Hydrothermal synthesizing sludge-based magnetite catalyst from ferric sludge and biosolids: Formation mechanism and catalytic performance

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

Tannery sludge with high chromium content has been identified as hazardous solid waste due to its potential toxic effects. The safety disposal and valorization of the tannery sludge remains a challenge. In this study, the chromium stabilization mechanism was systematically investigated during chromium-rich tannery sludge was converted to biochar and the removal performance of the sludge biochar (SBC) for Cr(VI) from tannery wastewater was also investigated. The results showed that increase in pyrolysis temperature was conductive to the stabilization of Cr and significant reduction of the proportion of Cr(VI) in SBC. It was confirmed that the stabilization of chromium mainly was attributed to the embedding of chromium in the C matrix and the transformation of the chromium-containing substances from the amorphous Cr(OH)3 to the crystalline state, such as (FeMg)Cr2O5. The biochar presented high adsorption capacity of Cr(VI) at low pH and the maximal theoretical adsorption capacity of SBC produced at 800°C can reach 352 mg Cr(VI)/g, the process of which can be well expressed by Langmuir adsorption isotherm and pseudo second order model. The electrostatic effect and reduction reaction were dominantly responsible for the Cr(VI) adsorption by SBC800. Overall, this study provided a novel strategy for the harmless disposal and resource utilization for the solid waste containing chromium in leather industry.

Effect of Maillard reaction based on catechol polymerization on the conversion of food waste to humus

The pollution and harm of food waste (FW) are increasingly concerned, which has the dual attributes of pollutants and resources. This study aimed to improve the synthesis efficiency of FW humic substances (HS), and investigating the effect of catechol on the formation mechanism and structure of humic acid (HA) and fulvic acid (FA). Results indicated that catechol incorporation could enable to exhibit higher HS yield and more complex structure, especially the maximum particle size of FA reached 4800 nm. This was due to the combination of catechol with multiple nitrogenous compounds, which accelerated molecular condensation. Spectroscopic scans analysis revealed that Maillard reaction occurs first. Subsequently, Maillard reaction products and amino acids were combined with different sites of catechol, which leads to the difference of molecular structure of HS. The structure of FA is characterized by an abundance of carboxyl and hydroxyl groups, whereas HA is rich in benzene and heterocyclic structures. The structural difference was responsible for the disparity in the functional properties of FA and HA. Specifically, the presence of amino, hydroxyl, pyridine, and carboxyl groups in FA contributes significantly to its chelating activity. This research provides an efficient and sustainable unique solution for the high-value of FW conversion, and provides evidence for understanding the structural evolution of HA and FA.

Effects of production temperature on the molecular composition and seed-germination-promoting properties of sludge-based hydrochar-derived dissolved organic matter

Sludge hydrothermal carbonization demonstrates potential for converting sludge into multifunctional carbon materials for soil remediation. However, the influence of dissolved organic matter (DOM) with unclear molecular characteristics in sludge-based hydrothermal carbon on plant growth has not been sufficiently investigated. Herein, the effects of hydrothermal temperature on the molecular transformation pathways and plant-growth-promoting properties of DOM were investigated via FT-ICR MS-based molecular network analyses and seed germination experiments. Results indicated that the highest DOM yield was achieved at 220 °C. During low-temperature (180 °C) hydrothermal treatment, the hydrolysis of biopolymers, as well as the partial condensation and cyclization of small-molecule intermediates, occurred in the sludge. This process produced unsaturated CHNO compounds containing one or two N atoms, which promoted seed germination. Further, the toxicity of DOM to plants increased with rising hydrothermal temperature. This was accompanied by S doping and aromatization reactions, which mitigated the effects of plant growth hormones. This study provides theoretical support for the optimization of sludge hydrothermal treatment and production of plant growth hormones, enhancing the ecological value of sludge-based hydrochar.

Tracking of the conversion and transformation pathways of dissolved organic matter in sludge hydrothermal liquids during Cr(VI) reduction using FT-ICR MS

In this study, the remediation effects of two types of sludge (ferric-based flocculant and non-ferric-based flocculant) on Cr(VI)-polluted wastewater were evaluated to clarify the key components in sludge hydrothermal solutions responsible for reducing Cr(VI) and understand the underlying molecular-level transformation mechanisms. The results revealed that the primary reactions during the hydrothermal processes were deamination and decarboxylation reactions. Correlation analysis highlighted proteins, reducing sugars, amino groups, and phenolic hydroxyl groups as the major contributors. In-depth analysis of the transformation process of functional groups within dissolved organic matter (DOM) and synergistic redox process between Cr(VI) and DOM in hydrothermal solutions demonstrated that phenolic hydroxyl and amino groups gradually underwent oxidation during reduction of Cr(VI) by DOM, forming aldehyde and carboxyl groups, among the others. Time-dependent density functional theory calculations revealed notable shift of reducing functional groups from ground state to excited state following iron complexation, ultimately facilitating reduction reaction. Subsequent investigations, including soil column leaching and seed germination rate tests, indicated that synergistic redox interaction between Cr(VI) and DOM significantly reduced waterborne heavy metal and toxic organic pollution. These findings carry substantial implications for sludge treatment and remediation of heavy metal pollution in wastewater, offering valuable insights into effective environmental remediation strategies.

Magnetic Biochar Derived from Fenton Sludge/CMC for High-Efficiency Removal of Pb(II): Synthesis, Application, and Mechanism

Magnetic biochar composites (MBC) were developed by a simple one-step pyrolysis method using Fenton sludge waste solid and carboxymethyl cellulose sodium. Detailed morphological, chemical, and magnetic characterizations corroborate the successful fabrication of MBC. Batch adsorption experiments show that the synthesized MBC owns high-efficiency removal of Pb(II), accompanied by ease-of-separation from aqueous solution using magnetic field. The experiment shows that the equilibrium adsorption capacity of MBC for Pb(II) can reach 199.9 mg g^-1, corresponding to a removal rate of 99.9%, and the maximum adsorption capacity (q_m) reaches 570.7 mg g^-1, which is significantly better than that of the recently reported magnetic similar materials. The adsorption of Pb(II) by MBC complies with the pseudo second-order equation and Langmuir isotherm model, and the adsorption is a spontaneous, endothermic chemical process. Investigations on the adsorption mechanism show that the combination of Pb(II) with the oxygen-containing functional groups (carboxyl, hydroxyl, etc.) on biochar with a higher specific surface area are the decisive factors. The merits of reusing solid waste resource, namely excellent selectivity, easy separation, and simple preparation make the MBC a promising candidate of Pb(II) purifier.

Treatment of industrial ferric sludge through a facile acid-assisted hydrothermal reaction: Focusing on dry mass reduction and hydrochar recyclability performance

Large amounts of Fenton sludge and waste activated sludge (WAS) are mixed as ferric sludge (FS) in most industrial wastewater treatment plants. The treatment of such waste represents a challenge and quantity-dependent cost, so that a reliable way for FS waste reduction is required. In this study, we develop a facile acid-assisted hydrothermal treatment (HT) for the cost-efficient treatment of hazardous FS waste. Sulfuric acid was dosed at 0.25 mL/g dry solid (DS) to the HT process, which significantly increased the total solid mass reduction (TMR) by 25.1 % and dry mass reduction (DMR) by 104.4 %. The participation of sulfuric acid during the HT process changed the HT reaction pathway from dehydration to demethylation based on the analysis of the derivative thermogravimetric and Van Krevelen diagram. The addition of sulfuric acid improved the release of Fe from FS by 52.9 %, which contributed to the DMR. During the acid-assisted HT, Fe(III) was effectively reduced to Fe(II) within the produced hydrochar, which can be recycled for the Fenton reaction during the degradation of actual industrial wastewater such as pharmaceutical wastewater. Moreover, Sulfuric acid facilitated the generation of sulfonated hydrochar, which was efficient as an adsorbent for the complete removal of some metals such as Cu(II) - cation metal (98.8 %) and Cr(VI) - anion metal (99.9 %). This study firstly provides a novel and reliable approach for hazardous FS reduction and pointed out the recycling of hydrochar as the supplement for the Fenton reaction and adsorbents for some hazardous heavy metals.

Application of sludge biochar nanomaterials in Fenton-like processes: Degradation of organic pollutants, sediment remediation, sludge dewatering

In today's society, wastewater sludge has become solid waste, and the preparation of wastewater sludge into sludge biochar nanomaterials (SBCs) for resource utilization has become a promising method. SBCs have advantages over other biomasses, including their complex composition, wide range of raw materials, and especially the presence of various transition metals with catalytic properties. Heterogeneous Fenton processes using SBCs as catalyst carriers have shown great potential in the removal of pollutants. In this review, the synthesis methods of SBCs are reviewed and the effects of different synthesis methods on their physicochemical properties are discussed. Furthermore, the successful applications of raw SBCs, metal-modified SBCs, and Fenton sludge-SBCs in organic pollutant degradation, sediment remediation, and sludge dewatering are reviewed. The mechanisms occurring with different metals as active sites are explored, and the review shows that the degradation efficiency and stability of SBCs are very satisfactory. We also provide an outlook on the future development of SBCs. We hope that this review will help readers gain a clearer and deeper understanding of SBCs and promote the development of SBCs.

Iron valence state evolution and hydrochar properties under hydrothermal carbonization of dyeing sludge

Iron (Fe) migration mechanisms and hydrochar properties in dyeing sludge hydrothermal carbonization (HTC) are important topics in wastewater treatment. HTC treatment of sludge produces wastewater containing Fe so it is necessary to study the migration behavior of Fe during HTC treatment. This study investigated the basic properties and Fe migration behavior of hydrochar during HTC treatment supplemented with nitric acid (HNO₃). The results showed that the carbonization degree and yield of hydrochar treated with the HNO₃ solution (HHC) were much lower than those of hydrochar treated with ultrapure water (WHC). The variation of total Fe (TF) concentration indicated that the decomposition of organic material and dissolution of minerals in the aqueous release of Fe during the liquid phase, led to much lower TF concentrations compared to the original dyeing sludge. Fe release was further enhanced with the addition of HNO₃ and increase of temperature, rendering a much lower TF concentration of the HHC compared to the WHC. The variations of Fe³⁺ and Fe²⁺ concentrations indicated that the HTC-treated hydrochar contained more Fe²⁺, caused by Fe³⁺ reduction with hydroxyl methyl-furfural and glucose in the liquid and subsequent Fe²⁺/Fe³⁺ transferral to the solid hydrochar phase. X-ray diffraction (XRD) showed that the main Fe content in WHC was FeO(OH), while HHC contained mainly Fe(SO₄)(OH)•2H₂O and Fe₃O₄. XPS and XRF showed that Fe could more easily enter the internal pores of the hydrochar instead of being deposited on the surface. This study provided more insights on Fe migration behavior during HTC treatment.

Process water recirculation for catalytic hydrothermal carbonization of anaerobic digestate: Water-Energy-Nutrient Nexus

The process water (PW) from acid-catalyzed hydrothermal carbonization (HTC) is still an environmental burden due to the enriched organics, nutrients, and salts. This study proposed a novel strategy to valorize food waste digestate into multifunctional hydrochar by recirculating the PW in the HCl-catalyzed HTC process. The produced multifunctional hydrochar could be utilized as a high-quality solid fuel with HHV of 27.9 MJ kg^-1 (hydrochar without PW recirculation) and a slow-release fertilizer by converting the complex Ca and P compounds from the food waste digestate into a Ca-P deposit (hydroxyapatite) with more than a 93 % P recovery rate (hydrochar with PW recirculation). Adding fresh HCl in the HTC PW recirculation system only displayed a marginal catalytic impact on the hydrochar properties after two cycles of recirculation. This study demonstrated the importance of inherent Ca in the feedstocks and the dual role of HCl in the HTC with PW recirculation.

Effect of Maillard reaction on the formation of humic acid during thermophilic phase of aerobic fermentation

This study aimed to explore the main pathway of humic acid (HA) formation during the thermophilic phase (TP) of aerobic fermentation, clarify the contribution of Maillard reaction. These experiments were carried out on cow dung, chicken manure and rice straw. Results indicated that the maximum temperature reached 60.2℃ during TP led to a sharp decrease in microbial abundance, while the production of HA increased. The network analysis indicated that microorganisms did not participate in the formation of HA and may be dominated by abiotic pathways. In addition, proteins and sugars were consumed at the highest rate during TP, and the trends were similar to HA formation. These findings suggested that the formation of HA has relationship to Maillard reaction, because TP provided suitable reaction conditions for Maillard reaction. Therefore, these results elucidated the contribution of Maillard reaction in HA formation during TP, and provided theoretical support for directional humification.

Revealing carbon-iron interaction characteristics in sludge-derived hydrochars under different hydrothermal conditions

Hydrothermal conversion is seen as a potential sustainable solution for the disposal and utilization of sewage sludge. One-step hydrothermal carbonization was used to prepare iron-based sludge hydrochars, and the microstructure properties of hydrochars under different hydrothermal conditions were investigated, with emphasis on the inherent interaction mechanisms between carbon and iron. The aromaticity of hydrochars increased with increasing hydrothermal temperature and time, whereas the specific surface area and pore volume as well as magnetic characteristics of hydrochars were only contingent on temperature. Once the temperature reached 160 °C, Fe₂O₃ in sludge was completely transformed into Fe₃O₄ in hydrochars. Simulated experiments suggest that glucose is more advantageous than protein in the iron transformation and mesopore formation. The coexistence of glucose, protein, and FeCl₃ improved the aromaticity as well as specific surface area and pore volume of hydrochars. This study provides a basis for designing high performance iron-based sludge hydrochars.

Preparation of sludge-based materials and their environmentally friendly applications in wastewater treatment by heterogeneous oxidation technology

The sludge resource utilization and the high value-added development are environmentally friendly means for sludge treatment. With its rich organic substances and metals content, sludge can replace activated carbon and become a widely used carbon-based material, such as sludge-based activated carbon (SBAC). Meanwhile, as a heterogeneous catalyst, sludge-based catalyst (SBC) can solve the requirements of traditional Fenton catalysts for pH, metal ion leaching, and catalyst recycling. In this paper, combining the properties of SBAC/SBCs, the characteristics of the three methods of activation, support, and hydrothermal preparation of SBAC/SBCs are reviewed. In general, it is necessary to select an appropriate preparation method based on pollutants and environmental treatment goals. Furthermore, compared with other catalysts, SBC heterogeneous oxidation has obvious advantages in refractory organic pollutants. And the reaction mechanism usually involves SO₄·^-, ·OH, O₂·^-, and ¹O₂ processes. Finally, some possible directions for future research involving environmentally friendly SBAC/SBCs are proposed.

Recent Development in Sludge Biochar-Based Catalysts for Advanced Oxidation Processes of Wastewater

Sewage sludge as waste of the wastewater treatment process contains toxic substances, and its conversion into sludge biochar-based catalysts is a promising strategy that merges the merits of waste reutilization and environmental cleanup. This study aims to systematically recapitulate the published articles on the development of sludge biochar-based catalysts in different advanced oxidation processes of wastewater, including sulfate-based system, Fenton-like systems, photocatalysis, and ozonation systems. Due to abundant functional groups, metal phases and unique structures, sludge biochar-based catalysts exhibit excellent catalytic behavior for decontamination in advanced oxidation systems. In particular, the combination of sludge and pollutant dopants manifests a synergistic effect. The catalytic mechanisms of as-prepared catalysts in these systems are also investigated. Furthermore, initial solution pH, catalyst dosage, reaction temperature, and coexisting anions have a vital role in advanced oxidation processes, and these parameters are systematically summarized. In summary, this study could provide relatively comprehensive and up-to-date messages for the application of sludge biochar-based catalysts in the advanced oxidation processes of wastewater treatment.

Preparation and application of Fe/biochar (Fe-BC) catalysts in wastewater treatment: A review

The removal of organic pollutants from water environments is a challenging problem. Fe-based BC (Fe-BC) composites are promising catalysts for generating reactive oxygen species (ROS) for environmental remediation considering their low costs and excellent physicochemical surface characteristics. The synthesis methods, properties, applications, and the mechanism of Fe-BC for removing pollutants are reviewed. Various methods have been used to prepare Fe-BC composites, and the synthetic methods and conditions used affect the properties of the Fe-BC material, thereby influencing its pollutant removal performance. The mechanisms of pollutant removal by Fe-BC are intricate and include adsorption, degradation and reduction. Fe loading on BC could improve the performance of BC by affecting its surface area, surface functional groups and electron transfer rate. Moreover, research gaps and uncertainties that exist in the use of Fe-BC were identified. Finally, the problems that need to be solved to make Fe-BC suitable for future applications are described.

Revealing the heating value characteristics of sludge-based hydrochar in hydrothermal process: from perspective of hydrolysate

Hydrothermal treatment (HT) is a promising method to convert sewage sludge to hydrochar biofuel. The heating value is directly correlated to the carbon content in hydrochar; however, the release of organic matter from sludge to hydrolysate and the transfer of the Maillard reaction products generated in the hydrolysate to the solid phase alter the carbon content in hydrochar. In this study, the relationship between hydrolysate and heating value of sludge-based hydrochar was presented, aiming to explain how the calorific value of hydrochar was affected by HT conditions. We adopted a direct combustion test to verify its clean combustion features. Hydrochar derived at 260 °C and residence time of 4 h (HC 260-4) exhibited the highest calorific value (HHV_daf = 26.23 MJ/kg) with an energy density of 1.43, and its fuel characteristics were similar to those of lignite. The increase in the HT temperature and residence time up to 260 °C and 4 h, respectively, was beneficial for enhancing HHV_daf. Conversely, further increase of the HT temperature to 300 °C and HT time to 6 h yielded a decrease in HHV_daf. Investigation of the underlying mechanism revealed that the protein and polysaccharide releasing from sludge to hydrolysate occurred the Maillard reaction (MR). The generated humic-like Maillard reaction product (MRP) was transferred to hydrochar, inducing an increase in the carbon content and calorific value and a decrease in the organic content of hydrolysate. As the carbohydrate content in the hydrolysate decreased, the MR was terminated, so no more MRP was transferred to hydrochar. At the same time, the protein was still continuously released at higher temperatures and longer residence times, yielding a decline in the HHV_daf. Moreover, clean energy utilization was verified from the reduced nitrogen content in hydrochar and lower CO and NO_x emission of HC 260-4 in the combustion test. After the HT, increased hydrophobicity and a lower fraction of bound water improved the dewaterability, which is of great significance for applying hydrochar as biofuel. The findings of this study provided a new perspective to explain the heating value generation of hydrochar and more direct evidence to assess its clean combustion properties, with promising perspectives for practical applications.

A review on fabricating functional materials by heavy metal-containing sludges

With the development of industry, sustainable use of natural resources has become a worldwide hot topic. Heavy metal-containing sludge (HMS) is a hazardous waste after wastewater treatment. At present, HMS is still treated by landfill or landfill after incineration. Considering the components, HMS usually contains various heavy metals and organic compounds, which is potentially used as a raw resource for catalyst production. This review thus concludes recent reports and developments in this field. First, basic technologies are summarized as component regulation, precursor formation, and structure transformations. Second, prepared materials are applied in various catalytic fields, such as gas purification, photocatalysis, electrocatalysis, and Fenton catalysis. During these processes, key factors are multi-metallic components, metal doping, temperature, and pH. They not only influence the formation of HMS-derived catalyst but also the catalytic activity. Furthermore, catalytic activities of HMS-derived catalysts are compared with those synthesized by pure reagents. An assessment and accounting are also supplied if raw resources are substituted by HMS. Finally, in order to apply HMS in a real application, more works must be devoted to the influence of trace metal doping on catalytic activities and stabilities. Besides, more pilot experiments are urgently necessary.