A novel strategy of tannery sludge disposal - converting into biochar and reusing for Cr(VI) removal from tannery wastewater

Impact of Alkali-Activated Tannery Sludge-Derived Geopolymer Gel on Cement Properties: Workability, Hydration Process, and Compressive Strength

The utilization of tannery sludge (TS) in construction materials not only effectively reduces pollution and resource consumption associated with waste disposal, but also promotes low carbon transformation in the building materials sector, further advancing sustainable development of green construction. This study aims to investigate the impact of sludge-based geopolymer gel on cementitious material performance, revealing the evolution mechanisms of material fluidity, setting time, hydration process, and compressive strength under the coupled effects of tannery sludge and alkali activation, thereby providing a reusable technical pathway to address the resource utilization challenges of similar special solid wastes. A series of alkali-activated composite cementitious materials (AACC) were prepared in the study by partially substituting cement with alkaline activators, TS, and fly ash (FA), through adjustments in TS-FA ratios and alkali equivalent (AE) variations. The workability, hydration process, and compressive strength evolution of AACC were systematically investigated. The experimental results indicated that as the TS content increased from 0% to 100%, the fluidity of fresh AACC decreased from 147 mm to 87 mm, while the initial and final setting times exhibited an exponential upward trend. The incorporation of TS was found to inhibit cement hydration, though this adverse effect could be mitigated by alkaline activation. Notably, 20-40% sludge dosages (SD) enhanced early-age compressive strength. Specifically, the compressive strength of the 0% TS group at 3 d age was 24.3 MPa, that of the 20% TS group was 25.9 MPa (an increase rate of 6.58%), and that of the 40% TS group was 24.5 MPa (an increase rate of 0.82%), whereas excessive additions resulted in the reduction of hydration products content and diminished later stage strength development. Furthermore, the investigation into AE effects revealed that maximum compressive strength (37.4 MPa) was achieved at 9% AE. These findings provide critical data support for realizing effective utilization of industrial solid wastes.

Fruit peel incorporated alginate based magnetic hydrogel bio-composite beads for removal of hexavalent chromium

High adsorption capacity, reusability and sustainability are the most important features sought in the adsorbent preferences to be used in wastewater treatment. In this research work, magnetic composite beads prepared from fruit peels (nectarine and orange) and alginate (ALG) as biopolymers (NAF and OAF) were synthesized by dropping and pH-precipitation method as alternative adsorbents. By encapsulating the adsorbent using alginate and imparting magnetic properties, the separation of the adsorbent from water after the adsorption process has been simplified. Fourier transform infrared spectroscopy analysis (FTIR), scanning electron microscopy (SEM) analysis, energy-dispersive X-ray (EDX)-mapping and X-ray diffraction (XRD) analyses were performed to examine the surface chemical structure and surface morphological structure of these new synthesized biosorbents. The calculated maximum adsorption capacities were 224.3 mg/g for OAF and 256.5 mg/g for NAF at 298 K and pH =2.0. It was observed that the adsorption process for both adsorbents was endothermic and spontaneous. Moreover, the adsorptions of Cr (VI) onto both adsorbents followed the pseudo-second order model and fit the Langmuir isotherm model better. OAF and NAF were found to be reusable with stable adsorption capacity for at least five cycles. Overall, this study demonstrates the performance of OAF and NAF in the removal of Cr (VI) ions from aqueous solutions, thus highlighting the promising potential of these magnetic bio-based adsorbents for sustainable water treatment.

Recent advances toward chromium oxidation and reduction reaction mechanisms during thermal treatment of solid waste: A critical review

Chromium is widely presented in industrial solid wastes like tannery sludge, electroplating sludge and metallurgical slag. These industrial solid wastes usually undergo thermal treatment process to reduce volume and toxicity. However, a significant amount of low-toxicity and low-mobility Cr(III) is determined to be oxidized to highly-toxic and highly-mobile Cr(VI) at high temperature, posing a greater threat to humans and the ecological environment. This paper summarizes the forms of Cr in solid wastes containing Cr, redox reactions mechanisms for different Cr forms, and methods to inhibit Cr(VI) formation during thermal treatment process. The Cr(III) compounds in solid waste containing Cr mainly include Cr(III) hydrates, Cr(III) oxides, Cr(III) hosting spinels and Organic-Cr(III). Cr(III) hydrates are usually oxidized at temperatures above 100 °C, even without the induction of alkali and alkaline earth metals. Compared to the direct reaction of Cr(III) oxides and spinels with O2, Cr(III) can be induced to oxidize at lower temperatures by alkali and alkaline earth metals. A large amount of Cr(III) is oxidized usually at 600-900 °C. Organic-Cr is generally pyrolyzed to CrO3(g), CrO2Cl(g) and Cr2O3(s) at high temperature. CrO2Cl(g) can be released directly into the atmosphere with CrO3(g), or captured by CaO to form CaCrO4. The reduction of Cr(VI) at high temperatures includes the decomposition of unstable Cr(VI) compounds driven solely by temperature, as well as reduction facilitated by acidic oxides. The reduction of Cr(VI) at high temperatures involves the decomposition of unstable Cr(VI) compounds, driven solely by temperature, as well as reduction facilitated by acidic oxides. Typical unstable Cr(VI) compounds include CrO3 and CaCrO4, which begin to decompose at temperatures above 270 °C and 1000 °C, respectively. Cr(III) oxidation and Cr(VI) reduction at high temperature are strongly dependent on the system basicity and the temperature. Subsequently, reducing oxygen content in atmosphere and the system basicity by adding common acidic oxides such as silicon dioxide, phosphate and sulfates exhibited a significant effect on inhibiting Cr(VI) formation during heating solid waste containing Cr. However, Cr oxidation and reduction mechanisms at molecular level have not yet been explored, and more effective measures to inhibit Cr(III) oxidation during thermal treatment of solid waste also should be developed in further works.

Effect of metal-modified sewage sludge biochar tubule on immobilization of chromium in unsaturated soil: Groundwater table fluctuations induced by rainfall

Many studies have studied biochar immobilizing chromium (Cr) in soil. However, few studies were conducted to reduce the environmental risks due to biochar aging in soil. In this study, we adopt FeCl3, MgCl2, and AlCl3 to activate sewage sludge to form modified biochar and produce biochar tubules. Then, the column experiments were carried out to study the effect of fluctuating groundwater table on Cr leaching behavior, total Cr, and fractions distribution with the insertion of biochar tubule. Results showed that the Cr immobilization performance was improved by metal-modification biochar, the biochar tubules can significantly decrease the Cr leaching amounts, retard the Cr downward migration in the soil, and there was a better effect with slightly Cr-contaminated soil. In addition, the immobilization effect is also impacted by the biochar's application mode and the hydrodynamic conditions. Detailedly, the Cr leaching amounts maximally decreased by 33.39%, the residual amounts maximally increased by 10.05% in the soil column, and the exchangeable (EX) and carbonates-bound (CB) fractions were maximally increased by 85.18%, 151.78% at the equal depth of soil column, respectively. BET, SEM-EDS, XRD, and FTIR analyses revealed that biochars' immobilization mechanisms on Cr involved reduction(predominately), physisorption, precipitation, and complexation. Risk assessment demonstrated that the sewage sludge biochar has very low environmental risk. This study indicates that the biochar tubule applied to immobilize Cr in soil has potential and provides new insights into reducing environmental risks due to biochar aging.

A systematic review of innovations in tannery solid waste treatment: A viable solution for the circular economy

Amidst growing global demand for leather goods, the efficient conversion of rawhide and skins into durable leather is crucial, yet approximately 80 % of these materials become solid and liquid waste during tannery operations. Improper management of tannery solid waste poses significant environmental risks, contaminating soil, groundwater, and surface water. This review explores thermochemical, biological, and phytoremediation methods for treating tannery solid waste, emphasizing their role in resource recovery and environmental sustainability. Thermochemical techniques like pyrolysis and gasification convert tannery solid waste into biochar, bio-oil, and syngas, which serve as soil amendments, renewable energy sources, or industrial feedstocks. Biological methods such as composting and anaerobic digestion decompose organic tannery solid waste components into nutrient-rich compost and biogas. Phytoremediation uses plants to remediate contaminants, including heavy metals, from tannery solid waste. These methods mitigate environmental pollution and support the leather industry's transition to sustainable practices, crucial for compliance with global regulations. Moreover, the review offers insights into current efforts and perspectives aimed at achieving a zero-waste policy, emphasizing the importance of a circular economy to alleviate the environmental burden associated with tannery operations and ensure their continued sustainability. Finally, a detailed discussion on the current challenges in terms of technology accessibility and economic feasibility was also discussed.

Detoxification of vancomycin fermentation residue by hydrothermal treatment and pyrolysis: Chemical analysis and toxicity tests

Vancomycin fermentation residue (VFR) is a by-product of the pharmaceutical industry with high ecotoxicity caused by the residual antibiotics, antibiotic resistance genes (ARGs), and heavy metals (HMs). In this study, the detoxification effect of hydrothermal treatment (HT) and pyrolysis for VFR was assessed using chemical analysis and toxicity tests. When VFR was subjected to HT and pyrolysis at ≥400 °C, more than 99.70 % of the residual vancomycin and all ARGs were removed. The HMs contents in VFR followed the order of manganese (676.2 mg/kg) > zinc (148.6 mg/kg) > chromium (25.40 mg/kg) > copper (17.20 mg/kg), and they were highly bioavailable and easily leached. However, HT and pyrolysis (≥400 °C) substantially reduced the bioavailable fractions and leaching properties of the HMs. After HT and pyrolysis at ≥ 400 °C, the potential ecological risk of HMs in VFR was reduced from considerable to moderate/low levels. The elutriate acute toxicity test suggested that HT and pyrolysis at ≥ 400 °C effectively reduced the toxicity of VFR to an acceptable level (p < 0.05). This study demonstrates that HT and pyrolysis (≥400 °C) are promising methods for treating VFR and detoxifying it, and the treated products are safe for further reutilization.

Tannery effluent treatments with mangrove fungi, grass root biomass, and biochar

Tannery effluents contain high amounts of polluting chemicals, such as salts and heavy metals released often to surface waters. New economic and eco-friendly purification methods are needed. Two adsorbing materials and five salt-tolerant fungal isolates from mangrove habitat were studied. Purification experiments were carried out using the pollutant adsorbents biochar and the biomass of vetiver grass (Chrysopogon zizanioides) roots and the fungi Cladosporium cladosporioides, Phomopsis glabrae, Aspergillus niger, Emericellopsis sp., and Scopulariopsis sp., which were isolated from mangrove sediment. They efficacy to reduce pollutants was studied in different combinations. Salinity, turbidity, total dissolved solids, total suspended solids, phenols, nitrogen, ammonia. Biological and chemical oxygen demand (BOD, COD) and several heavy metals were measured. The adsorbents were efficient reducing the pollutants to 15-50% of the original. The efficiency of the combination of biochar and roots was generally at the same level as the adsorbents alone. Some pollutants such as turbidity, COD and ammonium were reduced slightly more by the combination than the adsorbents alone. From all 14 treatments, Emericellopsis sp. with biochar and roots appeared to be the most efficient reducing pollutants to < 10-30%. BOD and COD were reduced to ca 5% of the original. The treatment was efficient in reducing also heavy metals (As, Cd, Cr, Mn Pb, Zn). The fungal species originating from the environment instead of the strains present in the tannery effluent reduced pollutants remarkably and the adsorbents improved the reduction efficiency. However, the method needs development for effluents with high pollutant concentrations to fulfil the environmental regulations.

An insight on the plausible biological and non-biological detoxification of heavy metals in tannery waste: A comprehensive review

A key challenge for the tannery industries is the volume of tannery waste water (TWW) generated during the processing of leather, releasing various forms of toxic heavy metals resulting in uncontrolled discharge of tannery waste (TW) into the environment leading to pollution. The pollutants in TW includes heavy metals such as chromium (Cr), cadmium (Cd), lead (Pb) etc, when discharged above the permissible limit causes ill effects on humans. Therefore, several researchers have reported the application of biological and non-biological methods for the removal of pollutants in TW. This review provides insights on the global scenario of tannery industries and the harmful effects of heavy metal generated by tannery industry on micro and macroorganisms of the various ecological niches. It also provides information on the process, advantages and disadvantages of non-biological methods such as electrochemical oxidation, advanced oxidation processes, photon assisted catalytic remediation, adsorption and membrane technology. The various biological methods emphasised includes strategies such as constructed wetland, vermitechnology, phytoremediation, bioaugmentation, quorum sensing and biofilm in the remediation of heavy metals from tannery wastewater (TWW) with special emphasize on chromium.

Biochar derivation at low temperature: A novel strategy for harmful resource usage of antibiotic mycelial dreg

Antibiotic mycelial dreg (AMD) has been categorized as hazardous waste due to the high residual hazardous contaminants. Inappropriate management and disposal of AMD can cause potential environmental and ecological risks. In this study, the potential of pleuromutilin mycelial dreg (PMD) as a novel feedstock for preparing tetracycline hydrochloride (TC) adsorbent was explored to achieve safe management of PMD. The results suggested that residual hazardous contaminants were completely eliminated after pyrolysis. With the increase of pyrolysis temperature, the yields, H/C, O/C, (O + N)/C, and pore size in PMD-derived biochars (PMD-BCs) decreased, while BET surface area and pore volume increased, resulting in the higher stability of the PMD-BCs prepared from higher temperatures. The TC adsorption of the PMD-BCs increased from 27.3 to 46.9 mg/g with the increase of the pyrolysis temperature. Surprisingly, pH value had a strong impact on the TC adsorption, the adsorption capacity of BC-450 increased from 6.5 to 71.1 mg/g when the solution pH value increased from 2 to 10. Lewis acid-base interaction, pore filling, π-π interaction, hydrophobic interaction, and charge-assisted hydrogen bond (CAHB) are considered to drive the adsorption. This work provides a novel pathway for the concurrent detoxification and reutilization of AMD.

Removal of Cr(vi) in wastewater by Fe-Mn oxide loaded sludge biochar

Sludge biochar loaded with Fe-Mn oxides (FMBC) was prepared and employed to remove Cr(vi) from wastewater. The influences of solution pH, co-existing ion, contact time, adsorption temperature and Cd(vi) concentrations on removing Cr(vi) by FMBC were investigated. The Cr(vi) adsorption on FMBC had strong pH dependence. Additionally, Na+, Mg2+, Ca2+, SiO32-, NO3- and Cl- ions exhibited no influence on Cr(vi) removal efficiency for FMBC, whereas there were inhibition effects of Pb2+, Cu2+, Ni2+, CO32-, SO42-, and PO43- on removing Cr(vi). The Cr(vi) adsorption from solution for FMBC was well described by models of pseudo-second-order and Langmuir, and the largest Cr(vi) removal capacity of FMBC reached 172.3 mg g-1. FMBC had good capacity for treating electroplating wastewater and mineral dissolving wastewater containing Cr(vi). After five regenerations, the 50 and 5 mg L-1 Cr(vi) removing efficiency of FMBC was 82.34% and 97.68%, respectively. The Cr(vi) removal for FMBC involved adsorption-reduction and re-adsorption of Cr(iii) generated by reduction. These results indicated that FMBC has good prospects for remediating Cr(vi)-containing wastewater.

Remediation of biochar-supported effective microorganisms and microplastics on multiple forms of heavy metals in eutrophic lake

In mineral-rich areas, eutrophic lakes are at risk of HMs pollution. However, few papers focused on the repair of HMs in eutrophic environment. Our study analyzed multiple forms of HMs, pore structure and microbial responses in the water-sediment system of eutrophic lake treated with biochar, Effective Microorganisms (EMs) or/and microplastics (MPs). As biochar provided an ideal carrier for EMs, the remediation of biochar-supported EMs (BE) achieved the greatest repairment that improved the bacterial indexes and greatly decreased the most HMs in various forms across the water-sediment system, and it also reduced metal mobility, bioavailability and ecological risk. The addition of aged MPs (MP) stimulated the microbial activity and significantly reduced the HMs levels in different forms due to the adsorption of biofilms/EPS adhered on MPs, but it increased metals mobility and ecological risks. The strong adsorption and high mobility of aged MPs would increase enrichment of HMs and cause serious ecological hazards. The incorporation of BE and MP (MBE) also greatly reduced the HMs in full forms, which was primarily ascribed to the adsorption of superfluous biofilms/EPS, but it distinctly depressed the microbial activity. The single addition of biochar and EMs resulted in the inability of HMs to be adsorbed due to the preferentially adsorption of dissolved nutrients and the absence of effective carrier, respectively. In the remediation cases, the remarkable removal of HMs was principally accomplished by the adsorption of HMs with molecular weight below 100 kDa, especially 3 kDa ∼100 kDa, which had higher specific surfaces and abundant active matters, resulting in higher adsorption onto biofilms/EPS.

Experimental and theoretical study to control the heavy metals in solid waste and sludge during pyrolysis using modified expanded vermiculite

Na+/K+/Mg2+/Ca2+ expansion-modified vermiculite and calcination expansion (700 °C, 800 °C and 900 °C)-modified vermiculite (700-Mg-V, 800-Mg-V and 900-Mg-V) were prepared as additives to control the emission of five heavy metals (Zn, Cr, Cu, Pb, and Cd) during the pyrolysis of municipal sewage sludge, paper mill sludge, municipal domestic waste, and aged refuse. Mg2+-Modified vermiculite obtained via thermally activated calcination at 800 °C retained 65% of heavy metals from all raw materials at 450 °C. Zn, Cr, and Cu retained nearly 90%. Although modified vermiculite could reduce the ecological risk, Cd had an ecological risk level higher than Zn, Cr, Cu, and Pb. The fine textural properties, laminated morphology, and expansion capacity of modified vermiculite were positively correlated with its retention of heavy metals. Heavy metals interacted with the (002) surface of vermiculite, and the reactions were mainly concentrated near the 17-O and surrounding atoms. The heavy-metal monomers were less capable of binding to the (002) surface of vermiculite than the oxides and chlorides of heavy metals. The effect of heavy-metal oxides and chlorides binding to the (002) surface of vermiculite was related to heavy metals.

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

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

Effects of temperature on the migration behaviour of arsenic and chromium in tannery sludge under CO2 gasification

To reduce heavy metals (HMs) contamination from tannery sludge, this study investigated the migration behaviour of arsenic (As) and chromium (Cr) at 700-900 °C using CO2 gasification. The HMs enrichment results showed that As contents of ash decreased (6.42→1.87 mg/kg) while Cr contents increased (41.40→78.24 mg/kg) over 700-900 °C. More Si-O bonds and fewer Ca-O bonds with increasing temperature in ash primarily determined this migration behaviour of HMs. Meanwhile, the proportions of toxic As(III) and Cr(VI) declined from 96.02% and 64.26-76.96% and 21.24%, forming As(0) and Cr(III) with less toxicity. This reduction was conducted via two pathways: (i) carbon reduced As(III)/Cr(VI) and (ii) carbon reduced Fe(II)/Fe(III) to Fe(0), then Fe(0) reduced As(III)/Cr(VI) assisted with carbon via Fe(0)→Fe(II)→Fe(III). However, free calcium ions oxidized As(0)/Cr(III) to As(III)/Cr(VI) at 700 ○C. At higher temperatures, silicate glass conversion of ash immobilized free calcium ions and barely oxidized HMs. Furthermore, this study identified the positive effect of increasing temperature on enhancing the stability of HMs in ash by transforming bioavailable HMs into non-bioavailable HMs, which decreased the leaching toxicity and environmental risk. Regarding HMs emissions control and cold gas efficiency, CO2 gasification treatment of tannery sludge is most effective at 800 °C.

The Fate of Heavy Metals and Risk Assessment of Heavy Metal in Pyrolysis Coupling with Acid Washing Treatment for Sewage Sludge

Pyrolysis is an emerging and effective means for sludge disposal. Biochar derived from sludge has broad application prospects, however, is limited by heavy metals. In this study, the fate of heavy metals (HMs) in pyrolysis coupling with acid washing treatment for sewage sludge was comprehensively investigated for the first time. Most of the HMs redistributed in the pyrolyzed residues (biochar) after pyrolysis, and the enrichment order of the HMs was: Zn > Cu > Ni > Cr. Compared with various washing agents, phosphoric acid presented a superior washing effect on most heavy metals (Cu, Zn, and Cr) in biochars derived at low pyrolysis temperature and Ni in biochars derived at high pyrolysis temperature. The optimal washing conditions for heavy metals (including Cu, Zn, Cr, and Ni) removal by H3PO4 were obtained by batch washing experiments and the response surface methodology (RSM). The total maximum HM removal efficiency was 95.05% under the optimal washing specifications by H3PO4 (acid concentration of 2.47 mol/L, L/S of 9.85 mL/g, and a washing temperature of 71.18 °C). Kinetic results indicated that the washing process of heavy metals in sludge and biochars was controlled by a mixture of diffusion and surface chemical reactions. After phosphoric acid washing, the leaching concentrations of HMs in the solid residue were further reduced compared with that of biochar, which were below the USEPA limit value (5 mg/L). The solid residue after pyrolysis coupling with acid washing resulted in a low environmental risk for resource utilization (the values of the potential ecological risk index were lower than 20). This work provides an environmentally friendly alternative of pyrolysis coupling with acid washing treatment for sewage sludge from the viewpoint of the utilization of solid waste.