Insights into the speciation of heavy metals during pyrolysis of industrial sludge

Co-pyrolysis of sewage sludge and phosphate tailings: Synergistically enhancing heavy metal immobilization and phosphorus availability

Phosphate tailings (PT) was used to reduce the release of heavy metals (HMs) during pyrolysis and the leachable rate of residual HMs, and simultaneously improve the bioavailability of phosphorus in the sludge-based biochar. The concentration of heavy metals and the fractions determined by BCR method was used to investigate the release and the transformation of Zn, Pb, Mn, Ni and Cu during pyrolysis involved with the effects of temperature and the addition of PT. The respective pyrolysis experiments shows that the release of Zn and Pb increases with temperature for both sewage sludge (SS) and PT, and the bioavailable fractions (F1 + F2) of Mn, Ni, and Cu increases with temperature for PT. During co-pyrolysis, blended samples released lower quantities of Zn and Pb and presented lower bioavailability of HMs than the individual SS or PT. A synergistic effect of co-pyrolysis was evident for volatile Zn and Pb. The decomposition of CaMg (CO3)2 from PT produced CaO, by which the volatile ZnCl2 and PbCl2 were transformed into ZnO and PbO with less volatility and higher reactivity with SiO2 and Al2O3 than the chlorides. Then SiO2 and Al2O3 from SS acted as the final stabilizer to immobilize the oxides. The final product combined with SiO2 and Al2O3, such as ZnSiO4 and ZnAl2O4, were detected. The addition of PT also introduced more Ca and P into sludge to produce biochar with higher concentration of apatite phosphorus with higher bioavailability.

Stabilization of PFAS-contaminated soil with sewage sludge- and wood-based biochar sorbents

Sustainable and effective remediation technologies for the treatment of soil contaminated with per- and polyfluoroalkyl substances (PFAS) are greatly needed. This study investigated the effects of waste-based biochars on the leaching of PFAS from a sandy soil with a low total organic carbon content (TOC) of 0.57 ± 0.04 % impacted by PFAS from aqueous film forming foam (AFFF) dispersed at a former fire-fighting facility. Six different biochars (pyrolyzed at 700-900 °C) were tested, made from clean wood chips (CWC), waste timber (WT), activated waste timber (aWT), two digested sewage sludges (DSS-1 and DSS-2) and de-watered raw sewage sludge (DWSS). Up-flow column percolation tests (15 days and 16 pore volume replacements) with 1 % biochar indicated that the dominant congener in the soil, perfluorooctane sulphonic acid (PFOS) was retained best by the aWT biochar with a 99.9 % reduction in the leachate concentration, followed by sludge-based DWSS (98.9 %) and DSS-2 and DSS-1 (97.8 % and 91.6 %, respectively). The non-activated wood-based biochars (CWC and WT) on the other hand, reduced leaching by <42.4 %. Extrapolating this to field conditions, 90 % leaching of PFOS would occur after 15 y for unamended soil, and after 1200 y and 12,000 y, respectively, for soil amended with 1 % DWSS-amended and aWT biochar. The high effectiveness of aWT and the three sludge-based biochars in reducing PFAS leaching from the soil was attributed largely to high porosity in a pore size range (>1.5 nm) that can accommodate the large PFAS molecules (>1.02-2.20 nm) combined with a high affinity to the biochar matrix. Other factors like anionic exchange capacity could play a contributing role. Sorbent effectiveness was better for long-chain than for short-chain PFAS, due to weaker, apolar interactions between the biochar and the latter's shorter hydrophobic CF2-tails. The findings were the first to demonstrate that locally sourced activated wood-waste biochars and non-activated sewage sludge biochars could be suitable sorbents for the ex situ stabilization and in situ remediation of PFAS-contaminated soil, bringing this technology one step closer to full-scale field testing.

Attempts to obtain clean biochar from hyperaccumulator through pyrolysis: Removal of heavy metals and transformation of phosphorus

The sound disposal of the ensuing heavy metal-rich plants can address the aftermath of phytoremediation. In this study, the first attempt was made to obtain heavy metals-free and phosphorus-rich biochar from phytoremediation residue (PR) by pyrolysis, and the effects of chlorinating agent type, chlorine dosage, and pyrolysis residence time on heavy metal removal, phosphorus (P) transformation, and biochar properties were investigated. The results showed that as chlorine dosage and pyrolysis residence time increased, added polyvinyl chloride (PVC) reduced the concentration of Zn in biochar to one-tenth of that in PR by intensified chlorination, where both Zn concentration (2727.50 mg/kg) and its leaching concentration (29.13 mg/L) met the utilization requirements, in which the acid-base property of biochar plays a key role in heavy metal leaching. Meanwhile, more than 90% of P in PR remained in biochar and the bioavailability of P in biochar enhanced with the decomposition of organic P to inorganic P, where the concentration of plant-availability P (Pnac) expanded from 1878.40 mg/kg in PR to 8454.00 mg/kg in biochar. This study demonstrated that heavy metal hyperaccumulator can be converted into heavy metal-free and phosphorus-rich biochar with promising applications, which provides new perspectives for the treatment of such hazardous wastes.

Competitive encapsulation of multiple heavy metals by magnesium potassium phosphate cement: Hydration characteristics and leaching toxicity properties

Magnesium potassium phosphate cement (MKPC) is increasingly used in the solidification/stabilization (SS) of heavy metal (HM) pollutants. However, research on composite HM pollutants remains limited. In this study, four heavy metals (Pb/Zn/Cu/Cd) were individually and simultaneously introduced into MKPC systems with different magnesium/phosphorus (M/P) molar ratios. The introduction of HMs altered the extent of hydration and morphology of MgKPO4·6H2O. Among the MKPC pastes, those with M/P = 2 and 3 had the highest HM solidification efficiency and strength, respectively. The HM solidification efficiency of all specimens exceeded 99 %. In samples with M/P = 3, the codoping of four HMs slightly increased the M/P ratio, thereby increasing MgKPO4·6H2O content and enhancing strength. Pb could generate additional low-solubility precipitates, such as PbHPO4, Pb3 (PO4)2, Pb5 (OH) (PO4)3, and Pb (OH)2, which easily accumulated in pores and were encapsulated by MgKPO4·6H2O, leading to the highest solidification efficiency of Pb by MKPC. Pb and Cu could also form the composite phosphate products Pb2Cu (PO4)3 (OH)·4H2O, thus promoting the S/S effect of Cu. Therefore, the use of MKPC with M/P ratio of 2-3 for the S/S of complex pollutants containing Pb and Cu is a promising approach.

Influence of inherent minerals on metalworking fluids sludge pyrolysis: Products characterization and heavy metals behavior

Safely and appropriately disposing of metalworking fluids sludge (MFS) remains a challenge owing to its highly hazardous properties, this work investigated MFS pyrolysis at various temperatures (500, 600, 700, 800, and 900 °C) for energy recovery and safety treatment of MFS. The experimental results indicated that inherent minerals at higher temperatures could enhance the gas yields and promote the qualities of oil and gas from MFS pyrolysis. The highest pyrolysis gas yield was achieved at 18.86 wt% after MFS pyrolysis at 900 °C. GC-MS analysis revealed that the inherent minerals facilitated a decrease in oxygenated and nitrogenated compounds within the oil, while simultaneously leading to a substantial increase in hydrocarbon contents. Notably, the highest content of aromatics (61.16%) was attained during pyrolysis at 900 °C. Moreover, inherent minerals improved carbon sequestration and the characteristics of biochar during the MFS pyrolysis. The leaching contents of heavy metals in biochars were reduced, thereby reducing the heavy metals associated environmental risk. This research suggests that the pyrolysis process was a promising approach for simultaneous energy recovery and MFS disposal with low environmental risk.

Enhancement of H2 and light oil production and CO2 emission mitigation during co-pyrolysis of oily sludge and incineration fly ash

The proper treatment and utilization of oily sludge (OS) and incineration fly ash (IFA) remains a significant challenge due to their hazardous nature. To attain effective recovery of petroleum hydrocarbons and synergistic disposal, this study investigated the co-pyrolysis of OS and IFA, resulting in successful energy recovery, CO2 mitigation, and heavy metal immobilization. Results revealed that the peak ratio of light oil to heavy oil fractions reached 179.42% with 20 wt% IFA addition, accompanied by the highest aromatic hydrocarbons selectivity of 30.72% and the lowest coke yield of 106.13 mg/g OS under the optimal temperature of 600 °C. In-depth analysis indicated that IFA inhibited the poly-condensation of macromolecular PAHs and promoted their cracking into light aromatic hydrocarbons. The addition of 50 wt% IFA significantly increased H2 yield (21.02 L/kg OS to 60.95 L/kg OS) and facilitated CO2 sequestration due to its higher content of Ca-bearing minerals. Moreover, high IFA ratios promoted the reduction of Fe species in OS to a low-valence state. Heavy metals in co-pyrolysis char were well immobilized into stable fractions with lower environmental risks. This work highlights the potential of co-pyrolysis as a viable approach for simultaneous disposal of multiple hazardous wastes and offers new insights for their utilization.

Influence of silica-aluminum materials on heavy metals release during paper sludge pyrolysis: Experimental and theoretical studies

It is of great significance to reduce the secondary risk of heavy metals during the pyrolysis of paper sludge. This study used kaolin and alumina-silica-based xerogels to control heavy metals released during sludge pyrolysis. Pyrolyzing a mixture of sludge and 7% kaolin at 400 °C achieved high retention rates for Cu (95.85%), Zn (95.97%), Pb (97.15%), Cd (84.23%), and Cr (84.05%) when the pyrolysis tail gas was treated with 9 g of xerogel. The addition of kaolin facilitated the transformation of Cu, Zn, Pb, and Cr from the unstable fraction to the stable fraction in pyrolysis biochar, reducing their leachability. The xerogels also played a crucial role in adsorbing and stabilizing the heavy metals. The results of thermodynamic equilibrium calculations showed that Pb(g), PbS(g), PbCl2(g), PbCl(g), Zn(g), ZnCl2(g), and Cd(g) were the main gaseous products of Zn, Pb, and Cd during paper sludge pyrolysis. The Pb atoms in PbCl2 and PbS, and the Zn atoms in ZnCl2 bond with the oxygen atoms on the kaolin surface by covalent bonds, while the Cl atoms in PbCl and the Pb atoms of elemental lead form ionic bonds with H and O atoms on the kaolinite surface, respectively. These experimental and simulation results offer new ideas for controlling heavy metals during sludge pyrolysis.

Non-negligibly negative role of e-waste-derived pyrogenic carbon in the soil washing of copper and polybrominated diphenyl ethers

The open incineration of electrical and electronic waste (e-waste) results in the accumulation of pyrogenic carbon in the soil. However, the effect of the e-waste-derived pyrogenic carbon (E-PyC) on the performance of soil washing at e-waste incineration sites remains unclear. In this study, the effectiveness of a citrate-surfactant mixed solution in removing copper (Cu) and decabromodiphenyl ether (BDE209) at two e-waste incineration sites was evaluated. The removal efficiencies of Cu (24.6-51.3%) and BDE209 (13.0-27.9%) were low in both soils and were not significantly improved by ultrasonic. Soil organic matter analysis, hydrogen peroxide and thermal pretreatment experiments, and microscale soil particle characterization demonstrated that the poor removal of soil Cu and BDE209 was due to steric effects of E-PyC on the release of the solid fraction of pollutants and the competitive sorption of the labile fraction of pollutants by E-PyC. Weathering of soil Cu weakened the influence of E-PyC but strengthened the negative impact of natural organic matter (NOM) on soil Cu removal by promoting complexation between NOM and Cu2+ ions. This study demonstrates that the negative effect of E-PyC on Cu and BDE209 removal by soil washing is non-negligible, which has implications for decontaminating e-waste incineration sites by soil washing.

Transformation and environmental risk of heavy metals in sewage sludge during the combined thermal hydrolysis, anaerobic digestion and heat drying treatment process

The design of this study was to investigate the solid-aqueous migration and chemical speciation transformation of heavy metals (HMs) in the sewage sludge during the combined process of thermal hydrolysis, anaerobic digestion and heat-drying. The results showed that most of the HMs were still accumulated in the solid phase of various sludge samples after treatment. After thermal hydrolysis, the concentrations of Cr, Cu and Cd increased slightly. All the HMs measured after anaerobic digestion were concentrated obviously. While the concentrations of all HMs decreased slightly after heat-drying. The stability of HMs in the sludge samples was enhanced after treatment. The environmental risks of various HMs were also relieved in the final dried sludge samples.

Co-Gasification Synergistic Characteristics of Sewage Sludge and High-Sodium Coal

A comprehensive study was conducted to assess the co-gasification characteristics of sewage sludge and high-sodium coal. As the gasification temperature increased, the CO₂ concentration was decreased, and the concentrations of CO and H₂ were increased, while the change of CH₄ concentration was not obvious. As the coal blending ratio increased, the H₂ and CO concentrations initially increased and then decreased, while the CO₂ concentration initially decreased and then increased. The mixture of sewage sludge and high-sodium coal shows the synergistic effect of co-gasification, and the synergistic effect was to promote the gasification reaction positively. The average activation energies of co-gasification reactions were calculated by the OFW method, and the average activation energy initially decreases and then increases as the coal blending ratio increases. Both fluidized-bed gasification and thermogravimetric analyzer gasification show that the optimum coal blending ratio is 0.6. Overall, these results provide a theoretical basis for the industrial application of sewage sludge and high-sodium coal co-gasification.

Pyrolysis of antibiotic mycelial residue for biochar: Kinetic deconvolution, biochar properties, and heavy metal immobilization

The safe disposal of antibiotic mycelial residue (AMR), a hazardous waste, is a pressing problem owing to the spread of antibiotic and heavy metal pollution. In this study, AMR pyrolysis at different temperatures and heating rates was investigated to prepare valuable biochar for heavy metal immobilization. The results showed that AMR decomposition mainly involved three pseudo-reactions, with average activation energies of 252.4, 149.8, and 219.7 kJ/mol, that fitted a three-dimensional diffusion model. Increasing the pyrolysis temperature and heating rate decreased the yield and volatile matter content of biochar, but the ash content, fixed carbon content, and aromaticity increased. The AMR-derived biochar had a favorable fuel property (18.1-19.8 MJ/kg) and stability against degradation in soil. Calcium oxalate hydrate, a major mineral in AMR, degraded during biochar formation. Furthermore, high pyrolysis temperature promoted the residual fractions of Cr, Cu, Zn, Cd, and Pb in biochar, more so than did the heating rate, inducing a low potential ecological risk. In particular, the leaching rate of Zn decreased from 46.9% in AMR to 0.3% in biochar obtained at 700 °C with a heating rate of 10 °C/min. This study elucidates the formation process and physicochemical properties of AMR biochar, which helps in the harmless utilization of AMR as a carbon resource.

Physico-chemical properties of waste derived biochar from community scale faecal sludge treatment plants

Background: The dumping of untreated faecal sludge from non-sewered onsite sanitation facilities causes environmental pollution and exacerbates poor public health outcomes across developing nations. Long-term mechanisms to treat faecal sludge generated from these facilities are needed to resolve the global sanitation crisis and realize the Sustainable Development Goal (SDG) 6 "ensure availability and sustainable management of water and sanitation for all" by 2030.  Pyrolysis of faecal sludge removes pathogens and generates biochar, which can be used as a soil enhancer. Methods: The properties of faecal sludge biochars from three full-scale treatment plants in India were determined via Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), energy dispersive x-ray (EDX) spectroscopy, crystal x-ray diffraction (XRD), proximate analyses, and BET surface area porosimetry. Results:  Results showed that all three biochars had low specific surface area, high alkaline pH values, high ash content, and negative surface charge. Fourier transform infrared spectra showed the same surface functional groups present in each biochar. X-ray diffraction analysis showed the mineral composition of each biochar differed slightly. Scanning electron microscopy analysis indicated a porous structure of each biochar with ash particles evident. Conclusions: Slight differences in the ash content, surface area, pH and mineral content was observed between the three biochars.

Leaching Characteristics of Heavy Metals in the Baghouse Filter Dust from Direct-Fired Thermal Desorption of Contaminated Soil

After thermal desorption, the total amount of heavy metals (HMs) is enriched in baghouse filter dust. To further understand the related environmental impact, the leaching characteristics under various conditions must be explored. Therefore, this study aimed to examine the leaching characteristics of seven HMs in the dust generated in the direct-fired thermal desorption process and to compare the differences in heavy metal leaching characteristics in the soil before and after thermal desorption. The leaching characteristics and bioaccessibility of seven HMs-arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), nickel (Ni), and zinc (Zn)-were analyzed in dust and in soil before and after thermal desorption. The activity of HMs in dust was strong. Therefore, environmental effects and effects on human health should be considered in the treatment of soil and dust after thermal desorption.

Physico-chemical properties of waste derived biochar from community scale faecal sludge treatment plants

Background: The dumping of untreated faecal sludge from non-sewered onsite sanitation facilities causes environmental pollution and exacerbates poor public health outcomes across developing nations. Long-term mechanisms to treat faecal sludge generated from these facilities are needed to resolve the global sanitation crisis and realize the Sustainable Development Goal (SDG) 6 “ensure availability and sustainable management of water and sanitation for all” by 2030.  Pyrolysis of faecal sludge removes pathogens and generates biochar, which can be used as a soil enhancer. Methods: The properties of faecal sludge biochars from three full-scale treatment plants in India were determined via Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), energy dispersive x-ray (EDX) spectroscopy, crystal x-ray diffraction (XRD), proximate analyses, and BET surface area porosimetry. Results:  Results showed that all three biochars had low specific surface area, high alkaline pH values, high ash content, and negative surface charge. Fourier transform infrared spectra showed the same surface functional groups present in each biochar. X-ray diffraction analysis showed the mineral composition of each biochar differed slightly. Scanning electron microscopy analysis indicated a porous structure of each biochar with ash particles evident. Conclusions: Slight differences in the ash content, surface area, pH and mineral content was observed between the three biochars.

Co-pyrolysis of sewage sludge and food waste digestate to synergistically improve biochar characteristics and heavy metals immobilization

Food waste digestate (FWD) is a desirable additive in sewage sludge (SS)-based biochar preparation owing to its high contents of intrinsic inorganic minerals and lignocellulosic compounds. In this study, we investigated the co-pyrolysis of SS with FWD at different mixing ratios (4:0, 3:1, 2:2, 1:3, and 0:4; SS:FWD w/w) at 550 °C to synergistically improve the biochar characteristics and immobilize the heavy metals in the SS. The results showed that co-pyrolysis of SS with FWD greatly increased the aromaticity and pH (by 13.22-26.56%) of the blended biochar, and significantly reduced the contents of total and bioavailable heavy metals. The addition of FWD effectively enhanced the conversion of heavy metals from less stable fractions to more stable forms, but led to the transformation of Cr from the residual fraction (F4) to the oxidizable fraction (F3) when the FWD:SS ratio was ≥ 3:1. Overall, the formation of co-crystal compounds, stable kaolinite, and metal oxides together with the enhancement of biochar characteristics during co-pyrolysis significantly reduced the heavy metal-associated ecological risk (potential ecological risk index lower than 15.51) and phytotoxicity (germination index higher than 139.41%) of the blended biochar. These findings suggest that high levels of mineral components in FWD greatly immobilize more heavy metals in biochar.

Combining impregnation and co-pyrolysis to reduce the environmental risk of biochar derived from sewage sludge

Sewage sludge derived biochar has great potential for agricultural application, whereas the risk of heavy metals in sewage sludge is a key challenge for utilization. This study investigated the synergetic effect of co-pyrolysis and ZnCl₂ impregnation treatment on the surface characteristics and potential ecological risk of heavy metals in sewage sludge derived biochar. It was concluded that ZnCl₂ impregnation led to an increase in biochar yield, O and S content, but decrease the ash content and pH. Additionally, the thermal stability of ZnCl₂-added biochar was decreased. Semi-quantitive specific functional groups analysis suggested that ZnCl₂ impregnation had a negative effect on the content of CO and C-O, while promoted the formation of aromatic CC. And the functional group of CO was retained in biochar by co-pyrolysis with Camellia oleifera shell due to its high content of potassium. For heavy metal passivation, ZnCl₂ impregnation decreased total content of heavy metals by chlorination, while the oxidizable fraction (F3) of heavy metals showed an increasing tendency. Results of potential ecological risk assessment indicated that combining ZnCl₂ impregnation and co-pyrolysis had great potential to reduce the ecological risk of heavy metals in sewage sludge derived biochar.

Molecular composition and biotoxicity effects of dissolved organic matters in sludge-based carbon: Effects of pyrolysis temperature

Sludge pyrolysis carbonization has shown potential to convert sludge biomass into multifunctional carbon materials. However, ecological risks of dissolved organic matters (DOMs) with obscure molecular characteristics retaining in sludge-based carbons (SBCs) have received little attention. This study investigated the impact of pyrolysis temperatures on the molecular conversion and biotoxicity effects of DOMs in SBCs. The results revealed that DOMs in SBCs_300-400 were mainly derived from depolymerization of biopolymers and the polycondensation and cyclization of small intermediate molecules, which mainly consisted of aromatic CHON compounds with 1-3 N atoms, featuring high unsaturation and molecular weights. High-temperature pyrolysis (500-800 °C) promoted the decomposition and ring-opening of aromatic CHON compounds into saturated aliphatic CHO compounds with 2-4 O atoms in SBCs_500-800. Noteworthily, SBCs_300-400-derived DOMs showed relatively strong biotoxicity on the growth and development of wild-type zebrafish embryos, pakchoi seeds, and Vibrio qinghaiensis Q67, which was significantly related to aromatic amines, phenols, and heterocyclic-N compounds in DOMs of SBCs_300-400. SBCs_500-800-derived DOMs were mainly straight-chain fatty acids and showed no observable acute biotoxicity. This study highlights the negative impact of DOMs in SBCs on the ecological environment, and provides the theoretical basis for controlling toxic byproducts in sludge pyrolysis process.

Reliable environmental trace heavy metal analysis with potentiometric ion sensors - reality or a distant dream

Over two decades have passed since polymeric membrane ion-selective electrodes were found to exhibit sufficiently lower detection limits. This in turn brought a great promise to measure trace level concentrations of heavy metals using potentiometric ion sensors at environmental conditions. Despite great efforts, trace analysis of heavy metals using ion-selective electrodes at environmental conditions is still not commercially available. This work will predominantly concentrate on summarizing and evaluating prospects of using potentiometric ion sensors in view of environmental determination of heavy metals in on-site and on-line analysis modes. Challenges associated with development of reliable potentiometric sensors to be operational in environmental conditions will be discussed and reasoning behind unsuccessful efforts to develop potentiometric on-site and on-line environmental ion sensors will be explored. In short, it is now clear that solely lowering the detection limit of the ion-selective electrodes does not guarantee development of successful sensors that would meet the requirement of environmental matrices over long term usage. More pressing challenges of the properties and the performance of the potentiometric sensors must be addressed first before considering extending their sensitivity to low analyte concentrations. These are, in order of importance, selectivity of the ion-selective membrane to main ion followed by the membrane resistance to parallel processes, such as water ingress to the ISM, light sensitivity, change in temperature, presence of gasses in solution and pH and finally resistance of the ion-selective membrane to fouling. In the future, targeted on-site and on-line environmental sensors should be developed, addressing specific environmental conditions. Thus, ion-selective electrodes should be developed with the intention to be suitable to the operational environmental conditions, rather than looking at universal sensor design validated in the idealized and simple sample matrices.

Application of co-pyrolysis biochar for the adsorption and immobilization of heavy metals in contaminated environmental substrates

Heavy metal pollution has been considered as a serious threat to the environment and human in the past decades due to its toxic and unbiodegradable properties. Recently, extensive studies have been carried out on the removal of heavy metals, and various adsorption materials have been successfully developed. Among, biochar is a promising option because of its advantages of various biomass sources, abundant microporous channels and surface functional groups, as well as its attractive economic feasibility. However, the application of pristine biochar is limited by its low adsorption capacity and nonregenerative property. Co-pyrolysis biochar, produced from the pyrolysis of biomass with the addition of another biomass or non-biomass precursor, is potential in overcoming the limitation of pristine biochar and achieving superior performance for heavy metal adsorption and immobilization. Therefore, this article summarizes the recent advances in development and applications of co-pyrolysis biochar for adsorption and immobilization of various heavy metals in contaminated environmental substrates. In details, the production, characteristics and advantages of co-pyrolysis biochar are initially presented. Subsequently, the adsorption behaviors and mechanisms of different heavy metals (including Hg, Zn, Pb, Cu, Cd, Cr, As, etc.) in flue gas and wastewater by co-pyrolysis biochar are reviewed, as well as factors influencing their adsorption capacities. Meanwhile, the immobilization of heavy metals in both biochar itself and contaminated soils by co-pyrolysis biochar is discussed. Finally, the limitations of current studies and future prospects are proposed. It aims at providing a guideline for the exploitation and application of cost-effective and environmental-friendly co-pyrolysis biochar in the decontamination of environmental substrates.

Comparative study on intercalation-exfoliation and thermal activation modified kaolin for heavy metals immobilization during high-organic solid waste pyrolysis

With the new municipal solid waste classification policy implemented in China, attention on achieving the waste-to-energy disposal of "dry waste" has been growing. Pyrolysis conversion of organic waste into value-added chemicals is a promising method to treat solid waste. However, after removing the non-combustible components of "dry waste", the obtained high-organic solid waste (HSW) contains various heavy metals, which requires urgent attention during thermochemical conversion. To mitigate heavy metals risk, kaolin was employed as additive during HSW pyrolysis, and intercalation-exfoliation and thermal activation modifications were performed on the kaolin to further immobilize and stabilize heavy metals in the derived chars. The characterization results illustrated that the interlayer spacing, pore volume and diameter of kaolin were expanded after intercalation-exfoliation modification, providing more opportunities for the adsorption of metals. The thermal activation method favored the transformation of kaolin into metakaolin via dehydroxylation to enhance its nonhexacoordinated Al proportion and chemisorption. During 450-650 °C, kaolin exhibited an effective solid enrichment performance for targeting heavy metals, and the intercalation-exfoliation and thermal activation modification further enhanced the adsorption capacity of the kaolin for Cd, Cr, Pb and Cr, Cu, Pb, Zn, respectively. Compared with Cu and Zn, additives demonstrated better stabilization effects for Cd, Pb, and Cr, transforming more bioavailable fractions to the residual speciation. Overall, a higher pyrolytic temperature (650 °C) and the addition of effective additives could simultaneously increase the residual fraction and decrease the bioavailable fraction of heavy metals in HSW-derived chars, reducing the potential ecological risk.

Optimizing environmental pollution controls in response to textile dyeing sludge, incineration temperature, CaO conditioner, and ash minerals

The dynamics of heavy metal speciation and flue gas emissions during the incineration of textile dyeing sludge (TDS) were quantified as a function of four addition levels of CaO, incineration temperature, and ash minerals using thermogravimetric analysis and experimental tube furnace. The TDS incineration was most improved with the addition of 10% CaO. The increased fractions of CaO coupled with the ash minerals changed the retention behaviors of eight heavy metals. The CaO addition increased the Cu, Zn, As, and Pb retentions, did not significantly change Cr, Mn, and Cd, but decreased the Ni retention. The CaO addition enhanced the speciation stability of Cu and transferred the Cr, Cd, and As speciations to the mobile fractions. The increased temperature weakened the Zn and Pb retentions and the speciation stabilities of As and Pb and turned the Cr, Mn, Ni, Cu, Zn, and Cd speciations into the stable fractions. The CaO addition inhibited HCN, NO, NO₂, COS, SO₂, CS₂, and SO₃ emissions from the TDS incineration. Neural network-based multi-response optimization was implemented to determine the optimal operational temperature for the TDS incineration and the reduction of the 12 gas emissions. The range of 640-755 °C with(out) 5% CaO appeared to be most beneficial in terms of both environmental quality and economic efficiency.

Chemical speciation and risk assessment of heavy metals in biochars derived from sewage sludge and anaerobically digested sludge

Dewatered sewage sludge (DSS) and anaerobically digested sludge (ADS) were pyrolyzed at 550 °C to investigate the characteristics of derived biochar and evaluate the risk of heavy metals (Cr, Ni, Cu, As, Cd, and Pb). The results showed that the pH value of the biochar derived from DSS (DSS-C) was slightly lower than that of the biochar derived from ADS (ADS-C), while DSS-C presented relatively higher specific surface area and total pore volume. DSS-C also showed higher H/C and lower O/C ratios than ADS-C, indicating a higher aromatic condensation and a lower polarity. Total concentrations of Cr, Ni, Cu, As, Cd, and Pb in DSS and ADS increased significantly after pyrolysis owing to the thermal decomposition of organic matter in the sludge, with corresponding rise of the Nemerow pollution index (NPI) of the biochars compared with the raw sludge. In addition, the sequential extraction procedure (BCR) analysis revealed that the pyrolysis process promoted the transformation of heavy metals from bio-available fractions to stable fractions. The potential environmental risk of heavy metals decreased from moderate and extremely high levels in the DSS and ADS to low risk and moderate levels in DSS-C and ADS-C after pyrolysis, respectively.

A review of glass ceramic foams prepared from solid wastes: Processing, heavy-metal solidification and volatilization, applications

The safe utilization of solid wastes containing heavy metals plays a crucial role in environmental preservation. As an efficient technology to achieve this goal, the preparation of glass ceramic foams from solid wastes can produce an excellent solidification effect on heavy metals. At present, there have been plenty of efforts made to achieve an excellent combination of such characteristics as mechanical strength, bulk density, thermal conductivity and so on, with the purpose to ensure the application in various high value-added fields. Due to the concentration on their application in the construction sector such as the use of thermal and acoustic insulation materials, some researchers seek to expand the scope of their applications. In this paper, a review is conducted into the methods used to prepare solid waste-based glass ceramic foams. Depending on the exact processing route, these methods can be categorized into two classes, which are powder sintering and inorganic gel casting. Not only heavy metals hinder the application of solid waste, they can also cause irreversible pollution to the wider environment. Solidification and volatilization represent the two routes associated with heavy-metal migration during the preparation of glass ceramic foams. Both traditional and innovative applications are indicated in this review. Furthermore, a discussion is conducted about the prospects and challenges facing different processing strategies, heavy-metal migration and applications.

Co-pyrolysis of sewage sludge and organic fractions of municipal solid waste: Synergistic effects on biochar properties and the environmental risk of heavy metals

The introduction of heavy metal-free biomass into the sewage sludge (SS) pyrolysis can effectively improve the biochar properties and reduce the bioavailability and toxicity of heavy metals (HMs) in blended biochar. Herein, this study aimed to understand the biochar properties and associated environmental risks of HMs, by comparing the residual contents from the co-pyrolysis of SS with various organic fractions of municipal solid waste (OFMSW) at 550 °C and pyrolysis alone at different temperatures between 350 and 750 °C. The results indicated that, compared with SS pyrolysis alone, co-pyrolysis of SS with various OFMSW (except PVC) lead to lower biochar yields but with higher pH values (increased between 21.80% and 31.70%) and carbon contents (raised between 33.45% and 48.22%) in blended biochars, and the chemical speciation analysis suggested that co-pyrolysis further promoted the HMs transformation into more stable forms which significantly reduce the associated environmental risk of HMs in the blended biochars (the values of RI lower than 55.80). The addition of PVC, however, impeded biochar properties and compromised HMs immobilization during SS pyrolysis.

In situ catalytic reforming of plastic pyrolysis vapors using MSW incineration ashes

The valorization of municipal solid waste incineration bottom and fly ashes (IBA and IFA) as catalysts for thermochemical plastic treatment was investigated. As-received, calcined, and Ni-loaded ashes prepared via hydrothermal synthesis were used as low-cost waste-derived catalysts for in-line upgrading of volatile products from plastic pyrolysis. It was found that both IBA and air pollution control IFA (APC) promote selective production of BTEX compounds (i.e., benzene, toluene, ethylbenzene, and xylenes) without significantly affecting the formation of other gaseous and liquid species. There was insignificant change in the product distribution when electrostatic precipitator IFA (ESP) was used, probably due to the lack of active catalytic species. Calcined APC (C-APC) demonstrated further improvement in the BTEX yield that suggested the potential to enhance the catalytic properties of ashes through pre-treatment. By comparing with the leaching limit values stated in the European Council Decision, 2003/33/EC for the acceptance of hazardous waste at landfills, all the ashes applied remained in the same category after the calcination and pyrolysis processes, except the leaching of Cl^- from the ESP, which was around the borderline. Therefore, the use of ashes in catalytic reforming application do not significantly deteriorate their metal leaching behavior. Considering its superior catalytic activity towards BTEX formation, C-APC was loaded with Ni at 15 and 30 wt%. The Ni-loading favored an increase in overall oil yield, while reducing the gas yield when compared to the benchmark Ni loaded ZSM catalyst. However, Ni addition also caused the formation of more heavier hydrocarbons (C20-C35) that would require post-treatment to recover favorable products like BTEX.

Functionalization of ultrasound enhanced sewage sludge-derived biochar: Physicochemical improvement and its effects on soil enzyme activities and heavy metals availability

Poor physicochemical characteristics and high heavy metals content are main limitations of applying sludge-based biochars in remediation studies. The present study attempts to combine two practical approaches of ultrasound pre-treatment with low-time and low-frequency and chemical functionalization using citric acid. The aims of this study are enhancement physicochemical characteristics and environmental applicability of sludge-derived biochar. The characteristics of obtained ultrasound-treated functionalized biochar (UFB), sludge-derived biochar (SDB) and sewage sludge (SS) were evaluated. Then, the effects of these additives on soil heavy metals availability, soil enzyme activities and soil physicochemical characteristics were investigated during a 2-month stabilization process. The results indicated that ultrasound pre-treatment and functionalization considerably increased pore volume, surface area, and surface functional groups of the biochar, but significantly decreased total heavy metals concentration and metals ecological risk index (Er). The results of soil amending showed that application of UFB decreased Pb, Zn and Cd availability in soil by 85.3, 82.9 and 30.6%, respectively. In all cases, except for Cd, the Pb and Zn availability decreased by UFB was two times greater than the availability decreased by SDB and SS. Compared to SDB, the UFB potentially enhanced the positive effect of additive on soil enzyme activities. The obtained results revealed that the feasible, uncomplicated physical and chemical techniques can be used as a valuable approach for enhancing the environmental applicability of sludge-derived biochar and management of the excessively produced sewage sludge in the world.

Migration characteristics of heavy metals during sludge pyrolysis

A comprehensive study was conducted to investigate the pyrolysis characteristics of municipal sludge, and the activation energy of sludge pyrolysis was determined using the Model-free method. The detailed migration characteristics of heavy metals in the pyrolysis products were also investigated at different pyrolysis temperatures (250-850 °C). The results demonstrate that sludge pyrolysis is a multi-step process; the activation energy of pyrolysis increased with the pyrolysis conversion rate, and the average activation energy was calculated as 79.59 kJ mol^-1. As the pyrolysis temperature increased, the char yield decreased, the tar yield increased then decreased, and the gas yield increased. At 850 °C, the thermal volatilities of heavy metals followed the sequence Cu < Cr < Ni < Mn < Pb < As < Zn < Cd = Hg. In addition, Cu, Cr, and Ni were seldom involved in migration during pyrolysis while As, Cd, and Hg readily migrated even at low pyrolysis temperatures. The results provide a theoretical basis for sludge pyrolysis technologies and heavy metals migration control.

Co-pyrolysis of monobasic potassium phosphate and plastic processing sludge: Characteristics and environmental risks of potentially toxic elements

A high concentration of potentially toxic elements (PTEs) can be frequently observed in the plastic processing sludge (PPS), thereby restricting its environmental applications. The main objective of this study was to investigate the effects of the co-pyrolysis of PPS and KH₂PO₄ (0, 5, 10 and 20 wt%) on the characteristics and environmental risks associated with the PTEs in PPS and derived chars. General characteristic analysis revealed that the char yield, ash content, pH, and particle size of the chars prepared with KH₂PO₄ were greater than those of the char prepared without KH₂PO₄ by 3.13-4.89 wt%, 2.95-4.4 wt%, 0.77-0.93, and 9.64-30.07 µm, respectively. The results of sequential extraction indicated that co-pyrolysis with KH₂PO₄ could considerably increase the distribution of PTEs in the F4 fraction (non-bioavailable) in PPS by 1.30-65.90% when compared with that obtained via co-pyrolysis with 5 wt% of KH₂PO₄. The toxic leaching tests indicated that the leaching concentrations of Cr, Ni, Cu, Zn, Cd, and Pb in the char prepared without KH₂PO₄ decreased to different extents when PPS was subjected to co-pyrolysis with KH₂PO₄, especially in case of co-pyrolysis with 5 wt% of KH₂PO₄. The range of decrease was 26.40-88.34%. However, in case of Cu, Zn, and Pb, the leaching concentration of the chars prepared with more than 10 wt% of KH₂PO₄ increased owing to the decomposition of (Cu Zn)PbVO₄(OH) in an acidic environment. The results obtained using Hakanson's equations revealed that the potential ecological risk associated with the PTEs in chars obtained by co-pyrolysis with KH₂PO₄ decreased, with a minimum decrease of 38.17%. In addition, the risk level associated with PPS reduced from considerable to low after co-pyrolysis with KH₂PO₄. The observations of this study imply that the co-pyrolysis of PPS with KH₂PO₄ can be a promising treatment for PTE immobilization.

Influence of thermal hydrolysis treatment on chemical speciation and bioleaching behavior of heavy metals in the sewage sludge

In this study, the transformation of chemical speciation of Cr, Mn, As and Cd in the sewage sludge before and after thermal hydrolysis treatment was investigated using modified BCR method. The effect of thermal hydrolysis treatment and chemical speciation change on the subsequent bioleaching behavior was also researched. The results showed that the concentrations of Cr, Mn, As and Cd in oxidizable fraction decreased in the sludge treated by thermal hydrolysis. Meanwhile, the proportions of Cr, Mn and As in the mobile fractions (acid-soluble/exchangeable and reducible fraction) all decreased, while Cd was concentrated in the sludge treated by thermal hydrolysis. The final pH value of bioleached sludge treated by thermal hydrolysis was lower than that in the bioleached raw sewage sludge. And faster increase of oxidation-reduction potential (ORP) was also found in the bioleaching process of the sludge treated by thermal hydrolysis. The removal percentage of Mn and Cd increased in the bioleached sludge treated by thermal hydrolysis. Thermal hydrolysis treatment can promote the bioleaching to some extent. Furthermore, the environmental risk of Cr, Mn, As and Cd in the bioleached sludge treated by thermal hydrolysis was all alleviated according to risk assessment analysis compared with the bioleached raw sewage sludge.

The impact of pyrolysis temperature on physicochemical properties and pulmonary toxicity of tobacco stem micro-biochar

Biochars (BCs) are currently widely used, yet their impact on human health is mostly unknown. We generated micro-tobacco stem-pyrolysed BCs (mTBCs) at different pyrolysis temperatures and assessed pulmonary toxicity in normal human lung epithelial BEAS-2B cells. mTBCs generated at 350 °C (mTBC350) and 650 °C (mTBC650) were analysed and compared for physicochemical properties and adverse effects. Pyrolysis temperature had a significant influence on chemical composition, particle size, specific surface area and aromatic carbon structure. mTBC650 displayed a highly ordered aromatic carbon structure with smaller particle size, high surface area (20.09 m2/g) and high polycyclic aromatic hydrocarbon and metal content. This composition could promote reactive oxygen species accumulation accompanied by greater cytotoxicity, genotoxicity and epithelial barrier malfunction in cultured cells. Thus, the risk of pulmonary toxicity owing to micro-BCs (mBCs) is affected by pyrolysis temperature. Long-term exposure to mBCs produced at high temperatures may lead to or exacerbate pulmonary disease.

Heavy metal stabilization and improved biochar generation via pyrolysis of hydrothermally treated sewage sludge with antibiotic mycelial residue

Hydrothermally treated sewage sludge was pyrolyzed at temperatures of 300, 500, and 700 °C with antibiotic mycelial residue addition ratios of 0, 10, 25, and 50 wt%. The results showed that co-pyrolysis could obviously improve biochar properties. Specifically, adding antibiotic mycelial residue increased the aromaticity and raised the higher heating value of the biochar, which indicates its better potential as fuel. The enrichment in functional groups improved the surface properties of biochar, indicating its better applicability. Additionally, the heavy metal concentrations in biochar were diluted by adding antibiotic mycelial residue, which led to lower toxic inputs to the environment. Moreover, heavy metals were transformed to more stable fractions after co-pyrolysis. A higher pyrolysis temperature and greater antibiotic mycelial residue amounts led to better immobilization of heavy metals, thus preventing their leaching to the environment. This work proposes a promising technique for the synergetic treatment of sewage sludge and antibiotic mycelial residue for improved biochar formation.

Fe-doped biochar derived from waste sludge for degradation of rhodamine B via enhancing activation of peroxymonosulfate

The disposal and management of waste sludge is a considerable challenge for environmental protection and resource utilization. Herein, sludge-based biochar material loaded with nano-Fe₃O₄ (xS@Fe-y) was fabricated via hydrothermal carbonization process and employed as catalyst to activate peroxymonosulfate (PMS) for degrading organic dyes in wastewater. Benefiting from the proper iron content, porous structure and the good dispersibility of iron on the catalyst surface, the proposed 5S@Fe-500 catalyst not only exhibited excellent catalytic activity and durability in the activation of PMS to degrade Rhodamine B (RhB), which was almost completely removed in 10 min (50 mL 50 mg L^-1), but also performed broad application prospects in pollutant degradation. More importantly, the free radical quenching test and electron spin-resonance spectroscopy (ESR) detection demonstrated that O₂•^-, SO₄•^-, OH and ¹O₂ were generated during the process of catalyst activation of PMS. Based on this, a possible reaction pathway for degrading RhB with the aid of 5S@Fe-500 was put forward. It is believed that this work offers a promising reuse method of converting the waste sludge to a high efficiency and low-cost nano magnetic catalyst to activate PMS for degrading refractory organic pollutants in aquatic surroundings.

The factors influencing sludge incineration residue (SIR)-based magnesium potassium phosphate cement and the solidification/stabilization characteristics and mechanisms of heavy metals

Magnesium potassium phosphate cement (MKPC) is prepared from MgO and KH₂PO₄ through an acid-base reaction and has been widely used in the rapid repairs of building structures and the solidification/stabilization (S/S) of heavy metals (HMs). The use of sludge incineration residue (SIR) rich in phosphorus resources to prepare SIR-based MKPC can achieve the reclamation of SIR and efficient HM S/S. Herein, based on the exploration of the optimal MKPC magnesia/phosphate ratio (M/P), the effects of SIR and HMs on the performance of the matrix and its interaction mechanism were comprehensively investigated. The results indicated that the compressive strength of the SIR-based MKPC increased first and then decreased with the gradual increase of SIR incorporation; the optimal was reached at 40.31 MPa when the SIR incorporation was 5 wt%. The peak signal and crystal lattice of Pb₂(PO₄)₃ indicated that there is a mixed effect between HMs (in SIR) and KH₂PO₄. The Visual MINTEQ analysis results also indicated that HMs are precipitated as HM phosphates. The formation of HM phosphates not only increases the M/P (with 30 wt% SIR, M/P increased by 0.019), affecting the microstructure and changing the compressive strength of the matrix, but also promotes the transformation of HMs from the bioavailable to the more stable residual forms. The residual forms of the six HMs were all above 84% after S/S. Therefore, the SIR-based MKPC preparation significantly immobilized the HMs; particularly, the leaching toxicities of Cu (96.6%) and Zn (96.3%) were alleviated.

Applying organic polymer flocculants in conditioning and advanced dewatering of landfill sludge as a substitution of ferric trichloride and lime: Mechanism, optimization and pilot-scale study

In this study, poly dimethyl diallyl ammonium chloride (PDADMAC) and polyacrylamide (PAM) were applied to substitute ferric trichloride (FeCl₃) and lime conditioning for advanced dewatering of landfill sludge (LS). Four response surface methodology (RSM) models were constructed for FeCl₃-lime, FeCl₃-PAM, PDADMAC-lime and PDADMAC-PAM, and identical dosages, namely 29.86, 57.91, 5.73 and 2.99 mg/g dry solids (DS) for FeCl₃, lime, PDADMAC and PAM, were obtained by solving the system of four RSM equations at water content of 60% to investigate conditioning mechanisms. Compared to FeCl₃-lime, PDADMAC-PAM conditioning had strong charge neutralization and bridging performance, and obtained conditioned LS with large flocs size, strong network structure and rapid dewatering rate. By integrating RSM with nonlinear programming for optimization, the total cost of PDADMAC-PAM route was saved by 7.9% and close to FeCl₃-lime, and the optimized condition with dosages of 1.93 and 3.47 kg/t DS was further confirmed by pilot-scale experiments. The results indicated that PDADMAC-PAM was a feasible substitute for FeCl₃-lime in sludge conditioning, and showed more advantage if dewatered sludge was further treated by incineration.

Co-disposal of incineration fly ash and sewage sludge via hydrothermal treatment combined with pyrolysis: Cl removal and PCDD/F detoxification

In this study, the incineration fly ash (IFA) of municipal solid waste (MSW) and municipal sewage sludge (MSS) was synergistically subjected to hydrothermal treatment coupled with pyrolysis (HTP). The regulation of Cl removal and the destruction and detoxification of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) were investigated. The results demonstrated that during hydrothermal treatment (HTT), the Cl removal rate increased with temperature, most of the soluble chlorides were removed, and the acid dissolution of Cl in the hydro-residue was significantly reduced. At hydrothermal temperatures exceeding 180 °C, the variation in the Cl removal rate decreased. Although a certain quantity of PCDD/Fs dissolved in the hydrothermal liquid, the total destruction rate achieved by HTT remained more than 90%. The detoxification rate did not exceed 60% owing to the formation of low-chlorinated PCDD/Fs. Subsequent pyrolysis of the hydro-residue further improved the Cl removal rate, which increased with pyrolysis temperature; the Cl content of pyro-char was reduced to 1.8% and that of the leached acid was less than 0.5 mg/g at 800 °C. In addition, PCDD/Fs in tar and pyrolysis gas were not detected under optimal conditions; the PCDD/F concentration of pyro-char was reduced to 0.17 ng I-TEQ/kg. The destruction and detoxification efficiencies of PCDD/Fs reached 98.49% and 92.50%, respectively. Thus, the method of HTP was conducive to the co-disposal of IFA and MSS.

Bottom ash derived from municipal solid waste and sewage sludge co-incineration: First results about characterization and reuse

This paper reports a complete characterization of the lowest fractions of bottom ash derived from co-combustion of municipal solid waste with sewage sludge (COBA), with the aim to suggest suitable reuse strategies of this by-product. X-Ray Microanalysis is coupled with mineralogical characterization, based on X-Ray Diffraction and Rietveld refinement, to extract information about COBA crystalline and amorphous phases. The composition of different particle size fractions shows that amount of amorphous increases with the increase of fractions sizes. In particular, the finest COBA size fraction (<300 μm) shows more leachable heavy metals (i.e. Pb, and Zn) compared to the investigated fraction with the highest sizes (1400 µm). On the basis of their composition, lowest particle size fractions show a better hydraulic behavior compared to bottom ash obtained from incineration of only municipal solid waste, suggesting possible attractive COBA applications, as for example, Portland cement substitution. In addition, COBA with size fractions in the range of 1000-1400 μm are proposed to be used to produce glass and ceramic. Finally, due to its high amount of reactive amorphous phase (about 73% for fraction size of 1400 μm) COBA is used, in combination with other by-products (coal fly ash and flue gas desulphurization residues), to stabilize municipal solid waste incinerator fly ash produced at the same incinerator plant, following the azure chemistry principle of use a waste to stabilize another waste.

Thermal effects

This review paper focuses on the researches published in 2019 in the field of thermal effects in wastewater and solid waste treatment. The content of this review paper includes five parts: wastewater and sludge treatment, nutrient removal and recovery, membrane technology, heavy metal removal and immobilization, and organic waste utilization. © 2020 Water Environment Federation PRACTITIONER POINTS: Thermal effect plays an important role in treatment of wastewater and sewage sludge. Recovery of nitrogen and phosphorus from wastewater and sewage sludge reduces environmental pollution and offers new products. Temperature improves removal and recovery of heavy metals and organic wastes.

Properties and adsorption mechanism of magnetic biochar modified with molybdenum disulfide for cadmium in aqueous solution

In this paper, we present the preparation of MoS₂-modified magnetic biochar (MoS₂@MBC) as a novel adsorbent by a simple hydrothermal method. MoS₂@MBC contains abundant S-containing functional groups that facilitate efficient Cd(II) removal from aqueous systems. We employed various characterization techniques to explore the morphology, surface area, and chemical composition of MoS₂@MBC; these included Brunauer-Emmett-Teller analysis scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction,. The results indicated the successful decoration of the surface of MoS₂@MBC with iron and MoS₂, and a higher surface area of MoS₂@MBC than that of unmodified biochar. Moreover, adsorption properties including thermodynamics and kinetics were investigated along with the effects of pH, humic acid, and ionic strength on the Cd(II) adsorption onto MoS₂@MBC. The O-, C-, S-, and Fe-containing functional groups on the surface of MoS₂@MBC led to an electrostatic attraction of Cd(II) and strong Cd-S complexation. The Langmuir and pseudo second-order models fitted best for the batch adsorption experiments results. The adsorption capacity of MoS₂@MBC (139 mg g^-1 on the basis of the Langmuir model) was 7.81 times higher than that of pristine biochar. The adsorption process was found to be pH-dependent. The experimental results indicated that MoS₂@MBC is an effective adsorbent for removing Cd(II) from water solutions. Further, the adsorption process involved the complexation of Cd(II) with oxygen-based functional groups, ion exchange, electrostatic attraction, Cd(II)-π interactions, metal-sulfur complexation, and inner-surface complexation. This work provides new insights into the Cd(II) ions removal from water via adsorption. It also demonstrates that MoS₂@MBC is an efficient and economic adsorbent to treat Cd(II)-contaminated water.

Developing a high-quality catalyst from the pyrolysis of anaerobic granular sludge: Its application for m-cresol degradation

This study proposes a novel approach for utilizing granular sludge discharged from anaerobic reactors to prepare an effective and stable catalyst for the removal of refractory contaminants in catalytic wet peroxide oxidation (CWPO). By implementing the response surface methodology, the experimental conditions for m-cresol degradation in CWPO with a HNO₃-modified sludge carbon (GSC-M) as catalyst were explored. The removal efficiencies for m-cresol and total organic carbon (TOC) were 100% and 91.4%, respectively, at the optimal conditions of 60 °C for 120 min with a pH of 3, H₂O₂ dosage of 1.85 g/L, and catalyst dosage of 0.75 g/L. A continuous experiment was conducted for 6 d to investigate the durability and catalytic performance of GSC-M, resulting in a TOC removal above 90% with the catalyst maintaining its original morphology. GSC-M catalyst exhibited excellent stability and low iron leaching (0.34%). The high catalytic degradation could be attributed to a high content of iron species, various types of surface functional groups, porous structures, and the π-π interaction between aromatic clusters in sludge carbon and the benzene ring of m-cresol. Interestingly, GSC-M catalyst exhibited magnetic properties which are beneficial for recycling. Based on the identified intermediates, a possible degradation pathway of m-cresol was proposed.

Pretreatment with Ochrobactrum immobilizes chromium and copper during sludge pyrolysis

To increase the degree of immobilization of heavy metals subjected to sludge pyrolysis, we investigated the effects of pretreating sludge with Ochrobactrum supplementation on the immobilization of chromium (Cr) and copper (Cu) during sludge pyrolysis. The sequential extraction procedure was used to test the metallic forms of Cr and Cu. The immobilization of Cr and Cu was characterized with X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, etc. Results show that: 1) the addition of Ochrobactrum (1-8%) can accelerate the mineralization process in blank sludge and can accelerate the conversion of the oxidizable forms of Cr and Cu into the residual forms subjected to pyrolysis; 2) pretreatment with Ochrobactrum supplementation can inhibit the volatilization of Cr and Cu during sludge pyrolysis, particularly in the case of a high concentration of Cu. Notably, the pretreatment with Ochrobactrum can reduce 20.38-85.09% of the potential ecological risk of Cr and Cu. The pretreatment with Ochrobactrum contributes to the immobilization of Cr and Cu subjected to sludge pyrolysis and thus can prevent pollution of the environment. The results of this study can be used for harmless disposal of municipal sludge.

Simultaneous heavy metal immobilization and antibiotics removal during synergetic treatment of sewage sludge and pig manure

The safe handling of heavy metals and antibiotics during waste disposal has attracted wide attention. In the present study, hydrothermally treated sewage sludge was used for co-pyrolysis with different concentration ratios of pig manure at 600 °C for heavy metal immobilization and antibiotic removal. Heavy metals (except Cd) were mainly retained in the biochar samples due to a high degree of decomposition characteristic of organic matter. Pyrolysis significantly immobilized the heavy metals via converting unstable F1 + F2 + F3 fractions (acid-soluble fraction + reducible fraction + oxidizable fraction) to stable F4 fraction (residual fraction), and more pig manure addition led to improved immobilization performance. After co-pyrolysis, the potential environmental risk of feedstocks reduced significantly and the addition of 50 wt.% of pig manure gave a minimum potential ecological risk index of 10.36 with a low risk of contamination. In addition, six types of antibiotics in feedstocks were decomposed completely during pyrolysis. The co-pyrolysis process showed numerous advantages in the synthetic treatment of sewage sludge and pig manure by reducing the heavy metal toxicity and antibiotic levels.

Synthesis of Fe-impregnated biochar from food waste for Selenium(Ⅵ) removal from aqueous solution through adsorption: Process optimization and assessment

Iron-impregnated food waste biochar (Fe-FWB) was synthesized for Se(Ⅵ) removal from aqueous solution. The effect and interactive effects of different parameters including pyrolysis time, temperature, and Fe concentration were explored using response surface methodology (RSM) to enhance conditions to achieve the highest Se(Ⅵ) removal using Fe-FWB. Pyrolysis time was not significant for Se(Ⅵ) adsorption capacity of Fe-FWB, but temperature and Fe concentration were found to be significant. The highest adsorption was achieved at 3.47 h and 495.0 °C with an Fe concentration of 0.44 M. Fe-FWB synthesized under optimum conditions were used to investigate the kinetic, equilibrium, and thermodynamic adsorption of Se(Ⅵ). Se(Ⅵ) adsorption reached equilibrium within 6 h, and both pseudo-second order and pseudo-first order models were suitable for describing kinetic Se(Ⅵ) adsorption. The Freundlich model was found to suitably fit the equilibrium adsorption data than the Langmuir model. The highest adsorption capacity of Fe-FWB for Se(Ⅵ) was 11.7 mg g^-1. Se(Ⅵ) adsorption on Fe-FWB was endothermic and spontaneous. The enthalpy change for Se(Ⅵ) adsorption was 54.4 kJ mol^-1, and the entropy change was negative at 15-35 °C. The increment of solution pH from 3 to 11 decreased the Se(Ⅵ) adsorption from 19.2 to 7.4 mg g^-1. The impact of interfering anions on Se(Ⅵ) adsorption followed the lineup: HCO₃^- > HPO₄^2- > SO₄^2- > NO₃^-. When compared to some adsorbents, the adsorption capacity of Se(Ⅵ) onto Fe-FWB was comparable even at neutral pH and the Fe-FWB was granular. These results indicate that Fe-FWB has prospective application in the removal of Se(Ⅵ) from aqueous solutions.

Insight into conditioning landfill sludge with ferric chloride and a Fenton reagent: Effects on the consolidation properties and advanced dewatering

The landfill sludge in storage reservoirs needs to be dewatered and disposed of for environmental and engineering purposes. The key factors are the high organic matter content and low permeability. Chemical conditioning is considered an efficient method for adjusting the properties of sludge. In this paper, two typical chemical agents, FeCl₃ and a Fenton reagent with different additive amounts, are studied and compared for dewatering and consolidation purposes. Compression experiments and consolidation experiments are compared, and the coefficient of compressibility and compression index are obtained and compared. Then, the sludge permeability, grain size distribution variations, specific resistance to filtration (SRF) and morphology observations are considered to analyse the treatment mechanism. The results indicate that the properties of landfill sludge will change as the curing time increases. FeCl₃ and Fenton are both effective in improving the consolidation and permeability properties of sludge. For the conditioning process, the optimum FeCl₃ content is 20%, and the process is dominated by coagulation if FeCl₃ is less than 20%; otherwise, it is dominated by hydrolysis. For the Fenton reagent, the optimum Fe²⁺ content and H₂O₂ content are 4% and 12%, respectively. The depolymerization effect of the Fenton reagent leads to the oxidation and recombination of the polar group on extracellular polymeric substances (EPSs). The results can be used to explain the conditioning mechanism of the effective agents of FeCl₃ and Fenton and compare the corresponding consolidation properties. The consolidation characteristics provide a reference for further application of vacuum preloading in the sludge disposal process.

Upcycling of groundwater treatment sludge to an erdite nanorod as a highly effienct activation agent of peroxymonosulfate for wastewater treatment

Groundwater treatment sludge is an Fe-rich waste continuously generated in large amounts through potable water production at groundwater treatment plants. In this study, the sludge was converted to erdite nanorod particles via a one-step hydrothermal route with only adding Na₂S. The sludge was a mixture of ferrihydrite, hematite and Si/Al oxides. After hyddrothemal treatment, erdite was primarily formed from ferrihydrite, which accounted for 91.2% of the Fe species in the sludge, whereas approximately 8.8% of hematite accounted for the Fe species that remained before and after the reaction. The produced erdite nanorods were approximately 200 nm in diameter and 1-3 μm in length. They also exhibited a superior efficiency in peroxymonosulfate (PMS) activation. Nearly 100% quinoline removal (initail concentration = 10 mg L^-1) was achieved when the eridite nanorods were used with PMS. The removal rate of quinoline was much higher than that of raw sludge, nano-scale zero-valent iron, FeS, hematite and magnetite. The erdite nanorods or the PMS alone had a quinoline removal rate of less than 20%. The erdite nanorods were spontaneously hydrolysed to generate Fe²⁺ for PMS activation and to form S species for the reductive cycling of Fe³⁺ to Fe²⁺, which likely promoted PMS activation. This study not only highlighted a facile method to recycle the sludge for erdite nanorod preparation but also presented a novel nanomaterial that could efficiently activate PMS for organic wastewater treatment.

Insight into dewatering behavior and heavy metals transformation during waste activated sludge treatment by thermally-activated sodium persulfate oxidation combined with a skeleton builder-wheat straw biochar

This work focused on dewatering performance and heavy metals (HMs) transformation during waste activated sludge (WAS) treatment by employing thermally-activated sodium persulfate (SPS) oxidation combined with a biochar made of wheat straw (WS-BC). Results demonstrated that the combined treatment was an efficient way to improve WAS dewatering, especially when SPS and WS-BC dosages were adjusted to 120 and 150 mg/g-volatile solids (VS), respectively. After being treated at 70 °C without pH adjustment, standardized-capillary suction time (SCST) was increased to 5.03, centrifuged weight reduction (CWR) was increased to 86.8% and moisture content (MC) was decreased to 42.6%, indicating an excellent dewatering performance. The underlying mechanism identified were: (1) SPS oxidation disintegrated extracellular polymers (EPS) at high temperature, thereby releasing EPS-bound water; (2) WS-BC acted as a skeleton builder conducive to form porous structure and provide free water release channels in WAS, which was favorable to improve dewatering properties of WAS; (3) thermally-activated SPS oxidation significantly improved the solubilization and reduced the leaching toxicity of HMs in WAS; and (4) the following WS-BC treatment further reduced the leaching toxicity of HMs in WAS. Therefore, the combined technology might be a promising strategy to improve WAS dewatering and reduce HMs risks in WAS.

Influence of pyrolysis temperature on the properties and environmental safety of heavy metals in chicken manure-derived biochars

Animal manures usually contain high contents of heavy metals (HMs) and thus pose a considerable threat to human health and environment when applied to soil. In this study, the effect of pyrolysis temperature (300 °C, 400 °C, 500 °C, 600 °C, and 700 °C) on the properties of biochar produced from chicken manure was studied. In addition, the response of speciation, bioavailability, leachability, and environmental risk of HMs in biochar to different pyrolysis temperature was investigated. The results showed that biochars pyrolyzed at high temperatures generally had high pH, ash content, surface area, and stability. As the pyrolysis temperature increased, the total concentrations of Cu, Zn, Cr, and Ni continually increased, whereas those of Pb and Cd initially increased and then decreased. Moreover, the transformation of the bioavailable fractions of HMs into stable fractions obviously increased with increasing pyrolysis temperature. Thus, pyrolysis at high temperature led to a further decrease in the bioavailability, leachability, and environmental risk of six HMs, as shown by DTPA, TCLP, and the potential ecological risk index. Overall, it is convincing that that 700 °C is the optimal temperature when considering the quality and environmental safety of biochar derived from chicken manure.

Catalytic oxidation of NO at ambient temperature over the chars from pyrolysis of sewage sludge

Sludge char (SC) was prepared by pyrolysis of sewage sludge, then nitric acid washing, potassium hydroxide activation, and hydrogen reduction methods were used to seek for the optimum treatment for improving the catalytic oxidation of NO at 30 °C. The optimum NO conversion of 65.6% was achieved when SC was activated and hydrogen-reduced, indicating the promising prospect of NO oxidation catalyst preparation from sewage sludge. The prepared SCs showed an intensive specific pore volume peak at the micropore size of 0.89 nm which is beneficial for NO oxidation. SC characterization like temperature programmed desorption of CO₂/NO/NO₂, in-situ diffuse reflectance infrared Fourier transform spectroscopy, etc. were conducted to reveal the catalytic oxidation mechanisms of NO. The results indicated that the oxygen-containing functional groups, such as carboxylic acid, carboxylic anhydrites and lactones, were largely removed by hydrogen reduction, leading to marked increases of surface basicity, specific surface area, and catalytic activity of SCs. The NO oxidation over the SCs can be explained quite well by the Eley-Rideal reaction model.

Efficient removal of tetrabromobisphenol A (TBBPA) using sewage sludge-derived biochar: Adsorptive effect and mechanism

Sewerage sludge-derived biochars (SSDBCs) with high adsorption capacity and excellent recyclability were synthesized to remove tetrabromobisphenol A (TBBPA) in aqueous system. Scanning electron microscopy, elemental mapping via energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy were used to characterize the morphology, composition, and microstructures. The maximum adsorption capacity of SSDBCs was about 87.02 mg g^-1 at 303 K and pH 7.5. The Langmuir isotherm demonstrated that the adsorption was mainly homogeneous and chemical processes. The kinetics of TBBPA removal well fitted the second-order dynamic model. Thermodynamic analysis showed that the adsorption was exothermic. The effect of π-π dispersive force and hydrogen bonding was proven as the main adsorption mechanism. Multiple cycle runs experiment revealed the excellent stability of recycled SSDBCs. This work provided a promising method of sludge resourceful treatment using an efficient, economic, cyclic, and convenient material for typical organic contaminant in the environment.

Alum sludge as an efficient sorbent for hydrogen sulfide removal: Experimental, mechanisms and modeling studies

This paper firstly reported a systematic study of using alum sludge (waterworks residue) for H₂S adsorption. Various trials were performed at ambient temperature in a fixed bed column to study the effects of H₂S flow rate, sorbent bed depth on the alum sludge adsorption efficiency of H₂S. The Breakthrough Curves were simulated by the Thomas model, Bed Depth Service Time model and Yoon-Nelson models. The mechanisms of H₂S adsorption onto alum sludge was examined by different physiochemical characterizations of exhausted and raw alum sludge. Moreover, the mass transfer coefficients were determined from mathematical descriptions of breakthrough curves. The alum sludge adsorption capacity was determined to be 374.2 mg of H₂S/g, slightly decreasing with the increasing flow rate and increasing with the increasing bed depth. All the three models successfully predict breakthrough curves which could be used for scaling-up purposes. The microporous structure, alkaline pH and the inherent metal species of the alum sludge promoted the formation of metal sulphate species. This study demonstrated that alum sludge could be used as cost-effective, largely available, and efficient sorbent for H₂S removal.

Leaching behavior and potential ecological risk of heavy metals in Southwestern China soils applied with sewage sludge compost under acid precipitation based on lysimeter trials

The ecological risk of heavy metals (HM) resulting from the use of sewage sludge compost (SSC) as an amendment to flower garden soil (FGS) and to abandoned phosphate mine soil (APMS) influenced by acid rain were simulated in lysimeter trials and the potential ecological risk index (PERI) was evaluated with minor modifications. The use of SSC indeed increased the mobility and release of HMs in FGS and APMS under conditions of acid rain. The leaching dynamics of HMs was found to be influenced by Fe/Al oxides and organic matter (OM) in the soil. The application of SSC as a fertilizer to barren APMS dramatically decreased the mobility of Cr, Cu and Pb by 51-56% due to their retention by particulate organic matter, while the leaching of As, Cd and Ni was increased as the result of competition with OM for available Fe/Al oxides (As) and proton-metal exchange reactions that occurred in HM-OM complexes (Cd and Ni). The ecological risk of FGS and APMS resulting from HM migration was actually low (PERI = 0.07-0.12), but the increased potential ecological risk resulting from the use of SSC were estimated to be moderate (a 16.0-33.5% increase in PERI for SSC-amended FGS) or high (a 140% increase in PERI for SSC-amended APMS). Ni, Cd and Cu were identified as the three main HMs responsible for increasing the ecological risk in soil which was mainly composed of fine-grained particles, whereas Cd and As were key ecological risks HMs in soil that was mainly composed of coarse-grained particles.