Dyeing Sludge Research Articles

Preparation of printing and dyeing sludge biochar/macromolecule polymer microsphere as a non-sacrificial persulfate activator with excellent reusability and reduced heavy metals leaching

Biochar-based environmental functional materials offer a promising approach for the resourceful utilization of printing and dyeing sludge (PADS). However, pollutant desorption and heavy metal leaching pose potential environmental risks. In this study, we developed a novel PADS biochar (PADSBC) macromolecule polymer microsphere with outstanding reusability and stability. This microsphere effectively activated peroxymonosulfate (PMS) for the degradation of sulfadiazine (SDZ) while significantly reducing heavy metal leaching compared with PADSBC. The SDZ removal efficiency was not influenced by the type of macromolecule polymers (calcium alginate (CA), cellulose nanofibers (CNF), and polyvinyl alcohol (PVA)) incorporated into PADSBC. In the CA-PADSBC800 microsphere/PMS system, 97 % of SDZ was removed within 40 min, primarily through 1O2-induced oxidative degradation (62 %) and adsorption by CA-PADSBC800 (29 %). Additionally, CA-PADSBC800 prevented SDZ desorption for over five consecutive cycles, while PADSBC800 exhibited 25 % desorption. The presence of HCO3- and humic acids slightly delayed SDZ removal, while Cl- slightly accelerated it. Doping CA, CNF, and CA + CNF into PADSBC800 reduced the leaching of Cu and Cr by over 90 %, and Ni by over 80 %, under both experimental conditions and simulated surface water and landfill leachate situations. The excellent performance of the PADSBC-macromolecule polymer microspheres was attributed to the presence of carboxyl, hydroxyl, and C = O functional groups.

Removal performance of Nitrogen, sulfur and chlorine pollutants during chemical looping combustion of textile dyeing sludge using red mud as an oxygen carrier

Textile dyeing sludge (TDS) is a solid waste produced by the textile industry, which contains a large number of N, S and Cl components. Direct heat treatment of TDS will cause a large number of N, S, Cl pollutants to be released. This study verified the possibility of chemical looping combustion (CLC) of TDS using alkaline WS900 red mud (RM) oxygen carrier, which is rich in Na/Ca, to remove N, S and Cl pollutants at the same time. Raising the reaction temperature in the range of 750–900 °C, prolonging the residence time and increasing the oxygen excess coefficient can promote the oxidation performance of WS900 to NOx precursor, while CO2 atmosphere inhibits the oxidation ability. After 10 cycles of CLC, the Cl fixation rate of WS900 is still close to 100% and its S fixation rate is more than 97%. It can maintain more than 90% of carbon conversion and 86% of CO2 selectivity. Compared with the pyrolysis condition, the removal rate of N pollutants is about 67%. Alkaline RM is also neutralized after absorbing acid gases during CLC, which is convenient for its further treatment and utilization.

Co-pyrolytic drivers and volatiles of 3D printing waste and textile dyeing sludge and their multi-objective optimization

Textile dyeing sludge (TDS) disposal poses a critical environmental challenge due to its complex and hazardous nature. This study explored the co-pyrolysis of 3D printing waste (3DPW), comprising photosensitive resin waste (PRW) and polycaprolactone waste (PCLW), with TDS as a potential co-circularity strategy. Drivers, interactions, and resultant products of the co-pyrolysis were characterized through thermogravimetric experiments. Kinetically, the co-pyrolysis process followed a second-order reaction (F2) model. The co-pyrolysis with PRW enhanced the releases of ketones, aldehydes, and esters, with 2-pentene (26.70%) and 2-ethylacrolein (34.67%) as the primary products. Conversely, the co-pyrolysis with PCLW reduced CO emission and generated tetrahydrofuran (27.28%) and cyclopentanone (44.58%). The co-pyrolysis with 3DPW offers a promising approach for mitigating environmental pollution by transforming hazardous wastes into value-added valuable precursors for fuels and organic chemicals.

Effect of lignin, cellulose and hemicellulose from biomass on sulfur release behavior from dyeing sludge combustion

Co-combustion with biomass is recommended for treating dyeing sludge (DS) from the view of minimizing sulfur emissions. However, the sulfur emission from DS combustion depends on the lignin (L)/cellulose (C)/hemicellulose (H) ratio in the biomass, which is frequently overlooked. This study reveals the effect of biomass on the combustion characteristics and sulfur evolutions of DS combustion using triphenylmethyl mercaptan (TM) and L/C/H as model representative compounds of DS and biomass, respectively. Results indicated that the combustion behavior of the TM–biomass mixture was dominated by TM and secondarily by L/C/H. Although L/C/H brought forward the TM combustion, these biomass components (especially L) deteriorated the combustibility and burnout performances of the combustion. L/C/H promoted CH3SH and SO2 release at low temperatures but inhibited these emissions at high temperatures. The sulfur retention ratios of L, C, and H were 7.53 %, −5.75 %, and 0.17 %. TM–L interaction initially promoted and later inhibited the co-combustion process and the CH3SH and SO2 release. L/C/H addition reduced the E values of the mixtures and the co-combustion kinetics were dominated by Dn models. Residue analysis also indicated a dominant role of L in sulfur fixation, mainly because L additive created abundant adsorption sites in the co-combustion residue.

Energy and exergy analyses, and elemental distribution of microwave-assisted auger pyrolysis of textile dyeing sludge and furfural residue

Microwave-assisted pyrolysis has emerged as a promising treatment method for textile dyeing sludge (DS) and furfural residue (FR), demonstrating the potential to address environmental issues and harness their energy. In this study, microwave-assisted auger pyrolysis of DS and FR was conducted, and the mass balance, elements distribution, energy, exergy and sustainability analysis were investigated. The results indicated that the distribution ratios of carbon (46.14–76.29%), hydrogen (15.04–47.96%), and oxygen (3.62–36.55%) in chars decreased with increasing temperature. As the pyrolysis temperature increased during FR pyrolysis, there was a higher thermal energy loss and a decrease in energy efficiency, energy yield, and exergy efficiency. The pyrolysis temperature and the type of solid wastes are key factors in determining both energy efficiency and exergy efficiency. The energy/exergy utilization efficiencies for FR pyrolysis (57.03–82.24%/55.90–81.82%) surpassed those for DS pyrolysis (26.46–38.93%/23.35–36.65%). The highest energy and exergy efficiencies for DS (i.e. 38.93% and 36.65%) and FR (i.e. 82.24% and 81.82%) were achieved at 450 °C. FR pyrolysis had a lower environmental impact than DS pyrolysis. This study aimed to provide insights into the treatment and disposal of sludge and FR through microwave pyrolysis, considering both energy and exergy aspects.

Volumetric scaling up self-sustained smouldering of printing and dyeing sludge with high moisture content

Smouldering is a novel low-energy, low-cost and high-effective technology for the disposal of high-moisture wastes. Most of the existing reports focus on the lab-scale studies and non-continuous operation, but the study about the large-scale device and continuous process is lacking. This work scales up the self-sustained smouldering from the lab-scaled device to the pilot-scale device for treating the printing and dyeing sludge (PDS) with high moisture (over 60%) under continuous operation. Firstly, the lab-scale smouldering experiments with different sand/PDS mixing ratios are performed, and the 4.5/1 mixing ratio is selected as a suitable mixing ratio. Then, the pilot-scale smouldering experiments are carried out and the influence of the discharging residual interval (50s, 35s, 20s) on the reaction stability is tested under continuous operation. Results indicate that the 35s for discharging residual interval may be suitable to maintain the high-temperature region at a relatively stable location under the studied operating condition. The flue gas concentrations are relatively stable during discharging residual, and the main component concentrations are 17.0–17.8% for O2, 2600∼3000 ppm for CO, 20∼35 ppm for VOCs, and 60∼80 ppm for NOx, 0.0415–0.0836ngTEQ/m3 for dioxin, respectively. The research in this study can give helpful information for the application of the continuous smouldering treatment in the future.

Preparation of high-strength ceramsite from coal gangue and printing and dyeing sludge: Design strategy and modelling mechanism

Disposal and harmless utilization of coal-based solid waste, industrial sludge, and other bulk industrial solid waste is a sustainable and economical approach. In this study, coal gangue and printing and dyeing sludge were recycled to reconstruct high-strength ceramsite using a simple and scalable method. The high silica and alumina contents in coal gangue provided a foundation for ceramsite preparation, and the iron oxide in the printing and dyeing sludge could reduce the sintering temperature. The optimal synthesis conditions were as follows: Coal gangue: printing and dyeing sludge = 40:60; sawdust content of 9mass%; sintering temperature of 1175 °C for 15 min; and preheating temperature of 400 °C for 10 min. The designed ceramsite exhibited high compressive strength of 27.26 MPa, low 1-h water absorption of 1.26%, and bulk and apparent densities of 998 and 1597 kg/m3, respectively; thus, it was categorized as high-strength ceramsite. In determining the modelling mechanism during the sintering process, the silicon and aluminum distributions on the ceramsite surface obtained under the optimal preparation conditions was highly consistent, forming a silicoaluminate that supported the ceramsite strength. Sawdust addition increased the pore structure of the ceramsite. The heavy metals contained in the raw materials were well-immobilized in the ceramsite, indicating that the prepared ceramsites were free from secondary contaminants. Use of bulk solid waste to prepare high-strength ceramsites is a feasible resourceful and harmless utilization method, which is of great significance for bulk solid waste disposal and reuse.

Microwave co-pyrolysis of industrial sludge and waste biomass: Product valorization and synergistic mechanisms

Microwave co-pyrolysis of textile dyeing sludge (TS) and furfural residue (FR) was investigated in this study. Comprehensive pyrolysis index (CPI) was employed to investigate pyrolysis performance and FR addition significantly improved co-pyrolysis. Product distribution showed that increasing FR ratio enhanced the bio-oil and gas with significant reduction in biochar. For bio-oil, FR addition neutralized the strong alkalinity and inhibited the formation of nitrogen compounds. Moreover, bio-oil maintained a very low oxygenates content (0.97–5.97%). For biochar, co-pyrolysis was favorable for the enrichment of C and H and mitigated the potential contamination. FR addition increased the calorific value and contributed to pore development of biochar. Online inspection displayed that co-pyrolysis drastically restrained the release of sulfur-compounds and nitrogenates in volatile, while enhanced the production of alkanes, aldehydes, and ketones. Moreover, the interaction between TS and FR was influenced by their major components. For heavy metals (HMs), Er was at low risk for most HMs and FR addition ratios of 40 and 80 wt% appeared to be more favorable. Principal component analysis (PCA) showed that FR addition changed the decomposition behavior and affected co-pyrolysis process. This study also evaluated the co-pyrolysis synergistic mechanisms of TS and FR. Overall, microwave co-pyrolysis is a novel and effective method to reduce environmental pollution and produce high-quality products, providing new insights for the low consumption and efficient utilization of industrial solid waste.

Microplastics as emerging contaminants in textile dyeing sludge: Their impacts on co-combustion/pyrolysis products, residual metals, and temperature dependency of emissions

As emerging contaminants in textile dyeing sludge (TDS), the presence and types of microplastics (MPs) inevitably influence the combustion and pyrolysis of TDS. Their effects on the co-combustion/pyrolysis emissions and residual metals of TDS remain poorly understood. This study aimed to quantify the impacts of polyethylene (PE) and polypropylene (PP) on the transports and transformations of gaseous emissions and residual metals generated during the TDS combustion and pyrolysis in the air, oxy-fuel, and nitrogen atmospheres. Thermal degradation of the MPs in TDS occurred between 242–600 °C. MPs decomposed and interacted with the organic components of TDS to the extent that they increased the release of VOCs, dominated by oxygenated VOCs and hydrocarbons under the incineration and pyrolysis conditions, respectively. The presence of PE exerted a limited impact on the concentration and chemical form of metals, while PP reduced the residual amount of most metals due to the decomposition of mineral additives. Also, PP (with CaCO3 filler) reduced the acid-extractable content of cadmium, copper, and manganese in the bottom slag or coke but increased that of chromium. This study provides actionable insights into optimizing gas emissions, energy recovery, and ash reuse, thus reinforcing the pollution control strategies for both the MPs and TDS.

Effect of textile waste on incineration behavior of dyeing sludge: Combustion characteristics, gas emissions, kinetics

Co-combustion treatment of homologous solid waste is a promising direction in terms of clean and sustainable production, with this study introducing textile waste to improve the combustion performance of dyeing sludge (DS) via thermogravimetric and mass spectrometric. Results revealed that co-combustion behaviors of DS-textile waste mixtures were mainly divided into three stages (light volatile, heavy volatile and char). Among PA/PET promoted the incineration of light volatiles of DS but postponed the incineration of heavy volatiles and char. Meanwhile α-c facilitated DS incineration in whole temperature interval. The interactions between DS and PA/PET promoted heavy volatiles combustion and restrained char combustion, while that between DS and α-c presented a consistent inhibition on heavy volatiles and char combustion. The introduction of all textile waste improved combustibility, burnout performance and combustion stability of DS, especially for α-c (up to 12.6 times). H2O and CO2 were the main gas products from DS-textile wastes combustion, whose intensities were 3 or 4 orders of magnitude higher than other gas products. With PA/PET/α-c introduction, more main gas products (H2O and CO2) released from DS combustion. Meanwhile appropriate α-c obstructed SO2 release from DS combustion. PA/PET addition enhanced the activation energy of DS, while α-c lowered them. The dominant mechanism of co-incineration process were chemical reaction and heterogeneous reactions orderly.

Energetic and environmental optimizations and byproduct valorization of pyrolysis of textile dyeing sludge with FeCl3

Maximizing the circular economy of hazardous wastes is one of the imperatives not to overshoot the biogeochemical limits of Earth. This study aimed to characterize and valorize the textile dyeing sludge (TDS) pyrolysis in response to the addition of FeCl3 conditioner in the N2 and CO2 atmospheres. The increase fraction of FeCl3 decreased comprehensive pyrolysis index (CPI600) from 1.05 to 0.42 in the N2 atmosphere and from 0.71 to 0.41 in the CO2 atmosphere at 20 °C/min. The addition of FeCl3 diminished the pyrolysis performance, with a more pronounced inhibitory effect occurring in the N2 atmosphere than in the CO2 atmosphere. Regardless of the type of pyrolysis atmosphere, with or without FeCl3, all the TDS pyrolysis released CO, CO2, CH4, aromatics, NH3, and SO2. The main pyrolytic byproducts were nitrides and hydrocarbons, represented by ethanolamine (C2H7NO) and benzene (C6H6). Not only did 4 % FeCl3 addition facilitate the transformation of nitrides into hydrocarbons during the pyrolysis, but it also achieved the optimal energetic performance and emission reduction. Our findings provide novel and actionable insights into the byproduct valorization and pollution control during the TDS pyrolysis and the promotion of carbon capture, utilization, and storage.

Investigation on ash fusion characteristics during the co-gasification of coal and textile dyeing sludge

Investigation on ash fusion characteristics during the co-gasification of coal and textile dyeing sludge

Study on the pollutant emission characteristics of the co-combustion of high S/Cl waste mixed with recycled papermaking waste

Using solid waste to replace coal for power generation is one of the effective ways to reduce CO2 emissions, but the incineration of some high-sulfur and high-chlorine solid waste will lead to the increase of pollutant emissions. This paper proposed a new way to self-control pollutant emissions by co-incinerating different solid wastes with specific properties. In this work, the recycled papermaking waste residue (RPR) and sludge (RPS) with high calcium content were chosen to co-combustion with high-sulfur and high-chlorine solid waste, and the emission characteristics and control mechanism of pollutants under co-combustion were explored. The primary calcium compound in the recycled papermaking solid waste was CaCO3. When RPR and RPS were mixed at a ratio of 1: 1, HCl could be fully captured, resulting in the least emissions. Pollutant emissions was controlled by adding RPR and RPS to high-sulfur textile dyeing sludge and polyvinyl chloride. The sulfur-fixing efficiency rose from 65 % to 91 % and the chlorine-fixing efficiency rose from 69 % to 93 % with an increase in Ca/(S + 0.5Cl) from 1 to 3. As the reaction temperature rose from 700 °C to 1000 °C, the sulfur-fixing efficiency and chlorine-fixing efficiency initially increased and subsequently dropped. The CaCO3 in the material was rapidly decomposed at 800 °C, which accelerated the positive reaction rate with SO2 and HCl, so that the sulfur-fixing efficiency and chlorine-fixing efficiency reached the maximum, which were 89 % and 92 %, respectively. CaSO4 and CaClOH were the principal reaction products at the temperature range of 700 °C to 1000 °C.

Improvement of waste sludge dewaterability by a novel co-conditioning approach with polyaluminum chloride-sludge based carbon-polyacrylamide

A novel sludge conditioning method incorporating sludge-based carbon (SBC) as a filter aid into the coagulation and flocculation co-conditioning process is proposed to strengthen sludge dewaterability, namely the co-conditioning of polyaluminum chloride (PAC)-SBC-polyacrylamide (PAM). The results show that the co-conditioning method can effectively reduce the specific resistance to filtration (SRF), moisture content (MC) of the filter cake and sludge compressibility coefficient (CC), meanwhile increase the net sludge solids yield (YN) of municipal sludge (MS) and textile dyeing sludge (TDS), resulting in a significant improvement in sludge dewatering performance for two types of sludge. The response of sludge dewatering performance to different conditioning agents and the variation characteristics of sludge EPS in co-conditioning process suggest that the co-conditioning mechanism involves a synergistic effect of coagulation, adsorption, and flocculation, which process includes the aggregation and disintegration of microbial cell flocs caused by PAC, the skeleton-building action and biopolymer adsorption by SBC, and sludge floc agglomeration facilitated by PAM. In addition, SBC also optimizes coagulation by electrostatic neutralization, while strengthening flocculation by bridging biopolymers and PAM. Moreover, co-conditioning exhibits a remarkable ability to eliminate dissolved organic matter from the filtrate, leading to an enhancement in the filtrate quality and a reduction in the downstream burden for sludge filtrate treatment. Consequently, the PAC-SBC-PAM co-conditioning emerges as a promising method for sludge pretreatment.

Optimal metal immobilization of oxygen-enriched co-combustion of Zn/Cd hyperaccumulator and textile dyeing sludge with natural or modified kaolin

Eco-friendly disposal of post-harvest and metal-enriched hyperaccumulator biomass is critical for the industrialization of phytoremediation. The addition of Al/Si-based materials, such as sludge and natural mineral additives, is intended to solve the issue of HM metal stabilization in the oxygen-enriched combustion of hyperaccumulator and slow down ash sintering. This study aimed to quantify and reveal the effects of 10% kaolin or modified kaolin on the oxygen-enriched (co-)combustions of Sedum alfredii Hance (SAH) and textile dyeing sludge (TDS) and their combustion kinetics, metal enrichment rates, and immobilization/stabilization mechanisms. The activation energies for the (co-)combustions of SAH and SAH mixed with 10% additives (ST31) were estimated at 75.84–260.97 kJ/mol and 65.58–327.59 kJ/mol, respectively. The (co-)combustion mechanism model was complex, including phase boundary, diffusion, and chemical reaction order. The co-combustion of SAH with 10% modified kaolin increased Cd and Zn immobilization by approximately 66.6% and 3.2%, respectively, compared with the theoretical data at 550 °C. The co-combustion of ST31 with 10% modified kaolin increased Cd immobilization by approximately 160.9% compared with the theoretical data at 750 °C. Obtained via thermal-acid modified treatment, modified kaolin increased the immobilization efficiencies of Zn and Cd through aluminosilicate reactions to form the stable structures (Ca–Zn–Si, Ca–Zn–Al, K–Zn–Si, Zn–Al, Zn–Si, Cd–Al, and Cd–Si) as well as physical absorption. According to the combination of the experimental, multi-objective optimization, and simulation results, SAH mixed 10% modified kaolin at 550 °C and ST31 mixed with 10% modified kaolin at 750 °C were the ideal co-combustion environment. This study can provide new insights into how to best reduce the environmental risks of the hyperaccumulator disposals through the oxygen-enriched combustion technology.

Regulation Mechanism of Solid Waste on Ash Fusion Characteristics of Sorghum Straw under O2/CO2 Atmosphere

Co-combustion of solid waste and biomass can alleviate biomass ash-related problems. To investigate the effects of solid waste on the ash fusion characteristics of biomass and its variation mechanisms under an oxidation atmosphere, an X-ray diffraction, thermogravimetric analyzer (TG), scanning electron microscope (SEM), and FactSage calculation were used to examine the ash fusion behaviors of sorghum straw (SS) with the addition of textile dyeing sludge (TDS) or chicken manure (CM). The ash fusion temperature (AFT) of SS increased gradually with the TDS ash addition; with CM ash addition, the AFT of SS mixtures increased rapidly (0–20%), decreased slightly (20–30%), and finally increased slowly (30–60%). The generations of high melting point (MP) minerals (e.g., KAlSi2O6, Fe2O3, and Fe3O4) led to an increase in the AFT of TDS-SS mixtures. The K+ in silicate was gradually replaced by Mg2+ or Ca2+, which caused the generations of high-MP minerals (e.g., Ca3MgSi2O8, Ca2MgSi2O7, and CaMgSiO4). The TG analysis showed that the additions of TDS or CM ash slowed down the weight loss of SS mixed ash due to the formation of high-MP minerals. The SEM and FactSage calculations were also explained with the AFT change and their variation mechanisms. The result provided effective references for the AFT regulation during the co-combustion of biomass and solid waste.

Catalytic degradation of crystal violet and methyl orange in heterogeneous Fenton-like processes

Metals-loaded (Fe3+, Cu2+ and Zn2+) activated carbons (M@AC) with different loading ratios (0.1%, 0.5%, 1%, 5% and 10%) were prepared and employed for catalytic degradation of dye model compounds (crystal violet (CV) and methyl orange (MO)) in wastewater by heterogeneous Fenton-like technique. Compared with Cu@AC and Zn@AC, 0.5% Fe3+ loaded AC (0.5Fe@AC) had better catalytic activity for dyes degradation. The effects of dyes initial concentration, catalyst dosage, pH and hydrogen peroxide (H2O2) volume on the catalytic degradation process were investigated. Cyclic performance, stability of 0.5Fe@AC and iron leaching were explored. Degradation kinetics were well fitted to the pseudo-second-order model (Langmuir-Hinshelwood). Almost complete decolorization (99.7%) of 400 mg L−1 CV was achieved after 30 min reaction under the conditions of CV volume (30 mL), catalyst dosage (0.05 g), H2O2 volume (1 mL) and pH (7.7). Decolorization of MO reached 98.2% under the same conditions. The abilities of pyrolysis char (PC) of dyeing sludge (DS) and metal loaded carbon to remove dye pollutants were compared. The intermediate products were analyzed and the possible degradation pathway was proposed. This study provided an insight into catalytic degradation of triphenylmethane- and aromatic azo-based substances, and utilization of sludge char.

Effect of electromagnetic induction drying on the drying–incineration process of dyeing sludge: focus on migration and conversion of sulfur

Secondary sulfur pollution in dyeing sludge (DS) during drying and incineration is a major environmental problem necessitating in-situ control. To robustly immobilise sulfur during drying–incineration, the authors introduce an electromagnetic induction (EMI) drying method and reveal the corresponding migration and conversion of sulfur in DS. The EMI-drying efficiency reached 10.69%/min, five times that of thermal drying. EMI drying increases the relative sulfoxide ratio from that of thermal drying. In a sludge–sulfur model, the proposed treatment promoted the oxidation and decomposition of organic sulfur without noticeably affecting the inorganic sulfur. The selective oxidation process during EMI drying promotes sulfur stabilisation in dried DS, decreasing the performance and stability of DS combustion. The sulfur-containing pollutants released during the incineration of DS mainly contain H2S, followed by CH3SH and SO2. EMI drying increases the outputs of SO2 and CH3SH but decreases the outputs H2S and total sulfur compared with the outputs of thermal drying. Under the sulfur-model conditions, EMI promoted the conversion of inorganic sulfur to sulfur-containing gases (especially H2S) during incineration. In contrast, the sulfur stabilised by partial oxidation of organic sulfur in the EMI-dried DS was not easily converted to gaseous sulfur during subsequent combustion. Overall, EMI inhibits the release of sulfur during the combined drying–incineration process of DS.

Co-pyrolysis of textile dyeing sludge/litchi shell and CaO: Immobilization of heavy metals and the analysis of the mechanism

To relieve the secondary contamination of heavy metals (HMs), the synergistic effect of co-pyrolysis of textile dyeing sludge (DS)/litchi shell (LS) and CaO on the migration of HMs was demonstrated in this study. The proportions of Cu, Zn, Cr, Mn, and Ni in the F4 fraction increased to 75%, 55%, 100%, 50%, and 62% at the suitable CaO dosages. When 10% CaO was added, the RI value of DLC-10% was reduced to 7.89, indicating low environmental risk. The characterizations of the physicochemical properties of biochar provided support for the HMs immobilization mechanism. HMs combined with inorganic minerals or functional groups to form new stable HMs crystalline minerals and complexes to achieve immobilization of HMs. The pH value and pore structure also play an important role in improving the immobilization performance of HMs. In conclusion, the results provided a new direction for the subsequent harmless treatment of HMs-enriched waste.

New model for evaluating greenhouse gas emissions from sludge treatment based on fossil and biogenic carbon migration

The treatment and disposal of massive amounts of sewage sludge are potential emission sources of greenhouse gases (GHGs) which strongly depend on the migration of fossil and biogenic carbon into GHGs. Owing to the lack of fossil carbon data and suitable estimation models, CO2 emissions generated from sludge combustion and biodegradation are considered carbon-neutral according to the Intergovernmental Panel on Climate Change (IPCC), increasing the inaccuracy of GHG emissions accounting. Therefore, this study aimed to build a new comprehensive assessment model to accurately evaluate GHG emissions from four typical sludge treatment and disposal approaches. Notably, fossil and biogenic carbon migration were considered, rather than the emission factor method when measuring direct GHG emissions. The results of 14C analysis, which quantified the contributions of fossil and biogenic carbon to the sludge samples, showed significant differences in the average fossil carbon fractions among the four types of sludge: 84.11% for chemical sludge, 25.99% for tannery sludge, 79.19% for textile dyeing sludge, and 45.97% for municipal sludge. According to our model, the route of safely landfilling deep-dewatered sludge possessed the highest GHG emission factors of 1.18–3.75 tCO2eq/t dry sludge. The optimal sludge treatment option was anaerobic digestion followed by land application. This treatment maintained a low and stable level of GHG emissions (0.48–0.54 tCO2eq/t dry sludge) and exhibited low sensitivity to changes in fossil carbon fraction and carbon content according to the sensitivity analysis results. For sludge with a high calorific value and a low fossil carbon fraction, the route of dry incineration + utilization of ash and slag as building materials was carbon-negative and more suitable than anaerobic digestion to achieve carbon neutrality. This study complements the fossil carbon data gaps of sludge in the IPCC Guidelines and provides a more accurate GHG emissions accounting model, which will support the sludge treatment industry in developing effective strategies to reduce GHG emissions.