Using incinerated sewage sludge ash as a high-performance adsorbent for lead removal from aqueous solutions: Performances and mechanisms

Recycling of sludge residue as a coagulant for phosphorus removal from aqueous solutions

Phosphorus pollution poses a significant challenge in addressing water contamination. The coagulant is one of the effective methods to remove phosphorus from wastewater. Abundant Al and Fe oxides in sludge residue make it have great potential to synthesize water treatment coagulants. However, the utilization of sludge residue for preparation of coagulant was seldom investigated. In this study, we fabricated a novel coagulant, polyaluminum ferric chloride (SM-PAC), using sludge residue as a raw material through acid leaching and polymerization processes. Characterization results confirm that the parameters of SM-PAC meet the specifications outlined in the national standard (GB/T 22627-2022). We investigated the effects of pH, dosage, initial phosphorus concentration, and contact time on the removal efficiency of SM-PAC. As anticipated, the prepared SM-PAC exhibited a significant efficacy in removing phosphorus, meeting the discharge standards set for municipal sewage. Furthermore, the adsorption kinetics analysis suggests that the predominant mode of phosphorus adsorption on SM-PAC is chemical adsorption. Furthermore, the SM-PAC was employed in the actual wastewater treatment plant and exhibited excellent efficiency in phosphorus removal. The utilization of SM-PAC can not only effectively address the issue of sludge disposal but also achieve the goal of "treating waste with waste." It is expected that the proposed method of reusing sludge residue as a resource can provide a sustainable way to synthesize a coagulant for phosphorus removal.

Adsorption Efficiency of Cadmium (II) by Different Alkali-Activated Materials

The objective of this study was to demonstrate the potential utilization of fly ash (FA), wood ash (WA), and metakaolin (MK) in developing new alkali-activated materials (AAMs) for the removal of cadmium ions from waste water. The synthesis of AAMs involved the dissolution of solid precursors, FA, WA, and MK, by a liquid activator (Na2SiO3 and NaOH). In concentrated solutions of the activator, the formation of an aluminosilicate gel structure occurred. DRIFT spectroscopy of the AAMs indicated main vibration bands between 1036 cm-1 and 996 cm-1, corresponding to Si-O-Si/Si-O-Al bands. Shifting vibration bands were seen at 1028 cm-1 to 1021 cm-1, indicating that the Si-O-Si/Si-O-Al bond is elongating, and the bond angle is decreasing. Based on the X-ray diffraction results, alkali-activated samples consist of an amorphous phase and residual mineral phases. The characteristic "hump" of an amorphous phase in the range from 20 to 40° 2θ was observed in FA and in all AWAFA samples. By the XRD patterns of the AAMs obtained by the activation of a solid three-component system, a new crystalline phase, gehlenite, was identified. The efficiency of AAMs in removing cadmium ions from aqueous solutions was tested under various conditions. The highest values of adsorption capacity, 64.76 mg/g (AWAFA6), 67.02 mg/g (AWAFAMK6), and 72.84 mg/g mg/g (AWAMK6), were obtained for materials activated with a 6 M NaOH solution in the alkali activator. The Langmuir adsorption isotherm and pseudo-second kinetic order provided the best fit for all investigated AAMs.

Recycling of waste glass and incinerated sewage sludge ash in glass-ceramics

Disposal of waste glass and incinerated sewage sludge ash (ISSA) in landfills is a waste of resources and poses significant environmental risks. This work aims to recycle waste glass and ISSA together to form value-added glass-ceramics. The physical and mechanical properties, leaching behaviour, and microstructure of the glass-ceramics produced with different proportions of waste glass powder (WGP) and ISSA were investigated. Thermodynamic calculations were performed to predict the formation of crystalline phases and the phase transformation involved. The results showed the potential of WGP and ISSA as raw materials in glass-ceramics production. WGP effectively densified the microstructure of the glass-ceramics by forming a viscous phase. As WGP content increased, the total porosity of glass-ceramics decreased whereas the density increased, accompanied by the formed anorthite transforming into wollastonite. The incorporation of WGP densified and refined the pore structure of the glass-ceramics, thereby improving the mechanical properties and reducing the water absorption. The glass-ceramics produced with a 50:50 blend of WGP and ISSA exhibited the highest compressive strength of 43.7 MPa and the lowest water absorption of 0.3 %. All fabricated glass-ceramics exhibited innocuous heavy metal leaching. The co-sintering of ISSA and WGP can produce additive-free glass-ceramics, characterized by reduced energy consumption and notable heavy metal immobilization capacity. These materials hold promise for utilization in construction as building materials.

Sustainable and efficient technologies for removal and recovery of toxic and valuable metals from wastewater: Recent progress, challenges, and future perspectives

Due to their numerous effects on human health and the natural environment, water contamination with heavy metals and metalloids, caused by their extensive use in various technologies and industrial applications, continues to be a huge ecological issue that needs to be urgently tackled. Additionally, within the circular economy management framework, the recovery and recycling of metals-based waste as high value-added products (VAPs) is of great interest, owing to their high cost and the continuous depletion of their reserves and natural sources. This paper reviews the state-of-the-art technologies developed for the removal and recovery of metal pollutants from wastewater by providing an in-depth understanding of their remediation mechanisms, while analyzing and critically discussing the recent key advances regarding these treatment methods, their practical implementation and integration, as well as evaluating their advantages and remaining limitations. Herein, various treatment techniques are covered, including adsorption, reduction/oxidation, ion exchange, membrane separation technologies, solvents extraction, chemical precipitation/co-precipitation, coagulation-flocculation, flotation, and bioremediation. A particular emphasis is placed on full recovery of the captured metal pollutants in various reusable forms as metal-based VAPs, mainly as solid precipitates, which is a powerful tool that offers substantial enhancement of the remediation processes' sustainability and cost-effectiveness. At the end, we have identified some prospective research directions for future work on this topic, while presenting some recommendations that can promote sustainability and economic feasibility of the existing treatment technologies.

Sewage sludge ash-incorporated stabilisation/solidification for recycling and remediation of marine sediments

Finding suitable disposal sites for dredged marine sediments and incinerated sewage sludge ash (ISSA) is a challenge. Stabilisation/solidification (S/S) has become an increasingly popular remediation technology. This study sheds light on the possible beneficial use of ISSA together with traditional binders to stabilise/solidify marine sediments. The performance of the binders on S/S of sediment 1 (clean) and sediment 2 (contaminated) was also compared. The results showed that the use of ISSA as part of the binder was effective in promoting the strength of the sediment with a high initial moisture content due to ISSA porous and high water absorption characteristics. The sediments treated with 10% cement and 20% ISSA attained the highest strength. Also, cement hydration as well as pozzolanic reactions between ISSA and Ca(OH)₂ made contributions to the strength development. This was supported by the microstructural analysis, in particular the porosity results. In terms of environmental impacts, two leaching tests (toxicity characteristic leaching procedure and synthetic precipitation leaching procedure) found that all the S/S treated sediment by 10% lime and 20% ISSA resulted in the lowest leachate concentrations under the on-site reuse scenario or under simulative acidic rainfall conditions. Therefore, recycling waste ISSA with lime can be used as an appealing binder to replace cement to stabilise/solidify dredged marine sediments for producing fill materials.

Novel recycling of phosphorus-recovered incinerated sewage sludge ash residues by co-pyrolysis with lignin for reductive/sorptive removal of hexavalent chromium from aqueous solutions

Incinerated sewage sludge ash (ISSA), a by-product generated from the combustion of dewatered sewage sludge, has been extensively studied as a secondary resource for phosphorus recovery by acid extraction methods. Recycling of the P-recovered ISSA residues is crucial to complete and sustain the whole process. In this study, the ISSA residue rich in iron was reused and co-pyrolyzed with lignin at 650, 850 and 1050 °C under N₂ atmosphere for the synthesis of a composite material to remove hexavalent chromium (Cr(VI)) from aqueous solutions. Characterization analysis including XRD, XPS, and FTIR showed that iron oxides in the residue were reduced to zero valent iron at 1050 °C that exhibits the optimal Cr(VI) removal performance. The Cr(VI) removal process was rapid and reached a plateau at around 30 min. The maximum removal rate was obtained at pH 2.0, which was conducive for the treatment of a synthetic Cr(VI)-containing wastewater in fix-bed column experiments, whereby Cr(VI) as well as total Cr were continuously removed. Overall, this study proposed a new routine for the recycling of ISSA residue after phosphorus recovery by the acid extraction method and provided a value-added product for Cr(VI) removal from wastewaters.

Immobilization of high-Pb contaminated soil by oxalic acid activated incinerated sewage sludge ash

Identifying effective and low-cost agents for the remediation of Pb-contaminated soil is of great importance for field-scale applications. In this study, the feasibility of reusing incinerated sewage sludge ash (ISSA), a waste rich in phosphorus, under activation by oxalic acid (OA) for the remediation of high-Pb contaminated soil was investigated. ISSA and OA were mixed at different proportions for the treatment of the high-Pb contaminated soil (5000 mg/kg). The Pb immobilization efficacy was further examined by both the standard deionized water leaching test and the toxicity characteristic leaching procedure (TCLP). The overall results showed that the use of the ISSA alone and an appropriate mixture of the ISSA and OA could effectively reduce the leachability of Pb from soil. 20% ISSA together with 30% OA (0.2 mol/L) reduced leached Pb concentration by 99%. The main stabilization mechanisms were then explored by different microstructural and spectroscopic analytical techniques including SEM, XRD and FTIR. Apparently, OA released phosphate from the ISSA and Pb from soil via acid attack, which combined and precipitated as stable lead phosphate minerals. However, excessive OA could cause high leaching of phosphate and zinc from the ISSA. Overall, this study indicates that ISSA could be used together with OA to remediate high-Pb contaminated soil, but careful design of mix proportions is necessary before practical application to avoid excessive leaching of phosphate and zinc from the ISSA.

Pyrolysis of Solid Digestate from Sewage Sludge and Lignocellulosic Biomass: Kinetic and Thermodynamic Analysis, Characterization of Biochar

This study investigates the pyrolysis behavior and reaction kinetics of two different types of solid digestates from: (i) sewage sludge and (ii) a mixture of sewage sludge and lignocellulosic biomass—Typha latifolia plant. Thermogravimetric data in the temperature range 25–800 °C were analyzed using Flynn–Wall–Ozawa and Kissinger–Akahira–Sunose kinetic methods, and the thermodynamic parameters (ΔH, ΔG, and ΔS) were also determined. Biochars were characterized using different chemical methods (FTIR, SEM–EDS, XRD, heavy metal, and nutrient analysis) and tested as soil enhancers using a germination test. Finally, their potential for biosorption of NH4+, PO43−, Cu2+, and Cd2+ ions was studied. Kinetic and thermodynamic parameters revealed a complex degradation mechanism of digestates, as they showed higher activation energies than undigested materials. Values for sewage sludge digestate were between 57 and 351 kJ/mol, and for digestate composed of sewage sludge and T. latifolia between 62 and 401 kJ/mol. Characterizations of biochars revealed high nutrient content and promising potential for further use. The advantage of biochar obtained from a digestate mixture of sewage sludge and lignocellulosic biomass is the lower content of heavy metals. Biosorption tests showed low biosorption capacity of digestate-derived biochars and their modifications for NH4+ and PO43− ions, but high biosorption capacity for Cu2+ and Cd2+ ions. Modification with KOH was more efficient than modification with HCl. The digestate-derived biochars exhibited excellent performance in germination tests, especially at concentrations between 6 and 10 wt.%.

Novel recycling of incinerated sewage sludge ash (ISSA) and waste bentonite as ceramsite for Pb-containing wastewater treatment: Performance and mechanism

With rapid economic growth and urbanisation, the reuse and recycling of solid wastes has become a high priority for the sustainable development of modern cities. In this study, two typical solid wastes, incinerated sewage sludge ash (ISSA) and waste bentonite, were co-valorised to produce granular adsorbents through a simple and energy-saving pelletisation/sintering process. A mixture of ISSA and bentonite at a weight ratio of 3:1 was pelletised and sintered at 700 °C. The resultant ceramsite, with good mechanical strength, could effectively remove Pb(Ⅱ) from aqueous solutions. The adsorption kinetics can be described by the pseudo-first-order (PFO) model. The results indicated that the Pb(Ⅱ) adsorption process was dominated by electrostatic attraction, precipitation, and complexation. The isothermal data exhibited a good correlation with the Freundlich model, indicating that the adsorption process was non-ideal and spontaneous. The maximum adsorption capacity was approximately 21.6 ± 0.35 mg/g at 318 K. After 5 cycles of regeneration, the adsorbent maintained good adsorption performance. Moreover, the removal rate was not greatly affected by ionic strength. These findings demonstrate that the granular adsorbent prepared with ISSA and waste bentonite can be recognised as a promising adsorbent for Pb-containing wastewater treatment.

Use of thermally modified waste concrete powder for removal of Pb (II) from wastewater: Effects and mechanism

Exploring effective uses of waste concrete powder (WCP), produced from recycling of construction & demolition waste is beneficial to the environment and sustainable development. In this study, WCP was first treated thermally to enhance the ability to remove Pb (II) from aqueous solutions. The experimental results revealed that the thermal treatment could enhance adsorption capacity due to modification of calcium bonding and pore structure of WCP. Preparation parameters such as temperature, particle size, and water-cement ratio were investigated to obtain the optimal operational conditions. Batch adsorption experiments were performed to explore influence factors of pH (1.00-6.00), ionic strength (0.05-2 mol/L), dosage (2-50 g/L), and temperature (25-45 °C). The pseudo-second-order kinetics model could adequately describe the adsorption process, and the Langmuir model was capable to predict the isotherm data well in the low concentration region (C₀ < 500 mg/L). The maximum uptake capacity for Pb (II) calculated by Langmuir model at 25, 35 and 45 °C were 46.02, 38.58 and 30.01 mg/g respectively, and the removal rate of Pb (II) was 92.96% at a dosage of 50 g/L (C₀ = 1000 mg/L). Precipitation, ion exchange, and surface complexation were identified to be the main mechanisms of Pb (II) adsorption through microscopic investigation by SEM-EDX, XRD, FTIR, XPS, and BET inspections. The study confirms that the WCP after thermal modification, can be selected as a promising adsorbent for the high performance and eco-friendliness.

Arsenate(V) removal from aqueous system by using modified incinerated sewage sludge ash (ISSA) as a novel adsorbent

Adsorption methods have been widely used in wastewater treatment due to its high removal efficiency, easy operation and handling, economic efficiency and little secondary pollution to the environment. In this paper, a high-iron containing incineration sewage sludge ash (ISSA) was modified by combined acid leaching and precipitation processes to improve its adsorption capacity of As(V). The effects of pH, time, temperature and ionic strength on the adsorption of As(V) were investigated by batch adsorption experiments. The results indicated that iron (mainly present as hematite) in the ISSA was rearranged to Fe(SO₄)OH. The modified ISSA showed an excellent adsorption potential for As(V) under acidic conditions and the adsorption capacity was around 9 times of the unmodified ISSA at pH 2-3. The adsorption process was fast during the first 2 h and reached an equilibrium at around 6 h. The Freundlich model could well fit the adsorption isotherm data, the presence of NO₃^- and Cl^- had a negligible influence on the As(V) removal by the modified ISSA, while PO₄^3- and SO₄^2- could significantly suppress As(V) removal via competitive adsorption. After 3 cycles of regeneration, the modified ISSA still showed a satisfying adsorption capacity. As(V) was removed by the modified ISSA mainly through ligand exchange reaction with hydroxyl oxygen (OH-) to form inner-sphere complexes. Therefore, the modified ISSA can be a promising material for As(V) removal from wastewater in particular due to the waste recycling potential.

Adsorption Characteristics of Stone-Bentonite Mixtures towards Zn: Equilibrium and Kinetic Tests

In the treatment of industrial polluted sites and the construction of landfill sites, anti-pollution barriers are usually used to prevent the diffusion of pollutants. In this paper, the adsorption characteristics of Zn ions by the rock-bentonite anti-pollution barrier were observed by means of static equilibrium and dynamic adsorption tests. The experimental results showed that the adsorption of Zn by stone chips—bentonite was close to the nonlinear Freundlich and Langmuir models. When the concentration of Zn ion is constant, the adsorption capacity increases with the increase in temperature. At a certain temperature, the adsorption removal rate decreases with the increase in concentration. Further study found that the adsorption of Zn from mixed soil was mainly an ion exchange process, and the adsorption mode of Zn from mixed soil was controlled by both intra-particle diffusion and membrane diffusion. Zeta potential, X-ray diffraction (XRD) and The Fourier Transform Infrared spectroscopy (FTIR) showed that with the increase in concentration, the mixed soil adsorbed more metal ions, and the thickness of the double electric layer decreased. Moreover, the adsorption of Zn2+ by bentonite was mainly interlayer adsorption and ion exchange. As an anti-pollution barrier material, the mixed soil of stone chips -bentonite can prevent the diffusion of pollutants, which has certain reference significance for engineering construction.

Alkaline modification of the acid residue of incinerated sewage sludge ash after phosphorus recovery for heavy metal removal from aqueous solutions

Enriched in phosphorus, sewage sludge ash has been extensively studied and applied as a secondary source for phosphorus recovery. Wet extraction, especially acid washing, is one of the most feasible methods to recover phosphorus from the ash due to its ease of operation, high efficiency and low cost. However, the management of the resultant acid residue was seldom addressed. In this study, special focus was paid to the reuse and recycling of the acid residue by an alkaline activation method. Its adsorption performance towards four different heavy metals in aqueous solutions was evaluated by batch and fixed-bed column adsorption experiments. The obtained material showed a high BET specific area (98.29 m²/g) and a total pore volume (0.114 cm³/g), and effectively removed Cd(II), Cu(II), Pb(II) and Zn(II) from aqueous solutions with the maximum adsorption capacity of around 26.8, 22.2, 53.3 and 13.5 mg/g respectively. It could be loaded in a fixed-bed column to continuously remove heavy metals especially for Pb(II). The proposed method to recycle the acid residue makes the wet extraction methods designing to recover phosphorus from incinerated sewage sludge complete without the generation of waste, which contributes to circular economy and a sustainable future.

Physico-chemical processes

By summarizing 187 relevant research articles published in 2019, the review is focused on the research progress of physicochemical processes for wastewater treatment. This review divides into two sections, physical processes and chemical processes. The physical processes section includes three sub-sections, that is, adsorption, granular filtration, and dissolved air flotation, whereas the chemical processes section has five sub-sections, that is, coagulation/flocculation, advanced oxidation processes, electrochemical, capacitive deionization, and ion exchange. PRACTITIONER POINTS: Totally 187 research articles on wastewater treatment have been reviewed and discussed. The review has two major sections with eight sub-topics.

Residuals, sludge, and biosolids: Advancements in the field

Advancements in the field of residuals, sludge, and biosolids have been made in 2019. This review outlines the major contributions of researchers that have been published in peer-reviewed journals and conference proceedings throughout 2019 and includes brief summaries from over 125 articles. The review is organized in sections including life cycle and risk assessments; characteristics, quality, and measurement including micropollutants, nanoparticles, pathogens, and metals; sludge treatment technologies including dewatering, digestion, composting, and wetlands; disposal and reuse including adsorbents, land application and agricultural uses, nutrient recovery, and innovative uses; odor and air emissions; and energy issues.

Using Sewage Sludge Ash as an Efficient Adsorbent for Pb (II) and Cu (II) in Single and Binary Systems

Incineration of sewage sludge produces every year huge amounts of sewage sludge ash. Due to its porosity and composition, sewage sludge ash can be used as an adsorbent for heavy metal ions removal. The present paper discusses the efficiency and feasibility of its use as an adsorbent for Pb (II) and Cu (II) removal in single and binary systems. Sewage sludge ash dosage, pH influence, equilibrium and kinetic studies were examined. The results show that sewage sludge ash is an effective and environmentally friendly adsorbent. The maximum adsorption capacity was 25.0 mg/g for Pb (II) and 7.5 mg/g for Cu (II). The presence of the competitive metal led to lower adsorption rate. The study concludes that sewage sludge ash is a promising adsorbent for Pb (II) and Cu (II) removal from wastewater presenting both economic and environmental benefits.

Recycling of incinerated sewage sludge ash as an adsorbent for heavy metals removal from aqueous solutions

The management of large quantities of incinerated sewage sludge ash (ISSA) is problematic. Environmental and economic benefits can be achieved by using ISSA as an adsorbent for heavy metals removal due to its exceptionally porous structure and active components. In this study, the feasibility of using ISSA to treat heavy metals (Cd(II), Cu(II) and Zn(II)) contaminated waters from both single- and binary-metal systems were investigated. The results showed that the pH of the solution played a pivotal role in the adsorption of heavy metals by ISSA and the optimal pH for the adsorption of these metals was around 6.00. The adsorption process of Cu(II), Cd(II), and Zn(II) in a single-metal system was similar and fast. The equilibrium data followed the Freundlich isotherm model and the corresponding adsorption capacity was 0.13, 0.11 and 0.06 mmol/g, respectively. However, the presence of other competitive metal ions had adverse effects on both the adsorption rate and the adsorption capacity for the target metal ions. The affinity of ISSA towards the metals followed the order of Cu(II) > Cd(II) > Zn(II). The difference in pH value and Ca or Na concentration of the solution after adsorption revealed that cation exchange played a fundamental role in the adsorption of the target metals, while electrostatic attraction and precipitation were insignificant. Over all, the application of ISSA as an adsorbent would be a promising option to both relieve the waste disposal pressure and mitigate the complex heavy metals pollution.