Effect of hydrothermal carbonization on migration and environmental risk of heavy metals in sewage sludge during pyrolysis

The effect of an acidic environment during the hydrothermal carbonization of sewage sludge on solid and liquid products: The fate of heavy metals, phosphorus and other compounds

The pH of sewage sludge is a crucial factor during the hydrothermal carbonization process that influences the characteristics of the resulting products and migration of certain compounds from the solid to liquid phase. Accordingly, this work is focused on examining the pH impact during the HTC process, in particular, pH equals 2, 3, 4, 5 and 6 on the individual hydrothermally carbonized products generated at 200 °C and 2 h residence time. For this reason, the chemical and physical indicators describing the post-processing liquid and hydrochar were determined. For instance, it was observed that the phosphorus content detected in the liquid, derived at pH2, rose significantly by 80%. Furthermore, decreasing the pH of sewage sludge had a significant impact on the ash content and the calorific value of the hydrochar. Additionally, changes in the specific surface area of hydrochar were noticed: pH = 5 and pH = 6 showed an increase of 20-30%, while for lower pH values a decrease of c.a. 26% was achieved. The distribution of heavy metals between the obtained fractions in the HTC process (solid and liquid) indicated that 92 to almost 100% of the tested heavy metals were transferred to the hydrochar. A significant effect of pH on the distribution between these fractions was observed only for Zn and Ni. For instance, for pH = 2, Zn and Ni in post-processing liquid were 34% and 29%, respectively. In addition, the sequential extraction of heavy metals from hydrochar was also performed in order to identify mobile and non-mobile phases. It was noticed that the acidic environment favours a higher amount of mobile heavy metals in hydrochar. The largest effect was observed for Cd, Pb, Cr and Cu, for which, at pH = 2, their respective amounts in the mobile fraction were 2.7; 3.6; 1.8; 6.2 times higher, compared to the hydrochar without pH correction.

Native desert plants have the potential for phytoremediation of phytotoxic metals in urban cities: implications for cities sustainability in arid environments

Arid regions can benefit from using native desert plants, which require minimal freshwater and can aid in remediating soil phytotoxic metals (PTMs) from traffic emissions. In this study, we assessed the ability of three native desert plants-Pennisetum divisum, Tetraena qatarensis, and Brassica tournefortii-to accumulate phytotoxic metals (PTMs) in their different plant organs, including leaves, stems, and roots/rhizomes. The PTMs were analyzed in soil and plant samples collected from Dubai, United Arab Emirates (UAE). The results indicated significantly higher levels of PTMs on the soil surface than the subsurface layer. Brassica exhibited the highest concentrations of Fe and Zn, measuring 566.7 and 262.8 mg kg-1, respectively, while Tetraena accumulated the highest concentration of Sr (1676.9 mg kg-1) in their stems. In contrast, Pennisetum recorded the lowest concentration of Sr (21.0 mg kg-1), while Tetraena exhibited the lowest concentrations of Fe and Zn (22.5 and 30.1 mg kg-1) in their leaves. The roots of Pennisetum, Brassica, and Tetraena demonstrated the potential to accumulate Zn from the soil, with concentration factors (CF) of 1.75, 1.09, and 1.09, respectively. Moreover, Brassica exhibited the highest CF for Sr, measuring 2.34. Pennisetum, however, could not translocate PTMs from its rhizomes to other plant organs, as indicated by a translocation factor (TF) of 1. In contrast, Brassica effectively translocated the studied PTMs from its roots to the stem and leaves (except for Sr in the leaves). Furthermore, Pennisetum exclusively absorbed Zn from the soil into its leaves and stems, with an enrichment factor (EF) greater than 1. Brassica showed the ability to uptake the studied PTMs in its stem and leaves (except for Fe), while Tetraena primarily absorbed Sr and Zn into its stems. Based on the CF and TF results, Pennisetum appears to be a suitable species for phytostabilization of both Fe and Zn, while Brassica is well-suited for Sr and Zn polluted soils. Tetraena shows potential for Zn phytoremediation. These findings suggest that these plants are suitable for PTMs phytoextraction. Furthermore, based on the EF results, these plants can efficiently sequester PTMs.

Potential availability of metals in anaerobic mono- and co-digestion of pig manure and maize

This study aims to analyse the potential availability of essential metals including as Co, Fe, Ni, Zn, Mn, and Cu and non-essential metals such as Pb, Cr, and Cd within anaerobic mono- and co-digestion of pig manure and maize. The metals partitioning was determined using the Modified BCR (European Community Bureau of Reference) sequential extraction at defined intervals over a 45-days period, correlating changes in metals speciation with key digestion variables. The findings revealed that Cr, Cu, Fe, and Pb were predominantly associated with the oxidisable fraction, while Zn, Mn, and Cd were potentially available in both processes. Notably, NH4+-N and the VFAs, except propionic acid, correlated significantly with the available fractions of Co, Mn, Ni, Zn, Cr, and Pb during mono-digestion of pig manure. The wider pH range and the chemical properties of the feedstock in co-digestion resulted in varied correlations between the metals availability and the digestion variables.

Chemical speciation and environmental risk assessment of heavy metals in ash from smouldering combustion of oily sludge

Smouldering combustion of oily sludge (OS) was carried out to learn the characteristics of heavy metals (HMs) in ash products. Ash collected from four different height layers of the column reactor was analysed for the chemical speciation and environmental risk of six HMs, including Cr, Ni, Cu, Zn, As, and Pb. The results showed that after smouldering combustion, only 21.3-32.2 % of the total HMs was remained in the ash products. The retention of HMs in ash was closely relevant to the carbonaceous destruction efficiency of OS. Smouldering combustion led to the decrease of HMs in acid-soluble/exchangeable fraction from 21.5-49.3 to 0.8-19.8% and oxidizable fraction from 22.6-49.6 to 5.3-21.3, and the increase of reducible fraction from 13.6-38.0 to 30.5-89.1% and residue fraction from 7.8-27.3 to 24.1-63.6%. Upward migration of HMs during smouldering was evidenced by their occurrence in the top clean sand layer, which was dominated in acid-soluble/exchangeable and reducible fractions, accounting for 89.7-99.1% in total. Toxicity extraction and environmental risk studies indicated that smouldering combustion would effectively reduce the toxicity and pollution risk of HMs; however, attention should be paid to the disposal of the top sand layer after smouldering operation due to its high pollution risk of HMs according to the evaluation of Risk assessment code.

Fate of antibiotics and hormones during hydrothermal carbonization of poultry litter: degradation kinetics and toxicity assessment of filtrates and hydrochars

This study investigated degradation kinetics of five selected organic micropollutants (OMPs) present in poultry litter (namely: sulfadiazine, tetracycline, and doxycycline hyclate (antibiotics); estrone and 17-β-estradiol (hormones)) during hydrothermal carbonization (HTC) treatment as the temperature stepwise increased to 250 °C. All five pure OMPs were completely degraded before 250 °C was reached during the HTC process. Nevertheless, presence of poultry litter slowed down the degradation of OMPs. Through elemental mass balance calculation, it is noted that after 15 min (temperature less than 137 °C), 69-82% of organic carbon and 50-66% of organic nitrogen initially consisting part of the target antibiotics were fully mineralized. Both HTC filtrates and hydrochars obtained from poultry litter inhibited Escherichia coli and Bacillus subtilis growth. A combination of high salinity, high nutrients, dissolved organic carbon, and other ions in the filtrate as well as the adsorption of OMPs on hydrochars were probably the reason for the high toxicity.

Effect of hydrothermal carbonization on pyrolysis behavior, nutrients and metal species distribution in municipal sludge

In this study, the effect of hydrothermal carbonation (HTC) on the pyrolysis behavior and the distribution of nutrients and metal species of waste-activated sludge (WAS) was investigated. Results showed that the pyrolysis activation energy range of WAS decreased from 11 to 57 kJ/mol to 10-36 kJ/mol when the hydrothermal carbonization was at 160 °C. As indicated by thermodynamic parameters, the hydrothermal carbonization process reduces the pyrolysis reaction activity of the hydrochar. The results of the chemical analysis indicate that hydrothermal carbonization significantly enhances the release of phosphorus and nitrogen, with maximum recovery at a temperature of 200 °C. The standard measurement and testing protocol revealed that hydrothermal carbonization increased the content of non-apatite inorganic P fraction in hydrochar and enhanced the availability of P. Heavy metal analysis shows that hydrothermal carbonization can strengthen the stability of heavy metals in WAS.

Review on heavy metal contaminants in freshwater fish in South India: current situation and future perspective

The primary natural resource we use in our daily lives for a variety of activities is freshwater for drinking and various developmental goals. Furthermore, the pace of human population increase worldwide is rising rapidly and has a great impact on the Earth's natural resources. Natural water quality has diminished owing to various anthropogenic activities. Water is crucial to the life cycle. On the other hand, chemical and agricultural industries pollute heavy metals. Acute and chronic diseases caused by heavy metals, such as slow metabolism and damage to the gills and epithelial layer of fish species, are divided into two categories. Pollutants can also harm liver tissues and result in ulceration as well as diseases such as fin rot, tail rot, and gill disease. The most prevalent heavy metals are As, Cr, Pb, and Hg, which are systemic toxicants that affect human health. These metals are categorized as carcinogens by the US Environmental Protection Agency and the worldwide agency for cancer research because they cause organ damage even at low exposure levels. The focus of the current study is to review various freshwater sources of heavy metal pollution.

Efficient separation of impurities Fe/Al/Ca and recovery of Zn from electroplating sludge using glucose as reductant

Electroplating sludge (ES), a hazardous waste containing heavy metals and Fe/Al/Ca impurities, is conventionally disposed of in landfills. In this study, a pilot-scale vessel with an effective capacity of 20 L was applied to recycle Zn from real ES. The sludge contained 6.3 wt% Fe, 6.9 wt% Al, 2.6 wt% Si, 6.1 wt% Ca, and 17.6 wt% Zn and was treated using a four-step method. First, ES was dissolved in nitric acid after washing in a water bath at 75 °C for 3 h to produce an acidic solution with Fe, Al, Ca, and Zn concentrations of 4527.2, 3116.1, 3357.7, and 21,275 mg/L, respectively. Second, the acidic solution was added with glucose at an Mglucose/Mnitrate ratio of 0.08 and hydrothermally treated at 160 °C for 4 h. During this step, nearly 100 % Fe and 100 % Al were simultaneously removed as a mixture containing 53.1 wt% Fe2O3 and 45.7 wt% Al2O3. This process was repeated five times, during which the Fe/Al removal and Ca/Zn loss rates remained unchanged. Third, the residual solution was adjusted with sulfuric acid, and over 99 % Ca was removed as gypsum. The residual Fe, Al, Ca, and Zn concentrations were 0.44, 0.88, 52.59, and 31,177.1 mg/L, respectively. Finally, Zn in the solution was precipitated as ZnO with a concentration of 94.3 %. Economic calculations showed that each 1 t of ES processed created revenue of about $122. This is the first study of high-value metal resource recovery using real electroplating sludge at the pilot scale. This work highlights the pilot-scale application of resource utilization of real ES and provides new insights into the recycling of heavy metals from hazardous waste.

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.

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

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

Advances in sewage sludge application and treatment: Process integration of plasma pyrolysis and anaerobic digestion with the resource recovery

Sewage sludge (SS) is an environmental issue due to its high organic content and ability to release hazardous substances. Most of the treatments available are biological, thermal hydrolysis, mechanical (ultrasound, high pressure, and lysis), chemical with oxidation (mainly ozonation), and alkali pre-treatments. Other treatment methods include landfill, wet oxidation, composting, drying, stabilization, incineration, pyrolysis, carbonization, liquefaction, gasification, and torrefaction. Some of these SS disposal methods damage the ecosystem and underutilize the potential resource value of SS. These challenges must be overcome with an innovative technique for the improvement of SS's nutritional value, energy content, and usability. This review proposes plasma pyrolysis and anaerobic digestion (AD) as promising SS treatment technologies. Plasma pyrolysis pre-treats SS to make it digestible by AD bacteria and immobilizes the heavy metals. The addition of Char to the upstream AD process increases the quantity and quality of biogas produced while enhancing the nutrients in the digestate. These two processes are integrated at high temperatures, thus creating concerns about their energy demand. These challenges are offset by the generated energy that can run the treatment plant or be sold to the grid, generating additional cash. Plasma pyrolysis wastes can also be converted into biochar, organic fertilizer, or soil conditioner. These combined technologies' financial sustainability depends on the treatment facility's circumstances and location. Plasma pyrolysis and AD can treat SS sustainably and provide nutrients and resources. This paper explains the co-process treatment route's techno-economic prospects, challenges, and recommendations for the future application of SS valorization and resource recovery.

Biochar derived from hydrolysis of sewage sludge influences soil properties and heavy metals distributed in the soil

Sewage sludge contains a large number of nutrients and dangerous substances, when sludge was processed into sludge hydrochar that was added to the soil, which not only solve the problem of sludge disposal, but also amend the soil and fix pollutants in the soil. However, it was lack of report on the effect of the sludge hydrochar on soil compositions and soil microorganism community structures until now. In the present study, the hydrothermal carbonization method is used to prepare hydrochar from sewage sludge at temperatures of 180 ℃ and 240 ℃ at durations of 6 h and 15 h in this paper. The effects of the prepared sludge hydrochar on soil-derived dissolved organic matter (DOM), the content of total dissolved nitrogen (TDN) and NO₃^--N in soil, and the community structure of soil bacteria and fungi were evaluated. Furthermore, the change rules in heavy metal speciation in soils treated with sludge hydrochar were investigated. With the increase in the preparation temperature and dosage of sludge hydrochar, the main components of DOM changed from soluble microbial byproducts to fulvic acid-like and humic acid-like fractions through UV and fluorescence characterization. The sludge hydrochar prepared at low temperature could significantly increase the contents of TDN and NO₃^--N in the soil. Affected by sludge hydrochar, the dominant phylum of the bacterial community changed from Proteobacteria to Actinobacteria, and the dominant phylum in the fungal community did not change, but its relative abundance increased. Finally, the sludge hydrochar obtained when the carbonization time was 15 h was more beneficial to reduce the total amount and available content of heavy metals in the soil. The study provides a basis for sludge hydrochar application for the soil amendment.

Microbiologically induced calcite precipitation for in situ stabilization of heavy metals contributes to land application of sewage sludge

Microbiologically induced calcite precipitation (MICP) has shed new light on solving the problem of in situ stabilization of heavy metals (HMs) in sewage sludge before land disposal. In this study, we examined whether MICP treatment can be integrated into a sewage sludge anaerobic digestion-land application process. Our results showed that MICP treatment not only prevented the transfer of ionic-state Cd from the sludge to the supernatant (98.46 % immobilization efficiency) but also reduced the soluble exchangeable Pb and Cd fractions by up to 100 % and 48.54 % and increased the residual fractions by 22.54 % and 81.77 %, respectively. In addition, the analysis of the stability of HMs in MICP-treated sludge revealed maximum reductions of 100 % and 89.56 % for TCLP-extractable Pb and Cd, respectively. Three-dimensional fluorescence, scanning electron microscopy-energy-dispersive X-ray spectroscopy, X-ray diffraction, and Fourier-transform infrared spectroscopy analyses confirmed the excellent performance of the ureolytic bacteria Sporosarcina ureilytica ML-2 in the sludge system. High-throughput sequencing showed that the relative abundance of Sporosarcina sp. reached 53.18 % in MICP-treated sludge, and the urease metabolism functional genes unit increased by a maximum of 239.3 %. The MICP technology may be a feasible method for permanently stabilizing HMs in sewage sludge before land disposal.

Hydrothermal treatment coupled with pyrolysis and calcination for stabilization of electroplating sludge: Speciation transformation and environmental risk of heavy metals

Electroplating sludge (ES) produced from treatment of electroplating wastewater is a hazardous waste due to its high content of heavy metals (HMs). This study investigates the feasibility of hydrothermal treatment (HT) coupled with pyrolysis and calcination as a method for safe disposal of ES by immobilizing the soluble fractions of target HMs in ES. The HMs before and after thermal processing were characterized to better understand their speciation transformation and environmental risk. Results showed that over 74% of HMs in ES were accumulated in the resulted solid residues and the other HMs were mainly released into the gas phase. The immobilization rates of HMs from the soluble fractions (F₁ and F₂) to stable fractions (F₃ and F₄) after the separate HT and HT coupled pyrolysis and calcination were up to 82.4%, 78.0% and 80.5%, respectively. HT coupled with high-temperature calcination outperformed HT in terms of converting low volatile HMs to stable residual speciations, such as Cu and Ni. HT coupled with pyrolysis showed the best effect in reducing the environmental risks of Cr. In terms of ecological risk index, the separate HT demonstrated an ideal immobilization effect and toxicity reduction for soluble fractions of HMs, especially for Zn and Mn.

Multiple environmental risk assessments of heavy metals and optimization of sludge dewatering: Red mud-reed straw biochar combined with Fe2+ activated H2O2

In this study, Fe-rich biochar (RMRS-BC) was prepared from red mud and reed straw to improve sludge dewatering and transformation of heavy metals (HMs, including Cd, Cr, Cu, Pb, and Zn). The optimal concentrations of RMRS-BC, Fe²⁺, and H₂O₂ to promote sludge dewaterability were identified by response surface methodology (RSM). The optimal dosages of RMRS-BC, Fe²⁺, and H₂O₂ were 74.0, 104.9, and 75.7 mg/g dry solids (DS), respectively. The corresponding capillary suction time (CST) and water content of sludge cake were 14.3 s and 51.25 wt%. For the improvement mechanism, heterogeneous and homogeneous Fenton reactions occurred due to RMRS-BC and Fe²⁺ activating H₂O₂. The extracellular polymeric substances (EPS) decomposed into dissolved organic matter (proteins and polysaccharides), thereby promoting the transformation of bound water to free water and further reducing the water content of the sludge cake. The research quantitatively assessed the environmental risk of heavy metals in the conditioned sludge cake based on bioavailability and ecotoxicity, pollution levels and potential ecological risks. Compound conditioning using RMRS-BC, Fe²⁺, and H₂O₂ could significantly improve the solubility and reduce the leaching toxicity of HMs. In general, RMRS-BC combined with Fe²⁺ to activate H₂O₂ provided an effective method to enhance sludge dewaterability and reduce HMs risk.

Removal of heavy metals from sewage sludge by chemical leaching with biodegradable chelator methyl glycine diacetic acid

The heavy metals (HMs) contained in sewage sludge are some of the largest obstacles that hamper the usage of sewage sludge in land application (e.g. fertilizer, soil improver). The conventional chelators, e.g., ethylenediaminetetraacetic acid (EDTA), were effective in the remediation of HMs polluted sewage sludge, but suffered from an evident drawback of low biodegradability. Therefore, the applicability of a new biodegradable chelator, methyl glycine diacetic acid (MGDA), to extract HMs from sewage sludge was carried out and compared with EDTA. The experimental parameters affecting the performance of MGDA were optimized. Leaching results showed that in general, MGDA exhibited higher Zn leaching efficiency and similar Cu, Ni and Cr leaching efficiencies with EDTA at same pH and dosage conditions. The maximum Zn, Cu, Ni and Cr leaching efficiencies of MGDA were 94.1% ± 4.5%, 58.2% ± 3.1%, 78.2% ± 2.3% and 54.6% ± 2.5%, respectively. The leaching efficiency plateaued within a reaction time of 4 h, but that of Cu and Ni showed a slightly decreasing trend during hours 4 to 10. In raw sewage sludge, the Zn and Cu were mainly presented in the organically bound fraction, i.e., 45.3 ± 3.2% of total Zn and 48.3 ± 1.4% of total Cu. The addition of MGDA and EDTA caused obvious distribution transformations in Zn and Cu from the organically bound fraction to soluble fraction. According to the reduced partition index calculation, the mobility of Zn, Cu, Ni, and Cr was not significantly lowered after the MGDA treatment. However, the HMs secondary pollution risk of the sludge was reduced due to the drop of the total HMs content after chelator leaching. Findings from this study suggest that MGDA could be a potential environment-friendly alternative for refractory chelators (e.g. EDTA) in the decontamination of HMs from sludge.

Influence of persulfate on transformation of phosphorus and heavy metals for improving sewage sludge dewaterability by hydrothermal treatment

Activated persulfate oxidation has been proven to be an efficient advanced sludge treatment technique to improve sludge dewaterability. This study investigates the influence of persulfate on the transformation of phosphorus (P) and heavy metals (HMs) during the hydrothermal treatment of sewage sludge. The hydrothermal temperature, time, and persulfate concentration are optimized by a Box-Behnken design to obtain the best sludge dewaterability, which is expressed by capillary suction time (CST). The highest CST reduction efficiency is 90.5% at the optimal hydrothermal temperature, time, and concentration of persulfate, which are 145 °C, 2 h, and 150 mg/g dry sludge (DS), respectively. The distribution and transformation of P and HMs with different persulfate concentrations (100-200 mg/g DS) during the hydrothermal process are investigated. Results show that more than 90% of the P and HMs in the sludge are retained in sludge cakes after the hydrothermal treatment. The addition of SPS can make the P in the sludge cakes transform into more stable P species according to the extraction capacity of sequential extracts. It can be found from the ecological risk indexes of the HMs that the addition of SPS during the hydrothermal treatment of sludge can reduce the environmental risk of HMs. This study provides insights into the P and HM distribution and transformation during hydrothermal treatment with persulfate, providing a reference for sludge recovery strategies.

Stabilization of heavy metals in biochar derived from plants in antimony mining area and its environmental implications

Heavy metals pollution in mining soils seriously threatens the ecological environment and human health worldwide. Phytoremediation is considered to be an ideal method to reduce the toxicity, mobility, and bioavailability of heavy metals in the soils. However, the disposal of plant-enriched heavy metals has become a thorny problem. To estimate the effect of pyrolysis on the stabilization of heavy metals in post-phytoremediation plant residues, different biochars were prepared from Conyza canadensis (CC), Gahnia tristis (GT), and Betula luminifera (BL) at different pyrolysis temperatures (300, 450, and 600 °C). Results indicated that pyrolysis was effective in the stabilization of heavy metals (Cr, Ni, As, Sb, Hg, and Pb) in plants and significantly (P < 0.05) decreased the bioavailability of most heavy metals. Among them, GT₆₀₀ prepared by pyrolysis of GT at 600 °C has the best stabilization effect on Sb, which increases the residual fraction by 7.32 times, up to 82.05%. The results of environmental risk assessment show that pyrolysis of biomass at high temperature (600 °C) can effectively mitigate the environmental impact of As, Sb, and Hg. Additionally, the reutilization potential of biochar produced by post-phytoremediation plant residues as adsorbents was investigated. The results of adsorption experiments revealed that all biochars have an excellent performance to adsorb Pb(II), and the maximum adsorption capacity is 139.16 mg g^-1 for CC₄₅₀. The adsorption mechanism could be attributed to complexation, electrostatic attraction, and cation exchange. This study demonstrates that pyrolysis is an effective and environment-friendly alternative method to stabilize heavy metals in plants, and their pyrolysis products can be reused for heavy metal adsorption.

Thermal treatment of sewage sludge: A comparative review of the conversion principle, recovery methods and bioavailability-predicting of phosphorus

Phosphorus is a nutrient that is essential to nature and human life and has attracted attention because of its very limited reserves. Dwindling phosphorus reserves and soaring prices have made the recovery of phosphorus from waste biosolids even more urgent. Waste activated sludge, as the final destination of most of the phosphorus in human domestic and industrial water, has been considered as a reliable source of phosphorus recovery. The thermal treatment method of sewage sludge is currently a relatively environmentally friendly disposal method, which mainly includes incineration, pyrolysis and hydrothermal carbonization. This paper reviews the methods for the recovery of different forms of phosphorus (wet chemical, thermochemical and electrodialysis) from solid products obtained from different sludge thermal treatment methods (incinerated sewage sludge ash, pyrolysis of sewage sludge char and hydrochar) and the bioavailability of the recovered phosphorus products. Incineration of sewage sludge is currently the most established and effective method for recovering phosphorus from the thermal treatment products of sewage sludge. One of the wet chemical methods has been applied on a commercial scale and is expected to be further developed for future industrial applications. Pyrolysis and hydrothermal carbonation still have many research gaps in this field. Based on their principles and laboratory performance, both of them have the potential to recover phosphorus and should be further explored.

Enhanced waste activated sludge dewaterability by the ozone-peroxymonosulfate oxidation process: Performance, sludge characteristics, and implication

Dewatering treatment is an essential step to diminish sludge volume, cut down transportation costs, and improve subsequent disposal efficiency. In this study, ozone-peroxymonosulfate (O₃/PMS) oxidation process was employed to ameliorate sludge dewaterability. Sludge capillary suction time (CST) and water content (Wc) of dewatered sludge cake could reduce from 70.5 s and 81.93% to 26.7 s and 65.65%, respectively, under the optimal dosage of 30 mg/g TS O₃ and 0.4 mmol/g TS PMS. The increased sludge zeta potential, particle size, and fluidity promoted sludge dewatering performance apparently. The decreased hydrophilic, fluorescent EPS components and proteins/peptides-like + Lipids percentage in EPS as well as the ratio of α-helix/(β-sheet + random coil) of treated EPS protein secondary structure was greatly responsible for the enhanced sludge dewaterability. SO₄^- and OH were detected in ozone-peroxymonosulfate process to crack sludge flocs, eliminate hydrophilic substances and liberate bound water. Moreover, the concentrations of both heavy metals and polycyclic aromatic hydrocarbons (PAHs) of sludge after O₃/PMS conditioning were decreased, and the stability and toxicity of heavy metals were also reduced, except Zn. In conclusion, this work offered a comprehensive insight based on ozone-peroxymonosulfate (O₃/PMS) advanced oxidation for improving the sludge dewaterability and environmental implication.

A review on recent disposal of hazardous sewage sludge via anaerobic digestion and novel composting

The high investment cost required by modern treatment technologies of hazardous sewage sludge such as incineration and anaerobic digestion have discouraged their application by many developing countries. Hence, this review elucidates the status, performances and limitations of two low-cost methods for biological treatment of hazardous sewage sludge, employing vermicomposting and black soldier fly larvae (BSFL). Their performances in terms of carbon recovery, nitrogen recovery, mass reduction, pathogen destruction and heavy metal stabilization were assessed alongside with the mature anaerobic digestion method. It was revealed that vermicomposting and BSFL were on par with anaerobic digestion for carbon recovery, nitrogen recovery and mass reduction. Thermophilic anaerobic digestion was found superior in pathogen destruction because of its high operational temperature. Anaerobic digestion also had proven its ability to stabilize heavy metals, but no conclusive finding could confirm similar application from vermicomposting or BSFL treatment. However, the addition of co-substrates or biochar during vermicomposting or BSFL treatment may show synergistic effects in stabilizing heavy metals as demonstrated by anaerobic digestion. Moreover, vermicomposting and BSFL valorization had manifested their potentialities as the low-cost alternatives for treating hazardous sewage sludge, whilst producing value-added feedstock for biochemical industries.

Products distribution and hazardous elements migration during pyrolysis of oily sludge from the oil refining process

Oily sludge is a hazardous waste due to the enrichment of nitrogen, sulfur, PAHs, and heavy metals. In this work, an oily sludge from oil refining factory was pyrolyzed at various temperatures of 250-850 °C in a fixed bed reactor focusing on product distribution and migration of hazardous compounds of PAHs, sulfur, nitrogen-containing compounds, and heavy metals. The mechanism of PAHs formation and migration of nitrogen, sulfur, heavy metals were elucidated by comprehensive analysis of the solid, liquid, and gas products. The distribution and risk analysis of heavy metals were also conducted. The pyrolytic products distribution was markedly affected by pyrolysis temperatures. A maximum oil yield was observed at 500 °C, which can further crack into gas due to secondary reaction. The pyrolytic gas was enriched in the order of CO₂ > CO > CH₄ > H₂. At lower temperatures, CO₂ was largely generated due to the elimination of oxygen-containing functional groups, while H₂ was mainly formed above 450 °C due to the recombination reaction. Higher temperatures promoted more N-/S-containing compounds into tar and gas phases. The N-/S-containing compounds mainly included NH₃, HCN, H₂S, SO₂, COS in the gas phase and amines, indoles, pyridines, nitriles, thiophenes in liquid phase. PAHs with 2-ring to 5-ring were mainly generated due to the secondary reaction at higher temperatures. Moreover, Pyrolysis caused the accumulation of heavy metals in chars. Cd presented a high potential risk while the other heavy metals in chars presented a low risk.

Full recycling of high-value resources from cabbage waste by multi-stage utilization

Cabbage waste (CW) was recycled for generating some potential high-value products by a multi-stage treatment technology. A novel multi-stage utilization process was successfully proposed which consisted of low-temperature extraction, medium-temperature thermolysis, and high-temperature activation. Plant extracts that contain fatty acids, alcohol, furan, and esters were first extracted from raw cabbage waste by ethanol at 70 °C. Pyrolytic oil was obtained by cabbage waste pyrolysis at different medium temperature conditions. The produced carbon residue was further activated at high temperature for environmental purification such as VOCs removal. The performance of this process was characterized by N₂ isothermal adsorption, Fourier transform infrared spectrometer (FTIR), thermogravimetric analysis (TG) and gas chromatography-mass spectrometry (GC-MS). Experimental results showed that the optimum temperatures for extraction, pyrolysis, and activation were 70 °C, 520 °C and 700 °C, respectively. Phenolic-rich pyrolysis solution with 50% phenolic contents could be obtained with the potential application of botanical pesticide. The produced biochar had a BET surface area of as high as 891.12 m²/g. The yields of biochar, pyrolytic liquid, and pyrolytic gas were 43.86%, 17.47%, 38.67%, respectively, and the process energy efficiency was over 42.7%. Applicability and feasibility of this process were also discussed in the aspects of energy quality balance, economy, and environment. The proposed multi-stage thermal-chemical process could be used as a full recycling method for biomass waste.

Treatment processes to eliminate potential environmental hazards and restore agronomic value of sewage sludge: A review

Land application of sewage sludge is increasingly used as an alternative to landfilling and incineration owing to a considerable content of carbon and essential plant nutrients in sewage sludge. However, the presence of chemical and biological contaminants in sewage sludge poses potential dangers; therefore, sewage sludge must be suitably treated before being applied to soils. The most common methods include anaerobic digestion, aerobic composting, lime stabilization, incineration, and pyrolysis. These methods aim at stabilizing sewage sludge, to eliminate its potential environmental pollution and restore its agronomic value. To achieve best results on land, a comprehensive understanding of the transformation of organic matter, nutrients, and contaminants during these sewage-sludge treatments is essential; however, this information is still lacking. This review aims to fill this knowledge gap by presenting various approaches to treat sewage sludge, transformation processes of some major nutrients and pollutants during treatment, and potential impacts on soils. Despite these treatments, overtime there are still some potential risks of land application of treated sewage sludge. Potentially toxic substances remain the main concern regarding the reuse of treated sewage sludge on land. Therefore, further treatment may be applied, and long-term field studies are warranted, to prevent possible adverse effects of treated sewage sludge on the ecosystem and human health and enable its land application.

Efficient conversion of sewage sludge into hydrochar by microwave-assisted hydrothermal carbonization

The treatment of sewage sludge (SS) is an environmental problem worldwide. In recent years, hydrothermal carbonization (HTC) of SS for hydrochar (HC) has attracted extensive attention. This study preliminarily explored the microwave-assisted HTC of SS for the first time. Increasing the reaction temperature (150-250 °C) and reaction time (0-120 min) resulted in a decrease in the HC yield, and it gradually increased with the rising solid-liquid ratio (0.03-0.25 g/mL). Compared with raw SS, the HC products possessed higher aromaticity, carbonization degree, porosity, and polarity, and lower content of soluble nutrients (N/P/K) and leachable heavy metals (Cu, Zn, Pb, Cd, Cr, and Ni), indicating a lower risk of nutrient and heavy metal loss. Attention should be paid to the total contents of Zn and Cd in HC exceeded the permitted value for use in cultivated land with edible crops. The use of CaO as a catalyst improved the yield of HC, made the HC and process water (PW) weakly alkaline, and further passivated the heavy metals in the HC. In the case of H₃PO₄, although the conversion of SS was enhanced (lower content of volatile organic matter in HC), the contents of soluble nutrients (N/P/K) in HC/PW increased, and the migration of Zn and Cd into process water was enhanced. The HCs obtained in this study had poor combustion properties, but higher ignition temperatures than raw SS. PW must be properly treated or recycled because it still contained high contents of organic matter and nutrients. This fundamental study provides basic insights into the microwave-assisted HTC of SS.

Simultaneous removal of heavy metals and bioelectricity generation in microbial fuel cell coupled with constructed wetland: an optimization study on substrate and plant types

A microbial fuel cell coupled with constructed wetland (CW-MFC) was built to remove heavy metals (Zn and Ni) from sludge. The performance for the effects of substrates (granular activated carbon (GAC), ceramsite) and plants (Iris pseudacorus, water hyacinth) towards the heavy metal treatment as well as electricity generation was systematically investigated to determine the optimal constructions of CW-MFCs. The CW-MFC systems possessed higher Zn and Ni removal efficiencies as compared to CW. The maximal removal rates of Zn (76.88%) and Ni (66.02%) were obtained in system CW-MFC based on GAC and water hyacinth (GAC- and WH-CW-MFC). Correspondingly, the system produced the maximum voltage of 534.30 mV and power density of 70.86 mW·m^-3, respectively. Plant roots and electrodes contributed supremely to the removal of heavy metals, especially for GAC- and WH-CW-MFC systems. The coincident enrichment rates of Zn and Ni reached 21.10% and 26.04% for plant roots and 14.48% and 16.50% for electrodes, respectively. A majority of the heavy metals on the sludge surface were confirmed as Zn and Ni. Furthermore, the high-valence Zn and Ni were effectively reduced to low-valence or elemental metals. This study provides a theoretical guidance for the optimal construction of CW-MFC and the resource utilization of sludge containing heavy metals.

An efficient biochar synthesized by iron-zinc modified corn straw for simultaneously immobilization Cd in acidic and alkaline soils

Synthetic functional biochar using agricultural waste as raw materials not only serves as an effective means for recycling waste but can also be employed for the remediation of heavy metal contaminated soil. However, the associated effect and mechanism underlying the immobilization of functional biochar in acidic and alkaline soils remain unclear. In this study, a novel iron-zinc oxide composite modified corn straw (Fe/Zn-YBC) was prepared and applied for the remediation of cadmium-contaminated acidic and alkaline farmland soils. The results showed that the addition of Fe/Zn-YBC increased the pH, cation exchange capacity (CEC), and dissolved organic carbon (DOC) in acidic soil, while increased the pH and DOC in alkaline soil. After immobilization for 42 d, the DTPA-Cd content in acidic and alkaline soils treated with Fe/Zn-YBC decreased by 12.77 %-57.45 % and 23.73 %-52.50 %, respectively. Fe/Zn-YBC treatment promoted the transformation of the exchangeable fraction into the Fe/Mn oxyhydroxide fraction of Cd, and increased the abundance and diversity of bacterial communities in the two soils. Furthermore, the SEM-EDS, XRD and FTIR results for Fe/Zn-YBC separated from the test soils showed that the distribution of Cd adsorbed on Fe/Zn-YBC was positively correlated with Fe, Zn, and O. Additionally, the Cd complexes (CdCO₃, CdZnFe₂O₄ and CdO) detected on Fe/Zn-YBC indicated that the primary immobilization mechanism of Fe/Zn-YBC involved the complexation of Cd and Fe, Zn oxides, and the precipitation of Cd and CO₃^2- in acidic and alkaline soils. The efficient remediation capacity and associated mechanism for this novel functional biochar provide insights for improved remediation of heavy metal contaminated farmland soil.

Characteristics of Hydrochar and Liquid Products Obtained by Hydrothermal Carbonization and Wet Torrefaction of Poultry Litter in Mixture with Wood Sawdust

Poultry farms with floor-standing poultry generate large amounts of poultry litter waste. The direct application of this waste as an organic fertilizer does not ensure sustainable and cost-efficient utilization of all waste fractions, and can also be linked to environmental hazards. Therefore, the development of new technologies is required for processing poultry litter into a safe product with higher added value. In this work, the characteristics of activated carbon derived from hydrochar, along with the liquid products obtained from hydrothermal carbonization (HTC) and the wet torrefaction (WT) of poultry litter, were investigated. Poultry litter (PL) was applied in a mixture with sawdust (SD) in the following ratios: 1:0 (PL/SD 1:0), 1:1 (PL/SD 1:1), 1:2 (PL/SD 1:2), and 2:1 (PL/SD 2:1). WT processing took place in an innovative fluidized bed system in a superheated steam medium with low overpressure (less than 0.07 MPa) at 300 °C and 350 °C for 30–45 min. Conventional HTC processing was performed in a water medium at 220 °C for 1–4 h. The hydrochar produced in the experiments was activated with steam for 1 h at 450–750 °C. The porosity characteristics of activated hydrochar were measured, including pore size, pore volume, and specific surface area, in view of potential industrial applications as an adsorbent. Additionally, the contents of 5-hydroxymethylfurfural (HMF), as high-value product, were determined in the liquid products obtained from HTC processing, as well as in the condensate obtained after WT processing. Specific surface areas of the activated hydrochars may still be too low for application as adsorbent material. Hence, its use as a biofertilizer and soil improver should be preferred. Interestingly, the liquid fraction obtained from the innovative WT process displayed a significantly higher 5-HMF content compared to the conventional HTC process.

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.

Influence of Temperature on Characteristics of Particulate Matter and Ecological Risk Assessment of Heavy Metals during Sewage Sludge Pyrolysis

The formation process of Particulate Matter (PM) during sludge pyrolysis at different temperatures (300–700 °C) and the ecological risks of heavy metals were studied. The results showed that the particulate matter is mainly condensed on the quartz film in a carbon-based organic matter when the pyrolysis temperature was between 200–500 °C in a volatilization process. Inorganic particles was found in the particulate matter when the temperature was raised to 500–700 °C in a decomposition stage. Heavy metals were enriched in particulate matter with increase in pyrolysis temperature. When the temperature reached 700 °C, the concentration of Pb and Cd in the particulate matter significantly increased. The ecological risk assessment showed that heavy metals in the sewage sludge had considerable ecological toxicity. When the pyrolysis temperature reached 700 °C, the ecological toxicity of those heavy metals enriched in the particulate matter decreased considerably.

Co-microwave pyrolysis of electroplating sludge and municipal sewage sludge to synergistically improve the immobilization of high-concentration heavy metals and an analysis of the mechanism

To improve the harmless treatment of high-concentration heavy metals (HMs) in electroplating sludge (ES), this study tried to combine the microwave pyrolysis technology and the addition of municipal sewage sludge (MS) to synergistically improve the immobilization of high-concentration HMs in ES. The results showed that the immobilization rate of HMs was less than 75% in ES pyrolysis biochar. Notably, the immobilization rate of HMs up to 98.00% in co-pyrolysis biochar. Finally, it was found by various characterizations that the organic carbon and inorganic minerals in MS played an important role in the immobilization of HMs through physical and chemical effects. HMs reacted with inorganic minerals to form HMs crystalline minerals (e.g., CuCl, Cu₂NiSnS₄, and NiSi₂, ZnS) to realize the immobilization of HMs. The addition of organic carbon was conducive to the formation of biochar with higher carbon crystallinity (I_D/I_G = 0.96) and larger specific surface area (52.50 m² g^-1), thereby enhancing the physical adsorption to HMs. Meanwhile, the complexation reaction between HMs and functional groups such as -OH, Si-O-Si could also further improve the immobilization of HMs. Therefore, this study provided a technical and theoretical basis for the harmless disposal of waste containing multiple HMs with high-concentrations.

Fabrication of Carbon-Alumina Composites via Catalytic Pyrolysis of Pine Sawdust on Aluminum Dross for Cr(VI) Removal

Carbon-alumina composites are prepared for the efficient removal of Cr(VI) from wastewater. Pristine and acid-treated alumina dross (AD and AAD) are copyrolyzed with pine sawdust to form the respective composites, ADPC and AADPC. Excellent absorption properties with Cr(VI) removal efficiency of 95.08% are demonstrated at 60 °C for an initial concentration of 6 μg/mL. The composites combine the merits of char, which provides a high surface-to-volume ratio with abundant functional groups on the surface, and alumina, which provides metal ions for coprecipitation. Carbon structures of pine, char, and composite were analyzed semiquantitatively using ¹³C NMR by a curve-fitting method. Cr(VI) adsorption is accurately described with chemisorption by the Langmuir isotherm model and a pseudo-second-order kinetic model. The results show that AADPC has more alcohol hydroxyl groups substituted to glucosyl units in amorphous cellulose assigned to the peak at 80 ppm and hemicellulose assigned to peaks at 97 and 101 ppm. Also, it has more phenolic groups in lignin distributed at syringyl units assigned to peaks at 129 and 146 ppm. These oxygen-containing functional groups have a significant influence on Cr(VI) adsorption and reduction to Cr(III) governed by the mechanisms of diffusion, adsorption, complexation, reduction, and coprecipitation. The results of this work provide a new direction for the reuse of biomass and industrial solid wastes to fabricate higher value-added products, i.e., adsorption materials for Cr(VI) removal and stabilization.

Stabilization of heavy metals loaded sewage sludge: Reviewing conventional to state-of-the-art thermal treatments in achieving energy sustainability

Sewage sludge has long been regarded as a hazardous waste by virtue of the loaded heavy metals and pathogens. Recently, more advanced technologies are introduced to make use of the nutrients from this hazardous sludge. Successful recovery of sludge's carbon content could significantly convert waste to energy and promote energy sustainability. Meanwhile, the recovery of nitrogen and trace minerals allows the production of fertilizers. This review is elucidating the performances of modern thermal treatment technologies in recovering resources from sewage sludge while reducing its environmental impacts. Exhaustive investigations show that most modern technologies are capable of recovering sludge's carbon content for energy generation. Concurrently, the technologies could as well stabilize heavy metals, destroy harmful pathogens, and reduce the volume of sludge to minimize the environmental impacts. Nevertheless, the high initial investment cost still poses a huge hurdle for many developing countries. Since the initial investment cost is inevitable, the future works should focus on improving the profit margin of thermal technologies; so that it would be more financially attractive. This can be done through process optimization, improved process design as well as the use of suitable co-substrates, additives, and catalyst as propounded in the review.

Hydrochar derived from municipal sludge through hydrothermal processing: A critical review on its formation, characterization, and valorization

Additional options for the sustainable treatment of municipal sludge are required due to the significant amounts of sludge, high levels of nutrients (e.g., C, N, and P), and trace constituents it contains. Hydrothermal processing of municipal sludge has recently been recognized as a promising technology to efficiently reduce waste volume, recover bioenergy, destroy organic contaminants, and eliminate pathogens. However, a considerable amount of solid residue, called hydrochar, could remain after hydrothermal treatment. This hydrochar can contain abundant amounts of energy (with a higher heating value up to 24 MJ/kg, dry basis), nutrients, and trace elements, as well as surface functional groups. The valorization of sludge-derived hydrochar can facilitate the development and application of hydrothermal technologies. This review summarizes the formation pathways from municipal sludge to hydrochar, specifically, the impact of hydrothermal conditions on reaction mechanisms and product distribution. Moreover, this study comprehensively encapsulates the described characteristics of hydrochar produced under a wide range of conditions: Yield, energy density, physicochemical properties, elemental distribution, contaminants of concern, surface functionality, and morphology. More importantly, this review compares and evaluates the current state of applications of hydrochar: Energy production, agricultural application, adsorption, heterogeneous catalysis, and nutrient recovery. Ultimately, along with the identified challenges and prospects of valorization approaches for sludge-derived hydrochar, conceptual designs of sustainable municipal sludge management are proposed.

The evaluation of immobilization behavior and potential ecological risk of heavy metals in bio-char with different alkaline activation

The bio-char was prepared by co-pyrolysis of municipal sewage sludge and biomass with chemical activation. The alkaline activating agents of KOH and K₂CO₃ were used to develop multilevel pore structure without heavy metal. The proximate analysis, ultimate analysis, SEM, and surface area and porosity analyzer were applied to present the physico-chemical properties and multilevel pore structure of bio-char. After impregnation pretreatment, the KOH provided more functional ingredients and reacted with C to expand pore structure for bio-chars. It was confirmed the specific surface area reached 2122.43 m²/g, and micropore area was 1674.85 m²/g after co-pyrolysis at 800 °C. Through the pretreatment of alkaline activation, the novel evaluation of heavy metal immobilization behavior in bio-char matrix were investigated by BCR sequential extraction and leaching tests. The KOH activation showed prominent immobilization behavior relatively, and the K₂CO₃ activation had more noticeable effects on leaching behavior. For Cu, Ni, Cr, Cd, Pb, and Zn, after co-pyrolysis at 900 °C, the proportion of unstable fraction decreased significantly, and the residual fractions of heavy metals were above 89.44% according to BCR sequential extraction procedure. Under optimal pyrolysis temperature, the E_r value of bio-char reduced to 41.93, and the potential ecological risks decreased from considerable risk to low risk to ensure the further eco-friendly application.

A novel activation-hydrochar via hydrothermal carbonization and KOH activation of sewage sludge and coconut shell for biomass wastes: Preparation, characterization and adsorption properties

A two-stage method of hydrothermal carbonization and chemical activation technology was applied to prepare a novel, large surface area and rich-pore structure activation-hydrochar from sludge sewage and coconut shell due to its mild, low-cost, and well-developed merits. The pore-making mechanism of activation-hydrochar was discussed by FT-IR, XPS, SEM, TG, TG-MS, XRD, and BET characterization. These results illustrated that the first stage of hydrothermal carbonization achieved the rich-pore structure hydrochar via dehydration, decarboxylation, deamination, and rearrangement reactions. The subsequent KOH activation was conducive to the pore-forming process. Specifically, the pore structure of activation-hydrochar was ameliorated and abundant active adsorption sites were obtained by the modification. The adsorption properties of activation-hydrochar on Methylene Blue (MB) and Congo Red (CR) were systematically investigated, and the max adsorption capacities of those were obtained with 623.37 mg/g and 228.25 mg/g, respectively. The pseudo-second-order kinetics and Langmuir models were both fit to elucidate the adsorption process for both dyes. Thermodynamics revealed adsorption performance accompanied by the spontaneous and endothermic processes. In general, the research clearly indicated the synthesis route for activation-hydrochar, and its further adsorption performance, capacity, and mechanism on MB and CR. This research demonstrated that activation-hydrochar with the abundant surface area and rich-pore structure made it a candidate for the production of effective adsorption material. It is prospective to achieve the utilization of wastes and its further application in wastewater treatment.

Insights into the effects of Zn exposure on the fate of tylosin resistance genes and dynamics of microbial community during co-composting with tylosin fermentation dregs and swine manure

Though heavy metals are widely reported to induce antibiotic resistance propagation, how antibiotic resistance changes in response to heavy metal abundances remains unclearly. In this study, the tylosin fermentation dregs (TFDs) and swine manure co-composting process amended with two exposure levels of heavy metal Zn were performed. Results showed that the bioavailable Zn contents decreased 2.6-fold averagely, and the removal percentage of total tylosin resistance genes was around 23.5% after the co-composting completed. Furthermore, the tylosin resistance genes and some generic bacteria may exhibited a hormetic-like dose-response with the high-dosage inhibition and low dosage stimulation induced by bioavailable Zn contents during the co-composting process, which represented a beneficial aspect of adaptive responses to harmful environmental stimuli. This study provided a comprehensive understanding and predicted risk assessment for the Zn-contaminate solid wastes deposal and suggested that low levels of Zn or other heavy metals should receive more attention for their potential to the induction of resistance bacteria and propagation of antibiotic resistance genes.

The migration, transformation, and risk assessment of heavy metals in residue and bio-oil obtained by the liquefaction of pig manure

The total contents and chemical speciation analysis of Zn, Cu, Pb, Cr, Mn, Ni, Cd, and As in pig manure (PM), liquefaction residues (LRs), and bio-oils (BOs) derived from PM by liquefaction with ethanol as a solvent at 180-300 °C were thoroughly investigated in this study. The environment risk assessment, leachability, and bioavailability of heavy metals in PM and LRs were studied. The results showed that more than 75% of heavy metals remained in LRs. The total contents of heavy metals in LRs were markedly elevated, but those in BOs gradually decreased with the increase in liquefaction temperature. Moreover, the acid soluble/exchangeable fraction and reducible fraction (F1 + F2) of heavy metals in LRs and BOs was significantly reduced, while oxidizable fraction and stable fraction (F3 + F4) desirably increased after liquefaction. Furthermore, the potential risk of heavy metals in LRs was decreased in comparison to that in PM, but the risk of Pb, Mn, and As had not been obviously reduced; therefore, the LRs from the liquefaction of PM should be pretreated before recycling. Temperatures from 220 to 260 °C were the optimum conditions for disposing of PM by liquefaction with ethanol.

Transformation and Mobility of Cu, Zn, and Cr in Sewage Sludge during Anaerobic Digestion with Pre- or Interstage Hydrothermal Treatment

Anaerobic digestion (AD) combined with hydrothermal treatment (HT) is an attractive technology for sewage sludge treatment and resource recovery. The fate and distribution of heavy metals in the sludge during combined HT/AD significantly affect the sludge final disposal/utilization options, yet such information is still lacking. This study systematically characterizes the transformation of important heavy metals Cu, Zn, and Cr in sewage sludge during AD with pre- or interstage HT (i.e., HT-AD or AD-HT-AD, respectively). Complementary sequential chemical extraction and X-ray absorption spectroscopy were used to characterize the speciation and mobility of metals. For the HT-AD system, both Cu and Zn predominantly occur as sulfides in HT hydrochars. Subsequent AD favors the formation of Cu₂S and partial transformation of nano-ZnS to adsorbed and organo-complexed Zn species. HT favors the formation of Cr-bearing silicates in hydrochars, whereas Fe(III)-Cr(III)-hydroxide and Cr(III)-humic complex are the predominant Cr species in AD solids. Similar reaction pathways occur in the AD-HT-AD system with some minor differences in metal species and contents, as the first-stage AD changed the sludge matrix. These findings have important implications for understanding the fate and mobility of heavy metals in sludge-derived hydrochars and AD solids.

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.

Experimental Investigation into the Effect of Pyrolysis on Chemical Forms of Heavy Metals in Sewage Sludge Biochar (SSB), with Brief Ecological Risk Assessment

Experimental investigations were carried out to study the effect of pyrolysis temperature on the characteristics, structure and total heavy metal contents of sewage sludge biochar (SSB). The changes in chemical forms of the heavy metals (Zn, Cu, Cr, Ni, Pb and Cd) caused by pyrolysis were analyzed, and the potential ecological risk of heavy metals in biochar (SSB) was evaluated. The conversion of sewage sludge into biochar by pyrolysis reduced the H/C and O/C ratios considerably, resulting in stronger carbonization and a higher degree of aromatic condensation in biochar. Measurement results showed that the pH and specific surface area of biochar increased as the pyrolysis temperature increased. It was found that elements Zn, Cu, Cr and Ni were enriched and confined in biochar SSB with increasing pyrolysis temperature from 300–700 °C; however, the residual rates of Pb and Cd in biochar SSB decreased significantly when the temperature was increased from 600 °C to 700 °C. Measurement with the BCR sequential extraction method revealed that the pyrolysis of sewage sludge at a suitable temperature transferred its bioavailable/degradable heavy metals into a more stable oxidizable/residual form in biochar SSB. Toxicity of heavy metals in biochar SSB could be reduced about four times if sewage sludge was pyrolyzed at a proper temperature; heavy metals confined in sludge SSB pyrolyzed at about 600 °C could be assessed as being low in ecological toxicity.

Characterization of heavy metals in textile sludge with hydrothermal carbonization treatment

Presence of heavy metals in sludge can severely limit land application due to their bioavailability. The current work studied distribution and risk as well as leaching toxicity of heavy metals in textile sludge treated with hydrothermal carbonization (HTC) at different conditions. Treatment temperature and time can significantly affect characteristics of heavy metals in sludge. For the treatment at 220 °C and 3 h, the content of Cu, Cr, Mn, and Zn existed in form F1 + F2 (water soluble and bound to carbonate and Fe-Mn oxides) dropped by 4.7, 7.1, 8.8, and 7.3%, while the content of Cu, Cr, and Mn in form F4 (bound to quartz, feldspars, etc) increased by 12.9, 19.1, and 10.6%, respectively. This effectively lowered the bioavailability and leaching rate of heavy metals in sludge. Addition of weak alkaline Al(OH)₃ could efficiently force the transformation of F1 to F4, possibly because of the increased pH value of sludge. HTC processing might be an effective way of fixing heavy metals in textile sludge.

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.

Co-transport of biochar colloids with organic contaminants in soil column

Co-transport of biochar (BC) colloids with coexisting organic contaminants (OCs) in soil involves complex interactions among BC colloids, OCs, and soil particles, which is significant for the environmental application and risk assessment of BC and yet has not been well addressed. This study explored co-transports of three typical OCs (i.e., phenanthrene (PHN), atrazine (ATZ), and oxytetracycline (OTC)) and BC colloids obtained from bulk BCs with different charring temperatures (200-700 °C) and particle sizes (250 nm, 500 nm, and 1 μm) in a soil column of 9 cm in height. Considerable transport of BC colloids alone was observed and the maximum breakthrough concentration (C/C_o) increased from 0.08 to 0.77 as the charring temperature decreased from 700 to 200 °C. The mobilities of PHN, OTC, and ATZ alone were very low but were greatly increased by co-transports with BC colloids, and their maximum C/C_o values were within 0.05-0.33, 0.03-0.44, and 0.05-0.62, respectively, in the absence and presence of various BC colloids. The enhancement effect of BC colloids on the OC transport decreased with increasing charring temperature or particle size of BC colloids. BC colloids mainly acted as a vehicle to facilitate the transport of OCs, and dissolved organic carbon from BC colloids also contributed to the increased mobility of OCs in dissolved form. These findings provide new insights into co-transport of BC colloids and contaminants in soil.

Enhancement of waste activated sludge dewaterability by ultrasound-activated persulfate oxidation: Operation condition, sludge properties, and mechanisms

The study proposed the ultrasound-activated persulfate oxidation as a novel approach to enhance sludge dewaterability. The results demonstrated that the reduction of water content of dewatered sludge cake was 16.5% and the capillary suction time was reduced to 39.5 s at the optimal conditions of 1.0 mmol/g-TS S₂O₈^2- and ultrasound energy density of 2.0 kW L^-1 within 15 min. The promotion of dewaterability was closely associated with the enlarged floc size, decreased viscosity, and near-neutral zeta potential. Meanwhile, the correlation analysis revealed that the protein in extracellular polymeric substances (EPS) governed sludge dewaterability, especially in loosely bound EPS. Three-dimensional excitation-emission matrix fluorescence spectroscopy, Fourier transform infrared spectroscopy and scanning electronic microscopy analysis revealed that ultrasound-activated persulfate oxidation treatment effectively degraded the gel-like EPS matrix and attacked cells, releasing the moisture which was trapped in EPS and cells. The aggregation of particles promoted the elimination of moisture. Furthermore, heavy metals in conditioned dewatered sludge cakes all satisfied the A level of agricultural land (GB4284-2018) requirements and the chemical speciation distribution of some heavy metals changed significantly.