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The removal of antimony (Sb) from textile and dyeing wastewater is a challenging task. To meet the discharge limit and the demand for water reuse, the currently available treatment process was complicated: first, coagulation with polyaluminum chloride (PAC) and polyacrylamide (PAM) prior to membrane filtration to remove turbidity and then coagulation with polyferric sulfate (PFS) after membrane filtration to remove Sb from the concentrate. Such a process is troublesome in operation and generates a large amount of hazardous sludge. Here, we report that using a titanium xerogel coagulant (TXC) prior to membrane filtration could simultaneously remove the turbidity and the residual Sb in one-step. With TXC as the coagulants, no after-membrane coagulation was needed. Moreover, the TXC coagulation significantly alleviated the membrane fouling. A 24-day running of a pilot-scale test demonstrates that addition of 60 mg/L of TXC reduced the residual Sb concentration in the effluent from 226–894 μg/L to below the discharge limit (

Hydrolysis and precipitation are the key processes that determine the coagulation performance of inorganic metal salts. The valence and concentration of metal salts, ligands, and solution pH all influence the coagulation performance. In order to get a full picture on the pros and cons of various inorganic metal salts as coagulants, this paper conducted thermodynamic equilibrium calculations and coagulation experiments to analyze and verify the hydrolysis-precipitation areas and speciation of four inorganic metal salts (aluminum, iron, titanium, and zirconium) in the absence and presence of ligands. In addition, the contributions of polynuclear hydrolysates in the hydrolysis and precipitation of the four metal ions were quantitatively described. Furthermore, the underlying mechanisms of the effects of common aquatic ligands on coagulation performance were discussed from the perspective of solution chemistry. The above results provide a theoretical basis for the development and application of inorganic metal salt coagulants, and are of great significance for understanding the coagulation performance and mechanisms accurately.

Laccase, as an important biocatalyst, has aroused wide public interests in the field of environment and catalysis. However, the application of laccase in large-scale is still limited due to its hig...

Antimony (Sb) pollution in the water environment caused by the large-scale mining of Sb ore and the wide use of Sb-containing products seriously endangers human health and poses a great threat to the ecological environment. Coagulation is one of the most cost-effective technologies for Sb pollution control in water/wastewater treatment and has been widely used. However, a comprehensive understanding of Sb pollution control by coagulation, from fundamental research to practical applications, is lacking. In this work, based on the current status of Sb pollution in the water environment, a critical review of the Sb removal performance and mechanism by coagulation and related combined processes was carried out. The influencing factors of Sb removal performance by coagulation are introduced in detail. The internal mechanisms and improvement strategies of Sb removal by oxidation/reduction-coagulation and coagulation-membrane filtration technologies are emphasized. Moreover, given the development of Sb-removing coagulants and the resource utilization of Sb-containing sludge, future perspectives of coagulation for Sb removal are discussed. As the first review in this field, this work will illuminate avenues of basic research and practical applications for Sb and Sb-like pollution control in water/wastewater treatment.

Temporal and spatial segregations are two fundamental requirements for the successful synthesis of nanoparticles (NPs). To obtain colloidally stable selenium nanospheres (SeNSs), surfactants or polymers are generally needed as structure-directing agents or stabilizers in the reduction approaches for SeNP synthesis. The addition of such chemicals sacrifices the purity of the obtained SeNPs and, therefore, is detrimental to the applications. Here, for the first time, we report that low-molecular weight (less than six carbons) diketones are excellent photoreductants for green and tunable synthesis of SeNPs, owing to their merits in temporal and spatial control. With simple diketones as the photoreductants, the resultant SeNPs were pure and colloidally stable with nice photoelectronic properties. This finding not only provides a useful strategy for the synthesis of SeNPs but also might be a milestone in the development of ketone photochemistry.

Titanium xerogel coagulant (TXC) has been proved as a promising coagulant in water treatment. However, the turbidity removal efficiency of TXC is unsatisfactory as there is a considerable amount of dissolved organic matter (DOM). To overcome this shortage, a cationic flocculant, poly(diallyl dimethyl ammonium chloride) (PDADMAC), was composited into the TXC. With the removal of Escherichia coli (E. coli) and the residual turbidity as evaluation index, the composition of PDADMAC to TXC was proved to result in an enhanced resistance to DOM load. The resultant PDADMAC-TXC (P-T) composites had a good pH-resistance. In the pH range of 4.0–10.3, the required dose of P-T composites kept at a low level, whereas much higher doses were needed for the TXC. At pH 8.7, the minimum dose required to achieve a 90% E. coli removal from a 10 mg/L DOM solution was reduced from 70 mg/L TXC to 15 mg/L P-T composites. The improvements could be attributed to the three effects of PDADMAC in the P-T composites: strengthened charge neutralization for negatively charged E. coli, enhanced adsorption bridging through the polymer chains, and weakened complexation between organic matter and titanium due to the hydrophobic interactions between the aliphatic chains of PDADMAC and the hydrophobic segments of TXC. The work here provides a useful strategy for the fabrication of DOM-resistant coagulants to meet the requirements for some special kinds of water.

Cu(II)-EDTA is a stable metal-organic complex in electroplating wastewater and is hard to be removed by the conventional precipitation method. UV decomplexation is a useful strategy for its removal. Here, we report that acetylacetone (AA) can enhance the decomplexation of Cu(II)-EDTA by facilitating the photo-Fenton reactions. Under UV irradiation, the pseudo-first-order decomplexation rate constant (k1) of Cu(II)-EDTA (0.3 mM) was enhanced 2–3 times by AA at a [AA]:[Cu(II)-EDTA] molar ratio of 1, whereas the photochemical consumption of AA was reduced 2–3 times by the presence of Cu(II)-EDTA. Based on intermediate species and product analyses, the first step in decomplexation was confirmed as direct photo-decarboxylation of Cu(II)-EDTA via the ligand to metal charge transfer (LMCT) process. Once Cu+ was formed, there would be Fenton-like reactions. AA facilitated the photo-Fenton reactions in two ways: (1) reducing Cu2+ to Cu+ and (2) providing more H2O2 via the redox cycle of AA. Besides, the degradation products of AA could also involve in the decomplexation of Cu(II)-EDTA through LMCT. The work here might provide a promising solution for the decomplexation of metal-organic complexes in industrial wastewater.

Basicity is an intrinsic factor of coagulants that plays important roles in turbidity removal. However, the effects of basicity on the coagulation performance of titanium (Ti) coagulants remain unclear. This knowledge gap is mainly due to the lack of highly polymerized Ti coagulants. In this study, we fabricated a series of Ti-based coagulants with basicity values in the range of 0-93% and evaluated their coagulation performance with simulated and real waters of low-turbidity. A positive correlation was observed between the basicity of the Ti coagulants and the coagulation performance, i.e., higher basicity led to higher turbidity removal, larger floc size, and faster settling rate. For the two titanium xerogel coagulants (TXC) of similar chemical structures, the one with a higher basicity (TXC-93%) worked better than the one of a lower basicity (TXC-78%). The advantage brought from the difference in basicity is of great significance in the purification of low-turbidity water. For the treatment of a surface river water, the required dosage of TXC-93% was about 1/2 to 1/3 of those of titanium tetrachloride (TiCl4) and polytitanium tetrachloride (PTC-21%), and was comparable to that of a high-quality commercialized polyaluminum chloride (PAC-72%). The results here provide a useful technical guidance for the mass production and application of Ti coagulants.

Coagulation is a conventional method in water treatment. In recent decades, with the rapid development of membrane filtration, the use of coagulation is facing some new challenges. How to minimize the membrane fouling became a leading-edge topic in the study of coagulation. Here, the performances of three types of titanium coagulants were evaluated in terms of both the coagulation removal of toxic micropollutants and the alleviation of membrane fouling. Three oxysalts and two antibiotics were taken as representatives of inorganic and organic micropollutants. As compared with titanium tetrachloride (TiCl4) and polytitanium chloride (PTC), titanium xerogel (TXC) with a higher polymerization degree showed much better performances in direct coagulation removal of oxysalts and antibiotics and in pre-coagulation for mitigating membrane fouling in both coagulation-sedimentation-ultrafiltration (CSUF) and in-line coagulation-ultrafiltration (CUF) processes. In the CSUF system, the membrane permeate flux with TXC pre-coagulation (89.5%) was much higher than those of TiCl4 (56.1%) and PTC (57.4%). After a 5 day continuous operation, the transmembrane pressure in the CUF system with TXC coagulation was increased only to 4.9 kPa, while those of PTC and TiCl4 were 12.2 and 18.5 kPa, respectively. The results here demonstrate that TXC is a promising coagulant for pollutant removal and membrane fouling alleviation, due to the following merits: better floc properties, weaker pH-dependence, and higher resistance to coordination with organic pollutants. The observation shed new lights on the fabrication and application of coagulants in a wide variety of scenarios.

Cu(II)-EDTA is highly stable in a wide pH range (3.0∼12.0) and hard to be removed by the conventional precipitation method. Fe(III) displacement/UV photolysis/alkaline precipitation [Fe(III)/UV/NaOH] has been proposed as a promising method for the removal of Cu(II)-EDTA. Nevertheless, a high dose of Fe(III) is needed in this combined process, resulting in the production of a large amount of hazardous sludge. The photochemistry of Fe(III) is known to be ligand-dependent. Fe(III)-oxalate complexes are strongly photoactive. However, the addition of oxalic acid to the Fe(III)/UV/NaOH process was of little help. Acetylacetone (AA) is a good chelating ligand for many metals and has been proved as an efficient photo-activator. By introducing a low dose of AA ([AA]/[Cu] = 1.5) into the Fe(III)/UV/NaOH process, the Fe(III) dosage ([Fe]/[Cu]) was reduced from 10.4 to 3.2. As a result, the chemical cost was reduced from 13.9 to 7.6 kW h/m3. Meanwhile, the energy cost in the UV photolysis was reduced from 1066.5 to 752.4 kW h/m3. Most importantly, the sludge yields were reduced from 8.3 to 2.7 kg/m3 in a simulated wastewater and from 101.8 to 30.8 kg/m3 in a real electroplating wastewater. Such a sludge reduction is of great significance in mitigating the load of landfill.

The importance of singlet oxygen (1O2) in synthesis of fine chemicals, inactivation of viruses and bacteria, degradation of pollutants, and photodynamic therapy has promoted the research on photocatalytic generation of 1O2. Metal-organic frameworks (MOFs), owing to the tunable structure, are good candidate photocatalysts for 1O2 generation. However, due to the poor visible-light absorption capacity and the invalid recombination of charge carriers, the 1O2 generation yields of most MOFs are still limited. Here, we demonstrate that β-diketone anchoring could improve the photocatalytic generation of 1O2 by MIL-125 through enhanced energy transfer. The enhanced energy transfer was attributed to both the narrowed bandgap and the diketone-derived oxygen vacancies, which function as the trapping sites for charge carriers. The results here shed light on the generation of 1O2 by visible-light-responsive MOFs and provide a new strategy for the design of MOFs-based photocatalysts.

Post-synthetic modification with β-diketone has been proven as an effective strategy to improve the visible light response activities of metal-organic frameworks (MOFs). Nevertheless, the effects of the anchored-diketones on the excitonic behaviors and photocatalytic mechanisms of MOFs are still unclear. Herein, two MOFs (MIL-101 with CrIII and MIL-101 with AlIII) were employed as prototypical materials to further understand this post-synthetic modification strategy. Diketone anchoring could enhance the photocatalytic generation of singlet oxygen (1O2) by the two MOFs in a joint mechanism: exciton-mediated energy transfer and photosensitization. Diketone anchoring induced triplet exciton was found playing an important role in the photocatalytic generation of 1O2. The photosensitization of the anchored-diketone also played a key role in the photocatalytic generation of 1O2. The results here provide a useful guidance for the development of MOFs-based photocatalysts and are helpful to realize the regulation of excitonic behaviors and photocatalytic mechanisms of MOFs.

Coagulation is one of the most widely used technologies in water/wastewater treatment. In recent years, titanium (Ti) coagulants have attracted increasing attention due to their outstanding performance and risk-free metal residues in coagulation. Nonetheless, there is a lack of a comprehensive understanding of Ti coagulants from fundamental research to practical application. To provide a full picture of Ti coagulation, the hydrolysis behaviors, coagulation mechanisms, application potential, and economic analysis of Ti coagulants were critically reviewed for the first time in this work. At present, there are three key issues in the development of Ti coagulation: 1) The hydrolysis controllability in the fabrication of pre-polymerized Ti coagulants remains a challenge; 2) New methods for Ti speciation analysis are still highly needed; 3) Scenarios of particular needs for low metal residue or high water clarity are to be found to make a breakthrough in the practical application of Ti coagulants. Cost reduction in the preparation of Ti coagulants and sludge recycling to value-added products are two important measurements to promote the future application of Ti coagulants in water/wastewater treatment. This review offers guidelines for the further research and development of Ti coagulants.

Fluoride (F−) pollution is a worldwide issue. Coagulation with aluminum (Al) salts is an efficient and economical method for the removal of F−. However, due to the strong complexation between Al3+ and F−, the residual F− and Al after coagulation usually exceed the limits. Zirconium (Zr) coagulants have drawn increasing attention due to their excellent flocculation ability for organic matter. In this work, the performance and mechanism of ZrCl4 coagulation for F− removal were investigated with the widely used Al2(SO4)3 as a reference. The optimum pH range is 4.0–6.0 for ZrCl4 and 8.0–10.0 for Al2(SO4)3. ZrCl4 was superior to Al2(SO4)3 for F− removal as the initial F− concentration was less than 30.0 mg L−1. Coexisting substances at environmental concentration levels showed negligible effects on F− removal by ZrCl4. Besides the better F− removal, another advantage of ZrCl4 over Al2(SO4)3 was the much lower residual metal concentration in the pH range of 4.0–11.0. The hydrolysis of Al2(SO4)3 was significantly inhibited due to the formation of Al-F complexes while the hydrolysis of ZrCl4 was not influenced even under strongly acidic conditions. Therefore, F− removal by Al2(SO4)3 was mainly achieved by preliminary complexation between Al3+ and F− and subsequent hydrolysis and polymerization of these complexes, while adsorption onto hydrolysates and ion exchange with surface hydroxyl groups were the main ways of F− removal by ZrCl4. The work here provides a new method for F− removal and may shed light on the application of Zr coagulants for other pollutants.

Titanium xerogel coagulant (TXC) worked better than titanium tetrachloride (TC) and polytitanium chloride (PTC) in a wider pH/dose range for the removal of turbidity. However, the underlying mechanisms were not comprehensively understood. In this work, the better coagulation performance of TXC than TC and PTC was systematically elucidated from the following aspects: the physicochemical properties of the three coagulants, the removal of turbidity and organic matter, and the complexation reactions in coagulation. The results demonstrate that the merits of TXC were attributable to the following characteristics: (1) the higher surface charge density/total surface site concentration/isoelectric point of TXC hydrolysates, (2) the formation of TXC hydrolysates with a net-work structure, and (3) the strong binding affinity of TXC hydrolysates to organic matter caused by the bonded acetylacetone in the TXC framework. In short, the hydrolysis behavior of TXC significantly differed from both its precursor, TC, and the prehydrolyzed PTC. The difference in the hydrolysis of TXC was derived from the gelation process, which led to the polymerization of Ti in a way different from prehydrolyzation. The elucidation of the hydrolysis mechanisms is useful for the better application of Ti-based coagulants and may shed light on the preparation of other metal salts.