate in a fixed-bed column. Optimum and stable permeability was obtained with a 50/50 mixture of andosol and bagasse. The maximum adsorption capacity obtained was 4.18 mg/g for a column with a bed depth of 1.8 cm and a flow rate of 4 mL/min. The experimental data fit best to Thomas and Adam-Bohart models. These experimental results were applied in the treatment of natural phosphate-containing water from Yaoundé Municipal Lake in Cameroon. Column performance increased by 60% due to the presence of Ca(2+) and Mg(2+) in the natural water. These cations form complexes with phosphate at the andosol surface. The standard enthalpy 15.964 kj/mol indicated that phosphate adsorption on andosol-bagasse mixture was an endothermic process. Kinetic experiments demonstrated that phosphate adsorption fitted better with a pseudo-second-order model.