Valorization of manganese-containing groundwater treatment sludge by preparing magnetic adsorbent for Cu(II) adsorption

Synthesis of a Magnetic Nanostructured Composite Sorbent Only from Waste Materials

Water pollution is a big problem for the environment, and thus depollution, especially by adsorption processes, has garnered a lot of interest in research over the last decades. Since sorbents would be used in large quantities, ideally, they should be cheaply prepared in scalable reactions from waste materials or renewable sources and be reusable. Herein, we describe a novel preparation of a range of magnetic sorbents only from waste materials (sawdust and iron mud) and their performance in the adsorption of several dyes (methylene blue, crystal violet, fast green FCF, and congo red). The preparation is performed in a hydrothermal process and is thus easily scalable and requires little sophisticated equipment. The magnetic nanostructured materials were analyzed using FTIR, VSM, SEM/EDX, XRD, and XPS. For crystal violet as a pollutant, more in-depth adsorption studies were performed. It was found that the best-performing magnetic sorbent had a maximum sorption capacity of 97.9 mg/g for crystal violet (methylene blue: 149.8 mg/g, fast green FCF: 52.2 mg/g, congo red: 10.5 mg/g), could be reused several times without drastic changes in sorption behavior, and was easily separable from the solution by simply applying a magnet. It is thus envisioned to be used for depollution in industrial/environmental applications, especially for cationic dyes.

Resource utilization of hazardous Cr/Fe-rich sludge: synthesis of erdite flocculant to treat real electroplating wastewater

Cr/Fe-bearing sludge is a hazardous solid waste, produced at mass production in smelting, plating and surface finishing industries. Such waste is commonly treated by chemical detoxification and safety landfill, whereas only a few Cr-rich sludge is recycled as a tanning reagent. In this study, a novel route was developed to recycle Cr/Fe-bearing sludge as erdite-bearing flocculant for wastewater treatment. Results showed that two sludges were irregular aggregates, one of which contained 1.6 wt.% Cr (short for LS) and the other contained 4.2 wt.% Cr (HS). After hydrothermal treatment, stable Cr(III)/S-bearing product was formed from the Cr(VI) reduction in the sludges. Conversely, erdite was generated in nanorod form with diameter and length of 200 nm and 0.5-1 μm from LS, respectively, whereas grew radially to 1.5-2.5 μm for HS. The two erdite-bearing products were spontaneously hydrolysed to Fe/S-bearing flocs and showed similar performance in the treatment of real electroplating effluent with 91.55, 1.94 and 0.25 mg/L of Zn, Ni and Cr, respectively. For instance, by adding 1 g/L product of LS, the release of Cr from the products did not occur, and the residual Zn, Ni and Cr in the effluent was 0.25, 0.65 and 0.17 mg/L, respectively, which met the discharge standard of the electroplating industry. With the two converted products, the residual Zn/Ni/Cr concentrations were apparently lower than those of the raw sludges and other common reagents (e.g. polymeric ferric sulphate, activated carbon and diatomite). Thus, such erdite-bearing products could serve as a flocculant and then be applied in electroplating wastewater treatment.

Engineered Magnetic Carbon-Based Adsorbents for the Removal of Water Priority Pollutants: An Overview

This review covers the preparation, characterization, and application of magnetic adsorbents obtained from carbon-based sources and their application in the adsorption of both inorganic and organic pollutants from water. Different preparation routes to obtain magnetic adsorbents from activated carbon, biochar, hydrochar, graphene, carbon dots, carbon nanotubes, and carbon nanocages, including the magnetic phase incorporated on the solid surface, are described and discussed. The performance of these adsorbents is analyzed for the removal of fluoride, arsenic, heavy metals, dyes, pesticides, pharmaceuticals, and other emerging and relevant water pollutants. Properties of these adsorbents and the corresponding adsorption mechanisms have been included in this review. Overall, this type of magnetic adsorbents offers an alternative for facing the operational problems associated to adsorption process in water treatment. However, some gaps have been identified in the proper physicochemical characterization of these adsorbents, the development of green and low-cost preparation methods for their industrial production and commercialization, the regeneration and final disposal of spent adsorbents, and their application in the multicomponent adsorption of water pollutants.

Synthesis of novel erdite nanorods for the activation of peroxymonosulfate during p-nitrophenol wastewater treatment

Fe-bearing salt and minerals are common reagents used in activating peroxymonosulfate (PMS) for Fenton-like oxidation in wastewater treatment. Fe-bearing reagents are used in mass production, which generate abundant Fe-bearing waste sludge in the absence of a reductant for Fe³⁺/Fe²⁺ cycling. Herein, a novel Fe/S-bearing mineral, erdite, was synthesized with a one-step hydrothermal route. The material exerted an Fe/S synergetic effect for p-nitrophenol degradation upon PMS activation and showed a one-dimensional structure similar to that of (FeS₂)_n^n-. It contained short rods with diameters of 100 nm and lengths ranging from 200 to 400 nm. It grew radically to 0.8-2 μm in length upon the addition of MnO₂. Ps-0.5, prepared by adding MnO₂ in an Mn/Fe molar ratio of 0.5, showed optimal efficiency in removing approximately 99.4% of p-nitrophenol upon PMS activation. Only 3.3% of p-nitrophenol was removed without MnO₂. The efficiency of p-nitrophenol removal through Ps-0.5 activation was higher than that through FeSO₄, nanoscale zero-valent iron (nZVI), CuFeS₂, and MnSO₄ activation. The formed erdite rods were spontaneously hydrolyzed to Fe/S-bearing flocs, in which an electron was used by structural S to reduce Fe³⁺ to Fe²⁺ upon PMS activation. The reduction resulted in a high p-nitrophenol removal rate. This study provided new insight into the development of an effective PMS activator in wastewater treatment.

The Tolerance of Anoxic-Oxic (A/O) Process for the Changing of Refractory Organics in Electroplating Wastewater: Performance, Optimization and Microbial Characteristics

In order to investigate the tolerance of an anoxic-oxic (A/O) process for the changing of refractory organics in electroplating wastewater, optimize the technological parameters, and reveal the microbial characteristics, a pilot-scale A/O process was carried out and the microbial community composition was analyzed by high-throughput sequencing. The results indicated that a better tolerance was achieved for sodium dodecyl benzene sulfonate, and the removal efficiencies of organic matter, ammonia nitrogen (NH4+-N), and total nitrogen (TN) were 82.87%, 66.47%, and 53.28% with the optimum hydraulic retention time (HRT), internal circulation and dissolved oxygen (DO) was 12 h, 200% and 2–3 mg/L, respectively. Additionally, high-throughput sequencing results demonstrated that Proteobacteria and Bacteroidetes were the dominant bacteria phylum, and the diversity of the microbial community in the stable-state period was richer than that in the start-up period.

Resource Recycling of Mn-Rich Sludge: Effective Separation of Impure Fe/Al and Recovery of High-Purity Hausmannite

Groundwater treatment sludge is a Fe/Mn-rich waste generated in mass production in a groundwater treatment plant for potable water production. The conventional disposal of sludge, such as direct discharge into river/lake, sea, and landfill, is not environmentally sustainable. Herein, a novel method was proposed to effectively separate Fe/Al and recover Mn via a combined hydrochloric acid leaching and hydrothermal route. The sludge contained 14.6% Fe, 6.3% Mn, and 11.5% Al and was first dissolved in 5 M HCl to prepare a leaching solution. Second, the leaching solution was hydrothermally treated, in which 97.1% Fe and 94.8% Al were precipitated as hematite and boehmite and more than 98% Mn was kept. Increasing the reaction temperature to 270 °C was beneficial for Fe/Al removal. With the consumption of abundant H+, the reaction of added glucose and nitrate accelerated as the temperature increased. An optimal pH was utilized for Fe/Al hydrolysis and crystallization, leading to extensive removal of Fe/Al. Third, the residual solution was adjusted to pH 8.3 with NaOH, and approximately, 99.2% Mn was removed as hausmannite with a Mn content of 63.6%. This method exhibited efficient separation of impure Fe/Al from Mn-rich groundwater treatment plant iron mud, and the recycled high-purity hausmannite was a marketable active pharmaceutical ingredient.

A facile calcination conversion of groundwater treatment sludge (GTS) as magnetic adsorbent for oxytetracycline adsorption

In this paper, groundwater treatment sludge (GTS) was recycled as a magnetic adsorbent via a facile calcination process without adding any reductant. The prepared magnetic adsorbents (MAs) were characterized by scanning electron microscopy (SEM), X-ray diffractometer (XRD) and X-ray photoelectron spectroscopy (XPS), vibrating sample magnenometer (VSM) and Mössbauer spectroscopy. The results showed that GTS comprised 33.2% Fe, 1.4% Al and 6.2% Si, and exhibited a weak saturation magnetization of 0.0008 emu/g. Without NaOH, the GTS calcinated at 700 and 500 °C were well magnetized with Ms of 20.1 and 7.1 emu/g, separately, but exhibited a low Ms of 0.43 emu/g at 300 °C. By adding NaOH powder, the Ms of GTS apparently increased to 4.9 emu/g after calcination at 300 °C, and further to 8.5 emu/g at 500 °C. In GTS, about 96.1% Fe was involved in ferrihydrite form. The Ms of calcinated GTS was accompanied with the phase transformation of ferrihydrite to maghemite. Si/Al oxides in GTS coordinated on the surface sites of ferrihydrite and inhibited the conjunction and phase transformation of adjacent ferrihydrite particles, but were effectively desorbed as in the presence of NaOH. Na500, preparing by calcinating GTS at 500 °C with NaOH, showed an optimal total surface sites (Hs) of 0.65 mmol/g. Oxytetracycline (OTC) was used as a target for studying the adsorption characteristics of synthetic magnetic adsorbents and a high adsorption capacity of oxytetracycline of 862.1 mg/g in comparison with the other calcinated GTS, and the adsorption data was consistent with the Langmuir model. By adding 6 g/L Na-500, approximately 100% of oxytetracycline and tetracycline and nearly 40% total organic carbon were removed from real pharmaceutical wastewater. With the method, GTS can be converted in mass production to magnetic adsorbent that exhibits effective application in pharmaceutical wastewater treatment.

Magnetic water treatment in a wastewater treatment plant: Part I - sorption and magnetic particles

The application of magnetic sorption to treat wastewaters is nowadays seen as a potential industrial method. In this work we apply magnetite particles to remediate real wastewater samples, with several contaminants competing for the same active sorption center at the same time. We also apply our studies at three different sampling points of a Wastewater Treatment Plant. In general terms, magnetite particles have shown a very good behaviour concerning the reduction of detergents and COD, while phosphates and total nitrogen, and the majority of heavy metals are high to moderately removed. The influence of the type of wastewater (i.e., sampling point) has also shown to be important especially for high concentration of contaminants.

Ecotoxicological assessment of micropollutant Diclofenac biosorption on magnetic sawdust: Phyto, Microbial and Fish toxicity studies

Diclofenac (DCF), a persistent pharmaceutical micropollutant which occurs in the ecosystems causing adverse effects on aquatic as well as terrestrial organisms. In this study, magnetic sawdust (MSD) was prepared using co-precipitation method for biosorptive removal of DCF from water. The MSD was characterized using various analytical techniques like microscopic and spectroscopic analysis. Magnetometer study confirms the ferromagnetic behavior of the biosorbent which is a key advantage in the separation of MSD after biosorption. The effect of experimental parameters was optimized in batch mode with evaluated maximum efficiency of 86.12 % at pH 6, biosorbent dosage 25 mg for 50 mg/L of DCF. Ecotoxicological assessment has been performed for the treated and untreated sample using plant seeds, microbes and zebra fish to check the adverse effects of DCF on these organisms. Evaluation of toxicity studies revealed that inhibition concentration of DCF for various seeds (60.91 mg/L to 43.11 mg/L), E. coli (48.82 μg/mL) and B. subtilis (31.55 μg/mL). The lethal concentration of DCF on the Danio rerio was found to be 156.99 mg/L. In contrast, significant increase in both the concentration measures of DCF after biosorption was observed making this biosorbent a potent alternative to other available treatment measures.

Effective recycling of Cu from electroplating wastewater effluent via the combined Fenton oxidation and hydrometallurgy route

Heavy metals, which commonly occur in complex forms, are difficult to remove in alkali electroplating wastewater effluent, and their resource recycling is rarely reported. Here, a Cu-bearing alkali wastewater effluent was effectively treated through Fenton oxidation, and the generated Fenton sludge was recycled into highly pure tenorite and hematite particles. The effluent contained 1.51 mg/L Cu and was subjected to Fenton oxidation, pH adjustment and coagulation. Amongst the three methods, Fenton oxidation showed superior efficiency to Cu removal, and the residual Cu in the effluent was 0.06 mg/L, thereby meeting the discharge standard for electroplating wastewater. However, Cu removal achieved less than 20% after pH adjustment and coagulation. Cu-bearing sludge, which was generated through the Fenton process, was dissolved in a mixture of hydrochloric and nitric acids. The dissolved solution contained 1.92 g/L Cu and 73.6 g/L Fe impurity. Impure Fe (67.4%) was removed as hematite aggregates after the solution was directly treated via a hydrometallurgy route, whilst 99.2% Cu was kept. When 0.5 mL of methanol was introduced to the hydrometallurgy system, nearly 100% Fe was removed as hematite nanoparticles with 94.8% purity, whilst more than 98% Cu was kept. The residual Cu was 1.88 g/L and precipitated as a tenorite block with a CuO content of 91.1% by adjusting the treated solution to pH 9. This study presented an environment-friendly method for enriching Cu from electroplating wastewater effluent without generating any waste.

Upcycling of Fe-bearing sludge: preparation of erdite-bearing particles for treating pharmaceutical manufacture wastewater

Groundwater treatment sludge is a type of solid waste with 9.0-28.9% wt.% Fe content and is precipitated in large quantity from backwash wastewater in groundwater treatment. The sludge is mainly composed of fine particles containing Fe, Si and Al oxides, such as ferrihydrite, quartz and boehmite. The Fe oxides mostly originate from the oxidation of ferrous Fe in groundwater, whilst the silicate/aluminium compounds mainly originate from the broken quartz sand filter in the backwash step. In general, the sludge is firstly coagulated, dewatered by filter pressing and finally undergoes harmless solidification before it is sent to landfills. However, this process is costly (approximately US$66.1/t) and complicated. In this study, groundwater treatment sludge was effectively recycled to prepare novel erdite-bearing particles via a one-step hydrothermal method by adding only Na2S·9H2O. After hydrothermal treatment, the quartz and boehmite of the sludge were dissolved and recrystallised to sodalite, whilst ferrihydrite was converted to an erdite nanorod at 160 °C and a hematite at 240 °C. SP160 was prepared as fine nanorod particles with 200 nm diameter and 2-5 μm length at a hydrothermal temperature of 160 °C. Nearly 100% OTC and its derivatives in pharmaceutical manufacture wastewater were removed by adding 0.1 g SP160. The major mechanism for the removal was the spontaneous hydrolysis of erdite in SP160 to generate Fe oxyhydroxide and use many hydroxyl groups for coordinating OTC and its derivatives. This study presents a novel method for the resource reutilisation of waste groundwater treatment sludge and reports efficient erdite-bearing particles for pharmaceutical manufacture wastewater treatment.

Recycling of High-Purity Strontianite and Hematite from Strontium-Bearing Sludge

Sr-bearing sludge is a hazardous waste that is commonly generated by nuclear power plants and mineral refining operations. In this work, Sr-bearing sludge was simulated and then cleanly recycled into high-purity strontianite with hematite nanoparticles as a byproduct via a novel hematite precipitation route. The sludge contained 26.1% Fe, 3.5% Sr, and Si impurities. After dissolution in 1.2 M nitric acid, the sludge was treated hydrothermally with the addition of glycol to precipitate Fe effectively. Without the addition of glycol, only 52% Fe was hydrothermally precipitated in the form of hematite aggregates. With the addition of glycol at the optimal M glycol/M nitrate molar ratio of 0.4, nearly 100% Fe was removed in the form of hematite nanoparticles with an average diameter of 50 nm, whereas over 98% of Sr was retained in the leachate. The generated hematite was highly purified with an Fe2O3 content of 95.23%. Sr was present at a high concentration of 3.9 g/L in the treated leachate and further precipitated in the form of strontianite with a purity of 96.8% through Na2CO3 addition. Tertiary butanol (TeB) exhibited a similar Fe removal rate as glycol even though its optimal M TeB/M nitrate molar ratio was 0.1, which was approximately one-fourth the optimal M glycol/M nitrate molar ratio. Fe removal involved spontaneous Fe3+ hydrolysis under hydrothermal conditions and was promoted by increasing the pH of the redox reaction between nitrate and glycol and/or TeB. The method reported here successfully enabled the resource recycling of Sr-bearing sludge to generate high-purity strontianite and hematite products without producing any secondary waste.

Upcycling of groundwater treatment sludge to magnetic Fe/Mn-bearing nanorod for chromate adsorption from wastewater treatment

Groundwater treatment sludge is a Fe/Mn-bearing waste that is mass produced in groundwater treatment plant. In this study, sludge was converted to a magnetic adsorbent (MA) by adding ascorbate. The sludge was weakly magnetised in the amorphous form with Fe and Mn contents of 28.8% and 8.1%, respectively. After hydrothermal treatment, Fe/Mn oxides in the sludge was recrystallised to siderite and rhodochrosite, with jacobsite as the intermediate in the presence of ascorbate. With an increment in ascorbate dosage, the obtained magnetic adsorbent had a significant increase in chromate adsorption but a decrease in magnetisation. When the M_ascorbate/M_Fe molar ratio was 10, the produced MA-10 was a dumbbell-shaped nanorod with a length of 2–5 μm and a diameter of 0.5–1 μm. This MA-10 showed 183.2 mg/g of chromate adsorption capacity and 2.81 emu/g of magnetisation. The mechanism of chromate adsorption was surface coprecipitation of the generated Cr³⁺ and Fe³⁺/Mn⁴⁺ from redox reaction between chromate and siderite/rhodochrosite on MA-10, separately. This study demonstrated an efficient recycling route of waste sludge from groundwater treatment to produce MA for treating chromate-bearing wastewater.

High-purity recycling of hematite and Zn/Cu mixture from waste smelting slag

In this study, Zn/Cu-bearing smelting slag was recycled via an integrated acid dissolution and hematite precipitation method. The slag was dissolved in nitric acid to generate an acid solution containing 23.5 g/L Fe, 4.45 g/L Zn and 2.81 g/L Cu, which was subjected to hydrothermal treatment with the addition of levulinic acid (LA). More than 99.95% of the initial Fe content was removed as hematite particles with diameters of approximately 200 nm, and the residual Fe concentration in the acid was 0.43 mg/L. The generated hematite contained 97.3% Fe₂O₃, 0.64% ZnO and 0.58% CuO. Greater than 99% of the initial Zn and Cu was retained in the acid and further precipitated as Zn/Cu-bearing solids by adjusting the solution pH to 9. The precipitated Zn/Cu-bearing solids contained 33.6% Zn and 21.7% Cu, whereas the Fe content was less than 0.2%. This paper is the first report of an environmentally friendly approach for recycling smelting slag without generating any hazardous waste.

Upcycling of Electroplating Sludge to Prepare Erdite-Bearing Nanorods for the Adsorption of Heavy Metals from Electroplating Wastewater Effluent

Electroplating sludge is a hazardous waste produced in plating and metallurgical processes which is commonly disposed of in safety landfills. In this work, electroplating sludge containing 25.6% Fe and 5.5% Co (named S1) and another containing 36.8% Fe and 7.8% Cr (S2) were recycled for the preparation of erdite-bearing particles via a facile hydrothermal route with only the addition of Na2S·9H2O. In the sludges, Fe-containing compounds were weakly crystallized and spontaneously converted to short rod-like erdite particles (SP1) in the presence of Co or long nanorod (SP2) particles with a diameter of 100 nm and length of 0.5–1.5 μm in the presence of Cr. The two products, SP1 and SP2, were applied in electroplating wastewater treatment, in which a small portion of Co in SP1 was released in wastewater, whereas Cr in SP2 was not. Adding 0.3 g/L SP2 resulted in the removal of 99.7% of Zn, 99.4% of Cu, 37.9% of Ni and 53.3% of Co in the electroplating wastewater, with residues at concentrations of 0.007, 0.003, 0.33, 0.09 and 0.002 mg/L, respectively. Thus, the treated electroplating wastewater met the discharge standard for electroplating wastewater in China. These removal efficiencies were higher than those achieved using powdered activated carbon, polyaluminum chloride, polyferric sulfate or pure Na2S·9H2O reagent. With the method, waste electroplating sludge was recycled as nanorod erdite-bearing particles which showed superior efficiency in electroplating wastewater treatment.

Resource Recovery of Waste Nd–Fe–B Scrap: Effective Separation of Fe as High-Purity Hematite Nanoparticles

Recycling rare-earth elements from Nd magnet scrap (Nd–Fe–B scrap) is a highly economical process; however, its efficiency is low due to large portions of Fe impurity. In this study, the effective separation of Fe impurity from scrap was performed through an integrated nitric acid dissolution and hydrothermal route with the addition of fructose. Results showed that more than 99% of the scrap was dissolved in nitric acid, and after three dilutions that the Nd, Pr, Dy and Fe concentrations in the diluted acid were 9.01, 2.11, 0.37 and 10.53 g/L, respectively. After the acid was hydrothermally treated in the absence of fructose, only 81.8% Fe was removed as irregular hematite aggregates, whilst more than 98% rare-earth elements were retained. By adding fructose at an Mfructose/Mnitrate ratio of 0.2, 99.94% Fe was precipitated as hematite nanoparticles, and the loss of rare-earth elements was <2%. In the treated acid, the residual Fe was 6.3 mg/L, whilst Nd, Pr and Dy were 8.84, 2.07 and 0.36 g/L, respectively. Such composition was conducive for further recycling of high-purity rare-earth products with low Fe impurity. The generated hematite nanoparticles contained 67.92% Fe with a rare-earth element content of <1%. This value meets the general standard for commercial hematite active pharmaceutical ingredients. In this manner, a green process was developed for separating Fe from Nd–Fe–B scrap without producing secondary waste.

A novel application of hematite precipitation for high effective separation of Fe from Nd-Fe-B scrap

Rare earths, e.g. neodymium (Nd), praseodymium (Pr) and dysprosium (Dy), are abundant in the rare earth sintered magnet scrap (Nd-Fe-B scrap), but their recycling is tedious and costly due to the high content of impurity Fe. Herein, a novel approach was developed to effectively recycle rare earths from the scrap via an integrated acid dissolution and hematite precipitation method. The scrap contained 63.4% Fe, 21.6% Nd, 8.1% Pr and 3.9% Dy. It was dissolved in nitric, hydrochloric and sulfuric acids, separately. Nearly all impurity Fe in the scrap was converted to Fe³⁺ in nitric acid but was converted to Fe²⁺ in hydrochloric and sulfuric acids. After hydrothermal treatment, the rare earths in the three acids were almost unchanged. From nitric acid, 77.6% of total Fe was removed, but total Fe was not from the hydrochloric and sulfuric acids. By adding glucose, the removal of total Fe was further increased to 99.7% in nitric acid, and 97% of rare earths remained. The major mechanism underlying total Fe removal in nitric acid was the hydrolysis of Fe³⁺ into hematite, which was promoted by the consumption of nitrate during glucose oxidation. This method effectively recycled rare metals from the waste Nd-Fe-B scrap and showed great potential for industrial application.

Editorial: Technically-based use of by-products as a tool to control pollution

This Virtual Special Issue of Journal of Environmental Management dealt with the recycling of waste and by-products, focusing on their use in controlling environmental pollution. The field of research was previously considered as promising, in view of its relevance and the increasing number of papers published in last years. And this Special Issue allows going a step ahead in the matter, with 90 submissions and a number of 48 high quality papers finally accepted and published. We think that it will be useful at a global level, especially for researchers, social partners, and social actors involved in environmental and public health issues related to environmental pollution.

Hydrothermal Conversion of Red Mud into Magnetic Adsorbent for Effective Adsorption of Zn(II) in Water

Red mud, a Fe-rich waste generated from the aluminum industry, was recovered as an adsorbent for wastewater treatment. The separation process of red mud from water after adsorption, including centrifugation and filtration, was complicated. This study demonstrated an alternative option to recycle red mud for preparing magnetic adsorbent via a facile hydrothermal route using ascorbic acid as reductant. Red mud is weakly magnetized and consists of andradite, muscovite, hematite, and cancrinite. After hydrothermal treatment, andradite in red mud was reductively dissolved by ascorbic acid, and transformed into magnetite and morimotoite. With increasing hydrothermal temperature, the dissolution of andradite accelerated, and the crystallite size of magnetite increased. When the hydrothermal temperature reached 200 °C, the prepared adsorbent P-200 showed a desirable saturation magnetization of 4.1 Am2/kg, and could be easily magnetically separated from water after adsorption. The maximum adsorption capacity of P-200 for Zn2+ was 89.6 mg/g, which is eight-fold higher than that of the raw red mud. The adsorption of Zn2+ by P-200 fitted the Langmuir model, where cation exchange was the main adsorption mechanism. The average distribution coefficient of Zn2+ at low ppm level was 16.81 L/g for P-200, higher than those of the red mud (0.3 L/g) and the prepared P-120 (1.48 L/g) and P-270 (5.48 L/g), demonstrating that P-200 had the best adsorption capacity for Zn2+ and can be served as a practical adsorbent for real-world applications. To our knowledge, this is the first study to report the conversion of red mud into a magnetic adsorbent under mild conditions.

Resource recovery of wastewater treatment sludge: synthesis of a magnetic cancrinite adsorbent

Water treatment sludge, which is mechanically dewatered and landfilled as solid waste, is considerably generated in water plants for potable water production. Herein, a novel route to hydrothermally convert this sludge into magnetic particles (MPs) is demonstrated. The sludge comprised amorphous aggregates with a relatively high Al/Si ratio of 3.7 and low Fe content of 8.5 wt%. After hydrothermal treatment, the Al/Si ratio of the MPs was approximated to 1, which was unaffected as the NaOH concentration increased from 2 M to 4 M or 6 M. The amorphous sludge was converted to MPs in the following order: spherical sodalite with a diameter of 3–5 μm, large spherical sodalite with a diameter of 5–10 μm and crystal dendritic cancrinite. Dendritic cancrinite was generated by recrystallisation of amorphous Al/Si oxides with spherical sodalite as the intermediate. With the addition of ascorbic acid, magnetisation of the weakly magnetised sludge increased from 0.11 emu g⁻¹ to 3.6 emu g⁻¹ and 14.8 emu/g by raising the NaOH concentration from 2 M to 4 M and 6 M. The magnetic property was related to the magnetite generated from the reduction of ferrihydrite and hematite in the sludge by the added ascorbic acid. Dendritic cancrinite exhibited an optimal surface site concentration of 0.31 mmol g⁻¹ and desirable adsorption capacity of tetracycline (TC) (482.6 mg g⁻¹), which were twice those of spherical sodalite prepared with 4 M NaOH. This study not only highlights the resource recovery of wastewater treatment sludge for MP preparation but also presents a new and effective adsorbent for treatment of TC-containing wastewater.

Water treatment sludge was directly converted into magnetic spherical sodalite and dendritic cancrinite particles, separately. These particles were efficient in tetracycline adsorption.