Fe-rich Sludge Research Articles

A novel electrocatalyst from TOCN/CGG hydrogel-supported Fe-rich sludge and its performance in treating azo dyes-contaminated water

Monolithic electrocatalysts are desired for the electro-Fenton oxidation system. We used a hydrogel consisting of TEMPO-oxidized cellulose nanofibers (TOCN) and cationic guar gum (CGG) to disperse and support Fe-rich sludge and finally obtained a Fe-doped biochar (denoted as C-Sludge@TOCN/CGG) after the freeze-drying and carbonization. This C-Sludge@TOCN/CGG exhibited a porous structure with evenly-distributed Fe due to the inherently three-dimensional porous structure of TOCN/CGG hydrogel and the abundant carbon content. Importantly, Fe and FeO existed in C-Sludge@TOCN/CGG due to the presence of TOCN and CGG during the pyrolysis. The electrochemical properties of C-Sludge@TOCN/CGG demonstrated its good electrocatalytic activity and stability with few side reactions. It had good performance in the electrocatalytic degradation of various azo dyes, attributed to the synergistic integration of TOCN/CGG-derived carbon matrix and carbonized Fe-rich sludge particles. Specifically, two transient radicals (i.e. ·OH and ·O2−) primarily improved the electrocatalytic degradation performance of C-Sludge@TOCN/CGG. This C-Sludge@TOCN/CGG also efficiently degraded a papermill-sourced wastewater containing direct red 23, direct yellow 11, direct black 19 and toner, in which the COD value decreased from 365.12 to 179.13 mg/L within 9 h. This work provides an example of utilizing renewable materials and solid waste to design electrocatalysts to address the wastewater issue.

Insight into the sulfite activation by Fe-rich sludge-derived biochar for efficient organic contaminant degradation: The role of iron species

Sulfate radical-based advanced oxidation processes driven by sulfite [S(IV)] activation offer a sustainable and cost-effective approach for environmental remediation. The utilization of abundant iron oxide species in Fe-rich sludge for S(IV) activation presents a cost-effective and eco-friendly solution, making it an attractive option for Fe-rich sludge disposal and water treatment. Herein, an efficient S(IV) activation system was developed by using Fe-rich sludge-derived biochar (FBC) for the first time. The performance of S(IV) activation by different pyrolyzed FBCs was evaluated in terms of tetracycline (TC) degradation. The highest degradation efficiency of TC was achieved 95.0 % in the S(IV)/FBC700 under the optimum conditions. The reactive radical species responsible for TC degradation in the system were confirmed to be SO4•−. The present work opens up new possibilities for the development of novel advanced oxidation processes utilizing the waste Fe-rich sludge.

Preparation of magnetic biochar used bioleached sludge to enhance tetracycline removal in a heterogeneous Fenton-like system

The environmental application of iron (Fe)-rich acidic sludge generated after bioleaching is still a challenge. Pyrolysis offers a promising avenue for converting this sludge into biochar. In this study, magnetic biochar (BLS-BC7) was prepared from acidic Fe-rich sludge and employed as a catalyst for tetracycline (TC) degradation by hydrogen peroxide (H2O2). Furthermore, compared with the original sludge-based biochar, BLS-BC exhibited a 2.3 times higher specific surface area, providing more active sites for the catalytic reactions. The scanning electron microscopy and Raman results indicated that BLS-BC7 was coated with a large number of particles on its surface, presenting a rough appearance with a high degree of defects, thereby providing more active sites for biochar. Characterization via photoelectron spectroscopy, elemental analysis, and Fourier transform infrared spectroscopy results indicated that BLS-BC contained a higher amount of oxygen functional groups, along with an increased content of CO and the formation of Fe–O, facilitating H2O2 activation to generate reactive oxygen species (ROS). Additionally, quenching experiments and electron paramagnetic resonance spectroscopy demonstrated the generation of multiple ROS such as 1O2, ·OH, and O2−·, in the BLS-BC7/H2O2 system. This resulted in a TC removal rate of 100 % within 120 min, with a mineralization rate of 53.4 %. The biochar exhibited easy recoverability and strong reusability, maintaining a TC removal rate of 91.3 % after five cycles of experiments. The X-ray diffraction patterns before and after the reaction showed that BLS-BC7 exhibited strong stability, with no apparent change in peak positions before and after use.

Co-gasification characteristics of Ca-rich sludge and Fe-rich sludge under CO2 atmosphere, and potential utilization of gasification residues as renewable catalyst in biomass pyrolysis

The co-gasification behaviors and performances of Ca-rich sludge (CS) and Fe-rich sludge (FS) were studied by thermogravimetric analyzer. The results showed that the comprehensive gasification index was highest at 40% CS ratio, which was 9.7 × 10−8 %2/(min2·°C3). The co-gasification resulted in more solid products generation due to that ash hindered contacts of gasifying agent and organics. By the sliding resistance furnace (SRF), in 80%CO2/20%N2, the evolution of gas compositions during the gasification process and impacts of different temperatures and blending ratios on co-gasification were evaluated. The evolution of gas compositions showed that the gas yields from CS gasification were higher than those from FS, and the interactions and ash catalytic effect during the co-gasification made the peak times earlier. The reaction temperatures and blending ratios had certain effects on the co-gasification process, affecting the mineral compositions and pores structures of sludge residues. Finally, considering the attributes of rich pores structure and Ca/Fe elements, the gasification residues can be used as ex-situ catalyst in the modification of biomass pyrolytic volatiles. Conducting the pyrolysis experiments by the SRF under N2 atmosphere, with adding gasification residues of sludge, the CO yield from biomass pyrolysis grew, but the yields of CO2, H2 and C2Hm reduced.

Enhanced sludge dewatering by PDMDAAC coupled with Fenton-like reaction initiated by Fe-rich sludge biochar with in-situ generation of H2O2: Fe/C structure as an electron shuttle

Fenton-like reaction has been used to enhance sludge dewaterability effectively. However, the unstable characteristic, ignitability, and corrosivity of H2O2 make it difficult for application. To address this challenge, a novel process of in-situ H2O2 generation by Fe-rich sludge biochar under aeration condition combined with poly-diallyldimethylammonium chloride (PDMDAAC) was proposed to enhance sludge dewaterability. Under optimized conditions, the capillary suction time and specific resistance to filterability of the conditioned sludge were reduced significantly compared with raw sludge (54.2 vs. 351 s, 0.249×1013 vs. 2.68×1013 m/kg, respectively). The carbon skeleton in Fe-rich sludge biochar can reduce the charge transfer impedance between Fe and O2, promoting in-situ generation of H2O2. The degradation of hydrophilic organics in extracellular polymeric substances of sludge floc and the PDMDAAC coagulation effect promoted the release of bound water. This study provides a strategy to enhance sludge dewaterability by Fenton-like reaction initiated by Fe-rich biochar with no H2O2 addition.

Catalytic Performance of Fe-Rich Sludge in Pyrolysis of Waste Oil Scum as Volatiles and Magnetic Char

Catalytic Performance of Fe-Rich Sludge in Pyrolysis of Waste Oil Scum as Volatiles and Magnetic Char

Iron-rich sludge biochar triggers sodium percarbonate activation for robust sulfamethoxazole removal: Collaborative roles of reactive oxygen species and electron transfer

The efficient removal of sulfamethoxazole (SMX) from swine farming wastewater is a major emerging environmental issue. This study successfully developed Fe-rich sludge biochar (Fe-BC) from dewatered iron-containing sewage sludge cake, in order to activate sodium percarbonate (SPC) for rapid SMX degradation in swine farming wastewater. Under optimal conditions of 0.5 g/L Fe-BC and 0.3 mM SPC at pH of 3.0, Fe-BC/SPC process achieved highly efficient SMX degradation within 5 min, reaching 100 % SMX degradation efficiency with kobs of 1.1439 min−1. The Fe-BC/SPC process exhibited recyclability and a high level of resistance to natural wastewater conditions. The main mechanism included adsorption and oxidation process. The SMX was probably adsorbed on the Fe-BC surface. Also, SPC activation by Fe-BC for SMX degradation was found to include a radical-driven pathway dominated by •OH and a non-radical pathway governed by direct electron transfer. Furthermore, the high rate of SMX degradation in the Fe-BC/SPC process was attributed to Fe2+ generation via the Fe3+/Fe2+ cycle and efficient electron transfer through oxygenated functional groups. Four SMX degradation pathways (i.e., hydroxylation, –NH2 group oxidation, ON bond and NS bond cleavage reactions) were proposed. The Fe-BC/SPC process exhibited high SMX removal performance and low-toxicity with swine farming wastewater. Overall, this study shows that the development of Fe-BC/SPC as a promising and efficient system for SMX degradation in practical swine farming wastewater treatment applications.

Synergistic 2,4-D Degradation by Fenton Reagent Obtained from Glow Discharge Plasma Formed H2O2 and DBD Pretreated Fe-Rich Sludge Catalyst

Synergistic 2,4-D Degradation by Fenton Reagent Obtained from Glow Discharge Plasma Formed H2O2 and DBD Pretreated Fe-Rich Sludge Catalyst

Valorization of waste Fe-rich sludge as erdite/KFeS2 rods under atmosphere condition and evaluation of their mutual transformation.

Pilot scale production of one-dimensional (FeS2)nn- rods was performed by using an automatic 20L vessel at 80°C under atmosphere condition with the resource utilization of Fe-rich sludge. The sludge was simulated at lab-scale with chemical pure of ferric trichloride. After the sludge treatment, the corresponding rods were not formed at room temperature. But by heating at 80°C, erdite rod was well-crystallized after 0.5h by only adding Na halite, and KFeS2 rod was crystallized weakly after 2h and highly at 10h with the addition of K halite. After 48h heating, the rods grow radially to 300nm for erdite, but to 5μm for KFeS2. However, at room temperature, erdite rod was converted to high crystallized KFeS2 in KOH water or ethanol solution, whilst the conversion of KFeS2 rod to erdite also occurred in NaOH water solution, but terminated in NaOH ethanol solution, without any morphology change. It is also noted that with the presence of both Na and K halite, the rod was an intermediate of erdite to KFeS2 with 1μm length after heating at 100°C but converted to 10-μm-length KFeS2 crystal at the temperature of > 120°C. The thermodynamic results confirmed that during the rod polymerization, the Fe(OH)3HS- formation was the sole rate-limiting step and showed a positive Gibbs value of 6.45kJ/mol at room temperature and negative values at the temperature of > 48°C. In summary, this method not only enabled the vaporization of waste Fe-rich sludge as value-added rods without generating any secondary waste but also showed a new route for the in situ conversion of erdite/KFeS2 rods at room temperature.

In situ formation of Ca(OH)2 coating shell to extend the longevity of zero-valent iron biochar composite derived from Fe-rich sludge for aqueous phosphorus removal

Despite being an effective and attractive functional strategy for aqueous phosphorus (P) removal, the use of zero valent iron (ZVI) biochar composites has been severely impeded by rapid self-erosion. We describe a new approach for extending the lifespan of Fe-rich sludge-derived ZVI biochar composites via CaCl2 modification. Preliminary results showed that composites obtained at 900 °C without modification (MBC900) and at 900 °C with 100 g Cl/kg addition (MBC900100) had the highest P removal efficiency. In subsequent batch experiments, MBC900100 exhibited more stable P adsorption capacities than MBC900 over a wide pH range (4–10) and at various dosages, which was enhanced by the presence of HCO3−. The theoretical maximum P adsorption capacities of MBC900 and MBC900100 were 227.14 and 224.15 mg g−1, respectively. Kinetic analysis indicated that chemisorption dominated the removal process. Continuous experimental data using the Yoon–Nelson model indicated that MBC900100 had a considerably longer half-penetration time. The primary mechanism of P removal by MBC900 was Fe/C micro-electrolysis. As the embedded CaO formed a dissolvable Ca(OH)2 shell in situ on the surface of MBC900100, the phosphate formed a precipitate with free Ca2+ before being removed via micro-electrolysis. Overall, CaCl2 modification successfully enhanced the longevity of the ZVI biochar composites.

Recovery of Fe from steel pickling wastewater as polymeric Fe/S rods for effective adsorption of phosphate from electrophoresis effluent

Pickling wastewater is rich in Fe(II) and commonly neutralised by limestone to precipitate Fe as waste Fe-rich sludge. Herein, an alternative reutilisation route was developed to recycle Fe from pickling wastewater as polymeric Fe/S rods for the effective removal of phosphate from the actual electrophoresis effluent. Results showed that irregular ferrous oxyhydroxide predominated in the precipitated sludge after the direct neutralisation of wastewater and was then converted into coarse spherical particles after alkaline heating at 80 °C for 24 h under atmospheric condition. After adding nitrite to oxide Fe(II) in pickling wastewater, ferric oxyhydroxide and magnetite were formed in the sludge and were converted into polymeric Fe/S-bearing rods with the same procedure. The rods had a one-dimensional polymeric (FeS2)nn- structure of KFeS2 crystal with 200 nm diameter and 2–5 μm length and was spontaneously hydrolysed in pH range of 3–9 to fine colloids with abundant hydroxyl/sulfhydryl groups. The optimal dosage of polymeric Fe/S-bearing rods was 10 g/L, where the phosphate removal was 99.6 %, and the remaining phosphate was only 0.47 mg/L in the treated electrophoresis effluent. During the KFeS2 rods synthesis, an alkaline supernatant was generated, but was recycled to prepare rods in the next round, and the prepared rods also showed high removal efficiency of phosphate. For instance, by adding 6.67 g/L prepared rods, the phosphate removal achieved by 94.5 %, approximated to that of fresh one (95.3 %). In summary, Fe was recycled from pickling wastewater as polymeric Fe/S-bearing rods without generating any secondary waste and the rods were effective in the treatment of phosphate-rich wastewater.

Review of resource utilization of Fe-rich sludges: purification, upcycling, and application in wastewater treatment

This paper discusses the resource utilization of Fe-rich sludges generated as waste products from water treatment, hydrometallurgy, surface finishing, and dye chemical industries. Apart from the conventional landfill disposal of such sludges, the work focuses on sludge purification for new commercial products, including iron red/black dyes, an iron concentrate powder, a polyferric flocculant, a catalyst, and a magnetic adsorbent. Among such purifications, a new strategy was developed to recycle Fe-rich sludges for a new Fe/S-bearing flocculant. Given that Fe-rich sludges may contain rare and/or heavy metals, the purification of sludges as high-purity hematite nanoparticles and other valuable products is detailed as a new insight. Accordingly, the mechanisms for the phase transformation of Fe-bearing minerals and the purification of valuable Fe oxides are deeply considered. The work summarizes the pilot- and/or field-scale application for recycling of Fe-rich sludge and proposes the development of a new Fe/S flocculant and a high-purity hematite product.

Dewatering Treatment and Catalytic Application of Sludge by Glow Discharge Plasma

In this paper, glow discharge plasma (GDP) was used to remove organic contaminants of sludge and improve sludge dewaterability. Fe-rich sludge as a catalyst combined GDP to form heterogeneous Fenton reaction, which was used to remove 2,4-dichlorophenoxyacetic acid (2,4-D) in aqueous solution. The surface micrographs and chemical composition of Fe-rich sludge was analysed by the Scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS). The effect of initial pH0 on hydrogen peroxide (H2O2) yield in GDP was discussed. The removal of 2,4-D was associated with the synergetic effect of adsorption and oxidative, and hydroxyl radicals (•OH) play a dominant role. This study provides an environmentally-friendly technology to improve sludge dewaterability and a potentially low-cost catalyst for 2,4-D removal in water, which is significant for the reuse of Fe-rich sludge.

Biosynthesis and recycling of magnetite nanocatalysts from Fe-rich sludge

Iron salts are widely used in municipal infrastructure to treat drinking water, prevent the corrosion of sewer lines and enhance phosphorus/nitrogen removal. The added iron salts deposit in sludge, leading to an increase in the cost of sludge disposal. Here, we demonstrated the feasibility of recycling magnetite nanoparticles (MNPs) produced from Fe-rich sludge under anaerobic conditions with indigenous microorganisms. The genus Geobacter was found to play the primary role in MNP synthesis. The produced MNPs were spherical with a size ranging from 12 to 24 nm. Compared to commercialized MNPs, the extracellular polysaccharides protected their surficial Fe2+ from being oxidized. The biosynthesized MNPs had a catalytic performance similar to that of commercialized MNPs but with high reusability after multiple cycles. These results demonstrate that the biosynthesis of MNPs is a novel and applicable way to recycle value-added products from Fe-rich sludge.

Chromium(VI) removal from synthetic solution using novel zero-valent iron biochar composites derived from iron-rich sludge via one-pot synthesis

In this study, Fe-rich sludge obtained from coagulation–flocculation of swine wastewater was processed into zero-valent iron (ZVI) biochar composites through one-pot pyrolysis at 700 °C under anoxic (MBCN700) and hypoxic (MBCA700) conditions. X-ray diffraction analysis revealed that α-Fe0 and γ-Fe0 were the dominant Fe species in MBCN700 and MBCA700, respectively. The effects of different process parameters, including composite dosage, initial Cr(VI) concentration, pH, dissolved O2 (DO), and contact time on Cr(VI) removal from synthetic solutions were evaluated. The results showed that both the acidic initial pH and DO contributed to Cr(VI) removal; the highest removal rate was observed at pH 3. Based on the Langmuir isotherm model, the predicted maximum removal capacity of MBCA700 was greater than that of MBCN700, primarily because of its core-shell structure, less corrosive ZVI, and higher degree of graphitization. X-ray photoelectron spectroscopy analysis coupled with fitting of the kinetic (pseudo-second-order) and isothermal (Freundlich) data suggested that Cr(VI) removal by MBCN700 occurred via adsorption, FeC micro-electrolysis, and co-precipitation. In contrast, the removal mechanism for MBCA700 included adsorption, reduction, and electrostatic attraction. Thus, ZVI biochar composites can serve as potential remediators of Cr(VI)-contaminated wastewater.

Low-temperature conversion of Fe-rich sludge to KFeS2 whisker: a new flocculant synthesis from laboratory scale to pilot scale

Herein, a KFeS2 whisker was formed in mass production at a low temperature, with waste cold-rolling sludge as Fe source, which exhibited good performance in the removal of Zn/Ni from real electroplating effluent. At laboratory scale, KFeS2 was generated at 80 °C by the hydrothermal method, and KFeS2 whisker grew radially with the extension of the reaction time. This method was applied at pilot scale, where a similar KFeS2 whisker was also produced with waste cold-rolling sludge as Fe source, and a residual brownish supernatant was observed after the reaction and then completely recycled in the next round for KFeS2 synthesis. After recycling five times, the produced KFeS2 whisker did not change. The drying and storage of KFeS2 have also been verified. Freeze drying and vacuum drying were applicable, whereas air drying was not profitable. Moreover, the efficiency of Zn/Ni removal using undried KFeS2 was similar to that of dried KFeS2. The efficiencies of Zn/Ni removal using KFeS2 were apparently higher those of common reagents for wastewater treatment.

Conversion of Fe-rich sludge to KFeS2 cluster: Spontaneous hydrolysis of KFeS2 for the effective adsorption of doxycycline

Fe-rich sludge is a solid waste considerably generated in coagulation, Fenton, and catalytic processes for wastewater treatment, in which it is commonly disposed in landfills. However, a limited portion of sludge is recycled as polymeric ferric flocculant and iron red. Herein, the Fe-rich sludge was simulated by the hydrolysis of FeCl3 and converted to a new one-dimensional KFeS2 cluster via a one-step hydrothermal route with the addition of K2S and KOH. The results showed that in the hydrothermal process, KFeS2 cluster grew radially from 2 μm to 10 μm with the increase in KOH concentration from 2 M to 5 M. The new cluster showed a high adsorption capacity of 2933.6 mg/L for doxycycline, which is 14 times that of sludge and higher than that of hematite nanoparticles, commercial polymeric ferric flocculant and pyrite. The adsorption isotherm complied with the Langmuir model, and the adsorption kinetics fitted well with the pseudo-second-order model. During adsorption, KFeS2 cluster was completely hydrolysed to release Fe/S-bearing colloids with a considerable number of Fe-SH/Fe-OH groups for the coordination of the NH2 group of tetracycline-type antibiotics, e.g. doxycycline. However, an inefficient removal of quinoline and p-nitrophenol was observed. With the proposed method, the used alkaline solution was completely recycled in the next round of KFeS2 synthesis without the generation of secondary waste. Such green method has potential applications in the resource utilisation of Fe-rich sludge.

Enhancing paper sludge dewatering by waste polyester fiber and FeCl3 for preparation of Fe-rich biochar

The paper sludge conditioning effects of waste polyester textile fibers as skeleton builders combined with ferric chloride (FeCl3) were evaluated and the sludge dewatering mechanism was explored. The catalytic effect of Fe-rich sludge biochar (Fe-SB) on enhancing sludge dewaterability was evaluated. Results showed the combined fiber-FeCl3 conditioning significantly promoted the sludge dewaterability compared with conditioner alone, leading to a 77.5% decrease in specific filtration resistance and a 68.9% increase in net yield. The decrease of extracellular polymeric substances (EPS) contents showed that the EPS were difficult to extract and sludge floc strength was enhanced as a result of chemical reactions such as complexation processes and charge neutralization. Hence, the enhancement of sludge dewaterability was primarily due to the sludge cake with a porous and incompressible structure formed by fiber and FeCl3, and the rigidity structure of fiber. Moreover, the Fe-SB prepared by fiber-FeCl3 conditioning sludge could effectively activate persulfate to enhance the sludge dewaterability, with water content of dewatered sludge decreasing by 14.6%. The Fe-SB had dual functions of the heterogeneous catalyst of persulfate and skeleton builder. This study presents a sludge recycling method that combined physicochemical conditioning and sludge biochar materials prepared by pyrolysis.

Coupling of KMnO4-assisted sludge dewatering and pyrolysis to prepare Mn,Fe-codoped biochar catalysts for peroxymonosulfate-induced elimination of phenolic pollutants

Conversion of waste sludge into biochar-based catalysts has been considered as a feasible alternative for sludge treatment, and dewatering of waste sludge is a key process for further treatment and disposal. In this study, KMnO4 was used as both a chemical conditioner to assist Fe-rich sludge dewatering and a chemical modifier to incorporate Mn oxides into the biochar catalysts. Results showed that the sludge dewatering was substantially improved when KMnO4 dosage was above 13% DS (dry sludge), and more than 99% of Mn was retained in the sludge cake. After pyrolysis of the sludge cake, a series of Mn,Fe-codoped biochar catalysts (assigned to x-Mn-FeBC) were obtained. The catalysts could activate peroxymonosulfate (PMS) well for complete removal of phenolic contaminants including phenol, bisphenol A (BPA) and 2,4-dichlorophenol (DCP) over a wide pH range (2.5–12) with the co-existence of water constituents. After 5 consecutive cycles, the catalytic performance of 13%-Mn-FeBC was decreased by half, but it could be fully recovered by simple calcination at 600 oC. It was proposed that radical and non-radical oxidation processes worked together for organic degradation, but non-radical pathways involving 1O2 oxidation and electron transfer between PMS and organic played more crucial roles. This study proposed a novel sludge treatment process by combining enhanced sludge dewatering with efficient catalyst preparation, and the as-prepared catalysts were promising in PMS activation and organic degradation.

Enhanced sludge dewaterability with sludge-derived biochar activating hydrogen peroxide: Synergism of Fe and Al elements in biochar

Reuse of sludge-derived Fe-rich biochar as sludge conditioner is an attractive route for management of waste activated sludge at source. Homogeneous and heterogeneous Fenton reactions have been proved in sludge conditioning with Fe-rich biochar activating H2O2 to enhance sludge dewaterability. The FeAl2O4 phase in Fe-rich biochar was first identified during pyrolysis of sewage sludge after adding both Fe2O3 and Al2O3, since Fe and Al elements are two of major metal elements in Fe-rich sludge. Compared with the Fe-rich biochar that did not comprise FeAl2O4 phase, the capillary suction time (CST) and specific resistance to filterability (SRF) of the sludge conditioned with the Fe-rich biochar comprising FeAl2O4 phase could be efficiently decreased by 23% and 44%, respectively. The results indicated that FeAl2O4 phase in Fe-rich biochar could improve sludge dewaterability by enhancing heterogeneous Fenton reaction. Synergistic effect between Fe and Al in FeAl2O4 contributed to weak the O–O bond in H2O2 and reduce the activation energy of H2O2 decomposition for enhancing ·OH generation, which could be explained by density functional theory (DFT) calculations for the first time. Thus, the decomposition rate of H2O2 and the amount of ·OH generation were obviously promoted by FeAl2O4 phase in sludge-derived biochar during sludge conditioning, attributing to the destruction of sludge flocs, the release of bound water, and the improvement of sludge dewaterability.