Preparation of functional geopolymers from municipal solid waste incineration fly ash: An approach combining experimental and computational simulation studies.

This study aims to evaluate the feasibility and safety of using municipal solid waste incineration fly ash (MSW-IFA) in the development of geopolymer-based solidification/stabilization (S/S) treatments. Geopolymers have garnered attention as a sustainable alternative to traditional cement, owing to their high strength, stability, and minimal CO2 emissions. In this study, a combination of experimental and simulation calculations was used to investigate the setting time, mechanical properties, environmental risks, hydration mechanisms and processes of municipal solid waste incineration fly ash-based polymeric functional cementitious materials (GFCM). The results demonstrate that the mechanical properties of GFCM are related to the changes in the mineral phases and the degree of compactness. Quantum chemical calculations indicate that the hydration products may be [Si(OH)4], [Al(OH)3(OH2)] and [Al(OH)4]-. It is possible that the heavy metals are embedded in the hydrated silica-aluminate by electrostatic interaction or chemisorption. Heavy metals may be embedded in hydrated silica-aluminate by electrostatic action or chemisorption. This study provides a feasible method for resource utilization and heavy metal stabilization mechanism of MSW-IFA.

Understanding the effects of A-site Ag-doping on LaCoO3 perovskite for NO oxidation: Structural and magnetic properties.

The partial substitution of A-site in perovskites is a major strategy to enhance the catalytic oxidation activity. This study explores the use of silver (Ag) to partially replace the lanthanum (La) ion at the A-site in LaCoO3 perovskite, investigating the role of Ag in the ABO3 perovskite structure, elucidating the nitric oxide (NO) oxidation mechanism over La1-xAgxCoO3 (x = 0.1-0.5) perovskites. La0.7Ag0.3CoO3 with an Ag-doping amount of 0.3, exhibited the highest NO oxidation activity of 88.5% at 275 °C. Characterization results indicated that Ag substitution enhanced the perovskite, maintaining its original phase structure, existing in the form of a mixture of Ag0 and Ag+ in the La1-xAgxCoO3 (x = 0.1-0.5) perovskites. Notably, Ag substitution improved the specific surface area, reduction performance, Co3+, and surface adsorption oxygen content. Additionally, the study investigated the relationship between magnetism and NO oxidation from a magnetism perspective. Ag-doping strengthened the magnetism of La-Ag perovskite, resulting in stronger adsorption of paramagnetic NO. This study elucidated the NO oxidation mechanism over La-Ag perovskite, considering structural and magnetic properties, providing valuable insights for the subsequent development and industrial application of high oxidation ability perovskite catalysts.

Behavior of heavy metals in municipal sludge during dewatering: The role of conditioners and extracellular polymeric substances.

Heavy metals, the main harmful substances in the sludge, are easily enriched, have adverse effects on the treatment and disposal of the sludge. In this study, two conditioners (modified corn-core powder, MCCP, and sludge-based biochar, SBB) were separately added and jointly added into municipal sludge to enhance sludge dewaterability. Meanwhile, diverse organics, such as extracellular polymeric substances (EPS), were released under pretreatment. The different organics had different effects on each heavy metal fraction and changed the toxicity and bioavailability of the treated sludge. The exchangeable fraction (F4) and carbonate fraction (F5) of heavy metal were nontoxic and nonbioavailable. When MCCP/SBB was used to pretreat the sludge, the ratio of metal-F4 and -F5 decreased, indicating that MCCP/SBB reduced the biological availability and ecological toxicity of the heavy metals in the sludge. These results were consistent with the calculation of the modified potential ecological risk index (MRI). To understand the detailed function of organics in the sludge network, the relationship between EPS, the secondary structure of the protein, and heavy metals was analyzed. The analyses revealed that the increasing proportion of ß-sheet in soluble EPS (S-EPS) generated more active sites in the sludge system, which enhanced the chelate or complex function among organics and heavy metals, thus reducing the migration risks.

Combined application of modified corn-core powder and sludge-based biochar for sewage sludge pretreatment: Dewatering performance and dissipative particle dynamics simulation.

Sludge is an inevitable by-product of municipal wastewater treatment processes, and its high moisture content poses a major challenge for its subsequent treatment and disposal. Previous studies have explored the effects of applying modified corn-core powder (MCCP) on dewatering sludge. Here, we characterized the effects of applying both MCCP and sludge-based biochar (SBB) on dewatering sludge. Analysis of the anti-shear ability of SBB revealed that SBB was a skeleton builder with high compressive strength, demonstrating that SBB could maintain the permeability of sludge under high-pressure filtration processes and facilitate the flow of bound water. Dissipative particle dynamics (DPD) was used to simulated the sludge flocculating process and verify the feasibility of the experiment. As the simulation progressed, the reaction in the sludge network reached equilibrium and the simulated structure of the sludge became loose. The dewatering performance and physicochemical properties of the treated sludge were studied to further characterize the effect of this combined technology. Compared with MCCP-sludge, MCCP&SBB-sludge, which was treated by 20% DS (mass of dry solids in sludge) of SBB and 20% DS of MCCP, achieved superior dewaterability. This combined method reduced the specific resistance of filtration by 76% and enlarged the net sludge solids yield by 138%. Further study of the properties of MCCP&SBB-sludge revealed a loose structure that resembled the structure recovered by the simulation, suggesting that the DPD simulation method simulated the sludge flocculating process successfully. Therefore, the combined application of MCCP and SBB was superior for sludge dewatering because of the synergistic effects of MCCP and SBB.

Inorganic flocculant for sludge treatment: Characterization, sludge properties, interaction mechanisms and heavy metals variations.

As an industrial waste, phosphogypsum was modified to produce flocculant for sludge dewatering. In this paper, characteristics of flocculant, properties of treated sludge, and interactions of sludge and flocculant were investigated. Results suggested that after modification, flocculant showed a positive electrical property and a porous structure. Besides, larger sludge flocs formed in treated sludge showed a higher settleability and filterability. Flocculant could narrow sludge colloid network by compressing its Electrical double-layer due to the presence of CaSO4. With potential change, the electronegative colloidal network cracked quickly and released sludge particles, active groups, unstable heavy metals and 82.91% of bound water. Moreover, porous adsorption between sludge particles and flocculant was found under molecular electrostatic potential and Van Der Waals force caused by flocculant addition. After modification, shear modulus of CaSO4, SiO2 and Al2O3 in modified phosphogypsum increased by 21%, 23% and 17%, respectively. This provided a strong skeleton support for sludge particles, which is significant to sludge dewatering. Particularly, through chelation, adsorption and rolling-sweeping process, risk level of unstable heavy metals excepting Cu in sludge filter cake was largely weakened. Immobilized rate of risky heavy metals was 23.96% (CdF1/F2), 39.92% (CrF1), 11.11% (PbF1/F2), 21.21% (ZnF1), 35.49% (NiF1/F2), and 78.61% (AsF1/F2), respectively. Therefore, this study provided significant insight for developing efficient method to promote bound water removal from sludge, and to stabilize risky heavy metals in sludge.

Role of extracellular polymeric substances in sludge dewatering under modified corn-core powder and sludge-based biochar pretreatments.

Extracellular polymeric substances (EPS) which wrapped on sludge particles were deemed to hinder the outflowing of combined water in sludge system. The complex composition of EPS was the bottleneck for revealing its relationship with sludge dewaterability. In this study, a combined modified corn-core powder (MCCP) and sludge-based biochar (SBB) condition was executed to treat sludge for enhancing dehydration performance, and the concentration and the form distribution of organics in EPS, the variances of protein secondary structures were investigated. Correlation between the sludge dewaterability and EPS components were performed, found strong correlations among the net sludge solids yield (YN) and the specific resistance of filtration (SRF) (R = -0.923), Zeta potential (R = -0.971). Furthermore, the relationship between the secondary structures of protein and dehydration performance were strong related. With the optimal dosage of SBB and MCCP, aggregated strands and α -helix were released, indicated that the unfolding and despiralization in soluble EPS (S-EPS) were improved, disordered the sludge network, reduced the flowing resistance of bound water, finally enhancing sludge dewaterability.

Chemical behavior of fluorine and phosphorus in chemical looping gasification using phosphogypsum as an oxygen carrier.

In China, the amount of phosphogypsum (PG) has exceeded 250 million tons with more than 55 million tons of growth rates each year. As the micro constituent, fluorine and phosphorus restrict the resourceful disposal of PG. This paper focused on chemical looping gasification (CLG) which used PG as an oxygen carrier, systematically investigated the gasification performance and chemical behavior of fluorine and phosphorus contained in PG during CLG process. Main conclusions are as follows. The main pollutant of chemical looping gasification process was HF, which was transformed from NaF. Phosphorus transformed from water-soluble phosphorus (Ca(H2PO4)2, Ca(HPO4)) into insoluble Ca3(PO4)2.20 reducing-oxidizing cycles were investigated, and a less and less fluorine content in oxygen carrier was found because its phase transformation from solid NaF to gaseous HF, and the phosphorus content in oxygen carrier changed slightly under the current conditions. The Ca3(PO4)2 particle layers existed in both the middle of the reduced solid particles and the middle of the cycled oxygen carrier particles, confirmed to actually act as a glue between the particles. Furthermore, transformation routes of fluorine and phosphorus during the CLG process were discussed and the generation of syngas in CLG process needed to be purified.

Dewatering performance of sewage sludge under pretreatment with modified corn-core powder.

The water retained inside the sludge flocs is the bottleneck for the dehydration of sewage sludge, which hindered the following treatment. In this study, corn core powder was modified (MCCP) using sodium hydroxide (NaOH) and cationic surfactant cetyltrimethylammonium bromide (CTMAB) to break the structure of extracellular polymeric substances (EPS) and cell membrane for enhancing the dewaterability of sewage sludge. The dewatering performance, the properties of the treated sludge, the composition and distribution of EPS were investigated to discuss the dewatering mechanism of sludge. Adding 20% DS (mass of dry solids in sludge) of MCCP reduced the moisture content (MC) and the specific resistance of filtration (SRF) of the sludge by 40% and 55%, respectively. Then, correlation analyses were performed between dewatering indices of sludge and sludge properties. A multiple linear regression model was established which indicated the relationship between MC and the key factors reflecting sludge dewaterability, demonstrating that larger particle size of sludge flocs and more total dissolved solids in filtrate may be propitious to reduce content of bound water in sludge.

Research on the variations of organics and heavy metals in municipal sludge with additive acetic acid and modified phosphogypsum.

Concentration and fraction distribution of organics and heavy metals in municipal sludge treated by modified phosphogypsum and acetic acid (signed as MPG/HAC) were studied. The results showed that MPG/HAC conditioning significantly produce synergistic enhancement effect to dissolution of unstable heavy metals wrapped in the stable colloid network. Simultaneously, after conditioning, about 45.16% of organics such as proteins, polysaccharides and humic acid in supernatant was degraded, thus dissociating large amount of active group which accelerated immobilization of dissolved heavy metals and weaken its toxicity. In addition, MPG with a porous structure could adsorb unstable heavy metals and transform them into residual fraction, leading to a considerable decrease in their mobility risk level. Besides, linear regression models showed that a strong oxidizability of sludge, and destruction of colloidal network could greatly promote dissolution of unstable heavy metals. Simultaneously, sludge oxidizability and organics degradation rate, and disintegration of extracellular polymeric substances (EPS) layer highly accelerate immobilization of unstable metals. Excepting Cd, environmental risk of Cr, Cu, Pb, Zn, Ni and As can be effectively weakened after conditioning. Additionally, MPG/HAC conditioning might be appropriate for stabilization of Cd, Cr and Zn in water supply sludge, especially for Zn.

Research on dewaterability and properties of sewage sludge under modified phosphogypsum and acetic acid pretreatments.

A combined modified phosphogypsum (MPG) and acetic acid (HAC) addition was performed to pretreat sewage sludge for better dewaterability, and dewatering performance, sludge flocs morphology, properties of treated sludge, composition and morphology distribution of extracellular polymeric substances (EPS), bound water release dynamic were investigated. Results suggested that combined addition could highly improve sludge dewatering compared to single addition. Furthermore, after pretreatment, stable sludge network was disintegrated and massive loosely bound EPS (LB-EPS) were destroyed into supernatant, thus increasing amount of organics with molecular weight between 1000 and 7000 Da in soluble EPS (SB-EPS). Simultaneously, about 9.5 kg kg-1 DS (Dry solids) of bound water was released from sludge network.

Investigation on extracellular polymeric substances, sludge flocs morphology, bound water release and dewatering performance of sewage sludge under pretreatment with modified phosphogypsum.

Modified phosphogypsum (MPG) was developed to improve dewaterability of sewage sludge, and dewatering performance, properties of treated sludge, composition and morphology distribution of EPS, dynamic analysis and multiple regression model on bound water release were investigated. The results showed that addition of MPG caused extracellular polymeric substances (EPS) disintegration through charge neutralization. Destruction of EPS promoted the formation of larger sludge flocs and the release of bound water into supernatant. Simultaneously, content of organics with molecular weight between 1000 and 7000 Da in soluble EPS (SB-EPS) increased with increasing of EPS dissolved into the liquid phase. Besides, about 8.8 kg•kg-1DS of bound water was released after pretreatment with 40%DS MPG dosage. Additionally, a multiple linear regression model for bound water release was established, showing that lower loosely bond EPS (LB-EPS) content and specific resistance of filtration (SRF) may improve dehydration performance, and larger sludge flocs may be beneficial for sludge dewatering.

Decomposing properties of phosphogypsum with iron addition under two-step cycle multi-atmosphere control in fluidised bed.

Phosphogypsum is a solid industry by-product generated when sulphuric acid is used to process phosphate ore into fertiliser. Phosphogypsum stacks without pretreatment are often piled on the land surface or dumped in the sea, causing significant environmental damage. This study examined the reaction characteristics of phosphogypsum, when decomposed in a multi-atmosphere fluidised bed. Phosphogypsum was first dried, sieved and mixed proportionally with lignite at the mass ratio of 10:1, it was then immersed in 0.8 [Formula: see text] with a solid-liquid ratio of 8:25. The study included a two-step cycle of multi-atmosphere control. First, a reducing atmosphere was provided to allow phosphogypsum decomposition through partial lignite combustion. After the reduction stage reaction was completed, the reducing atmosphere was changed into an air-support oxidising atmosphere at the constant temperature. Each atmosphere cycle had a conversion time of 30 min to ensure a sufficient reaction. The decomposing properties of phosphogypsum were obtained in different atmosphere cycles, at different reaction temperatures, different heating rates and different fluidised gas velocities, using experimental results combined with a theoretical analysis using FactSage 7.0 Reaction module. The study revealed that the optimum reaction condition was to circulate the atmosphere twice at a temperature of 1100 °C. The heating rate above 800 °C was 5 [Formula: see text], and the fluidised gas velocity was 0.40 [Formula: see text]. The procedure proposed in this article can serve as a phosphogypsum decomposition solution, and can support the future management of this by-product, resulting in more sustainable production.