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.

Zero-valent iron coupled calcium hydroxide: A highly efficient strategy for removal and magnetic separation of concentrated fluoride from acidic wastewater

The removal of concentrated fluoride in acidic wastewater by the conventional Ca(OH)₂ method is challenged by the insufficient efficiency and difficult separation of fine CaF₂ precipitate. Herein, we construct a strategy to tackle these challenges by coupling zero-valent iron (ZVI) with Ca(OH)₂. ZVI reduces fluoride concentration from 12,000 to 3980 mg L^-1 under optimal conditions primarily through the in-situ growth of porous FeF₂·4H₂O shell on its surface, which simultaneously assists fluoride removal via adsorption. The residual fluoride after ZVI treatment then decreases to 6.74 mg L^-1 via precipitation with Ca(OH)₂. Interestingly, the iron ions dissolved from ZVI also participate in the precipitation to form magnetite. This co-precipitation reinforces the fluoride removal and meanwhile endows the resulted precipitates with magnetism, thus enabling the perfect solid-liquid separation by the magnetic field before discharge. The application prospect of this coupling strategy is further verified by its ability in decreasing the concentrations of fluoride and other coexisting heavy metals (Zn²⁺, Cd²⁺ and Pb²⁺) in real smeltery wastewater below their discharge limitations. This study provides a promising strategy for the treatment of concentrated fluoride in acidic wastewater and also highlights ZVI as a good candidate to couple with conventional methods for enhanced pollution control.

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.

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.

Graphitic carbon nitride-doped sewage sludge as a novel material for photodegradation of Eriochrome Black T

The bio-resource utilization of sewage sludge is presented by preparation of novel waste sludge-doped graphite carbon nitride (g-C₃N₄) photocatalyst. The sludge flocs which constitute bacteria and organic substances served as a pore-forming framework in the catalyst, while the inorganic fractions including those transition metals and crustal metals can function as dopants for sludge-based g-C₃N₄ composite. The physicochemical properties of as-prepared catalyst were well analyzed by multiple characterizations. The composite catalyst showed higher surface area (50 m²/g) and more mesoporous structures (8.9 × 10^-2 cm³/g) as compared with pristine g-C₃N₄ (8.4 m²/g and 6.6 × 10^-2 cm³/g, respectively). The photoelectrochemical results showed that introduced sewage sludge impurities lowered down the photocarriers recombination efficiency and enhanced more efficient electron-hole separation by about 100 times. The photocatalytic activity was tested by degradation of typical dye Eriochrome Black T (EBT). The optimal sample improved removal of EBT by 56% in 90 min under ultraviolet irradiation (λ = 254 nm). According to the results of main metal ion leaching concentration and reuse tests, the composite catalyst exhibited excellent stability and repeatability.

Conversion of sewage sludge into environmental catalyst and microbial fuel cell electrode material: A review

At present, environmentally friendly and cost-effective disposal of sewage sludge (SS) is the major challenge of wastewater treatment that prompted the concept of sludge valorization. A recent technology, SS conversion into biochar as an efficient catalyst for environmental application, shows great promise to sludge valorization. This review presents the literature and advances of sludge biochar-based catalysts (SBCs), including their synthesis route, physiochemical characteristics, catalytic applications, reaction mechanisms, chemical stability, feasibility, and future aspects. Two major applications of SBCs such as organic pollutants degradation and employing as an electrode material in a microbial fuel cell (MFC) were summarized. The literature has indicated that carbonization of raw or organic/ inorganic-laden sludge produces various metal phase structure and surface functional groups which perform various catalytic reaction such as Fenton-like reaction, ozonation, H₂O₂/ persulfate activation, and photoreaction in the organic pollutants degradation tests. The degradation efficiency and chemical stability of SBCs have found very satisfying. Moreover, catalysts are highly recyclable, separable, and ensure negligible metal leaching. Secondly, high-temperature carbonized sludge exhibits excellent electrical conductivity which is suitable to use as MFC electrodes. The low-cost sludge biochar-based electrodes (SBEs) performance is comparable to many commercial electrodes. This new technology is concurrently advantageous for environmental pollution remediation, energy production, and harmful metals immobilization, which offer a new route towards SS valorization.

Sewage sludge-derived TiO2/Fe/Fe3C-biochar composite as an efficient heterogeneous catalyst for degradation of methylene blue

Novel TiO₂/Fe/Fe₃C-biochar composite, as a heterogeneous catalyst, has been synthesized by a single-step route, where sewage sludge (SS) and different ratios of nanoparticles (NPs: Fe and Ti) impregnated with chitosan using coagulation and flocculation techniques for subsequent thermal decomposition at 800 °C. The physiochemical properties of samples have been characterized thoroughly and employed in methylene blue (MB) degradation tests. It was found that NPs ratio and chitosan support have significant influences on the properties and catalytic activity of catalysts. Chitosan inclusion successfully improves the surface area and mesoporosity of composites, while high contents of Fe integration reduce surface area and active site (Fe₃C) due to Fe⁰ agglomeration. Though, Ti incorporation produces Ti³⁺ that activated photosensitivity. Catalyst with the high mesoporous surface, Ti³⁺, selective Fe₃C, and small Fe⁰ shows superior MB removal competency through concurrent adsorption, photodegradation, and H₂O₂ activation. Primarily OH and some O₂^- radicles participating in the degradation reactions evident from scavenging experiments. The maximum MB removal capacity evaluated as 376.9 mg L^-1 in neutral pH. Moreover, the catalyst exhibits excellent material stability, recyclability, easy separability, and low Fe-ion leaching (0.11 mg L^-1) after catalysis. This study provided new insight into a low-cost and environmentally friendly route of catalyst synthesis using SS, NPs, and chitosan, which concurrently advantageous to SS disposal and wastewater treatment.

Visible Light Driven Organic Pollutants Degradation with Hydrothermally Carbonized Sewage Sludge and Oxalate Via Molecular Oxygen Activation

Converting sewage sludge into functional environmental materials has become an attractive sewage sludge disposal route. In this study, we synthesize a sewage sludge-based material via a facile one-pot hydrothermal carbonization method and construct a visible light molecular oxygen activation system with hydrothermally carbonized sewage sludge (HTC-S) and oxalate to degrade various organic pollutants. It was found that iron species of HTC-S could chelate with oxalate to generate H₂O₂ via molecular oxygen activation under visible light, and also promote the H₂O₂ decomposition to produce ^•OH for the fast organic pollutants degradation. Taking sulfadimidine as the example, the apparent degradation rate of HTC-S/oxalate system was almost 5-20 times that of iron oxides/oxalate system. This outstanding degradation performance was attributed to the presence of iron-containing clay minerals in HTC-S, as confirmed by X-ray diffraction measurements and Mössbauer spectrometry. In the oxalate solution, these iron-containing clay minerals could be excited more easily than common iron oxides under visible light, because the silicon species strongly interacted with iron species in HTC-S to form Fe-O-Si bond, which lowered the excitation energy of Fe-oxalate complex. This work provides an alternative sewage sludge conversion pathway and also sheds light on the environmental remediation applications of sewage sludge-based materials.

Kinetic study of heavy metals Cu and Zn removal during sewage sludge ash calcination in air and N2 atmospheres

Heavy metal control is essential during the thermochemical recovery of phosphorus (P) from sewage sludge ash (SSA). For medium volatile heavy metals, i.e. Cu and Zn, the effect of chlorine additive was complicated and more sensitive to temperature variation. So, in the in-depth study on the removal kinetics of Cu and Zn was necessary. Thus, the studies described in this paper considered the experiments and kinetic models of Cu and Zn removal in SSA through calcination under different atmospheres and temperatures. The results showed that within 15 min, the removal of Cu and Zn was more effective at the same temperature in air than in N₂. The result is consistent with kinetic analysis: Reaction activation energy of both Cu and Zn in an air atmosphere is lower than in N₂. In addition, the reaction orders, energy and frequency factors of Cu and Zn removal reaction during SSA calcination at high temperature with air and N₂ atmosphere were calculated.

Plasmon-Induced Hot Electrons on Mesoporous Carbon for Decomposition of Organic Pollutants under Outdoor Sunlight Irradiation

In this study, a 4 in. CMK-8-Nafion membrane was fabricated using three-dimensional cubic ordered mesoporous carbon CMK-8 blended with a Nafion polymer. Plasmon-resonance hot electrons and holes from Au nanoparticles (NPs) combined with this CMK-8-Nafion membrane resulted in the effective decomposition of methyl orange (MO) due to the synergetic work of hot carriers with mesoporous carbon; a sample of Au/CMK-8-Nafion exposed to outdoor sunlight radiation for 150 min successfully removed 97% of MO. Fourier transform infrared spectroscopy (FTIR) was employed to examine the generation of hydroxyl groups (OH-) during decomposition. Finally, the spatial distribution of hydroxyl groups was also investigated across the different coverage densities of plasmonic Au NPs.

Simple synthesis of lithium-doped sulfated titania nanoparticles and their high visible light photocatalytic activity under negative bias electrostatic field

Li-doped TiO₂/SO₄²⁻ nanoparticles were successfully synthesized via a simple calcinination process in a vacuum environment using Ti(SO₄)₂ and LiBr as precursors, and were characterised by TEM, XRD, IR, DLS, XPS and UV-vis (DRS).