Performance and potential mechanism of Cr(VI) reduction and subsequent Cr(III) precipitation using sodium borohydride driven by oxalate.

The method of treating high concentrations of Cr(VI) alone by NaBH4 has proved feasible, but the effects of the coexistence of Cr(VI) and organic compounds have not been evaluated. The objective of this study was to explore the potential mechanism by which oxalate affects the reduction of high concentrations of Cr(VI) treated by sodium borohydride (NaBH4) and the subsequent precipitation of Cr(III). The results show that Cr(VI) reduction could be gradually promoted by oxalate (1.0-10 mM). Compared with the control solution, the reduction of Cr(VI) in a 10 mM oxalate solution could be increased from 56.6% to 99.1%. Particularly, the promotion of Cr(VI) reduction attributed to the enhancement of OH- production from NaBH4 hydrolysis due to the increasing concentration of C2O42- species, forming conjugated acid-base pairs in the form HC2O4--C2O42-, which provided an effective buffer. In 0.10-0.40 mM oxalate-Cr(VI)-NaBH4 systems, the resulting Cr(III) could precipitate at different levels within 20 h, and showed settlement rates in the range of 8.8% and 95.8%, but no precipitate was found in 1.0-10 mM oxalate-Cr-NaBH4 systems. This is related to whether there was a sufficient oxalate dosage, which could be complexed with Cr (III) at a molar ratio of 1:1. The precipitates were analysed by means of electron spin resonance (ESR), atomic force microscopy (AFM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR), indicating that Cr (III) could support oxalate coprecipitation. The results of the present study reveal the influence of oxalate on Cr(VI) reduction and subsequent Cr (III) precipitation, which are of great significance to the application of NaBH4 in the treatment of industrial wastewater containing Cr(VI)-oxalate.

Response surface methodology investigation into optimization of the removal condition and mechanism of Cr(â ¥) by Na2SO3/CaO.

The removal of Cr(Ⅵ) by chemical reduction-precipitation is widely applied in wastewater treatment plants. Nevertheless, the formation of Cr(OH)3 with gel properties has weak settlement performance, making it necessary to add a coagulant aid to reduce the settling time and improve the settling effect. In this investigation, a high concentration of Cr(Ⅵ) was removed using Na2SO3 as a reducing agent and CaO as a coagulant. An improved reduction and precipitation experiment was modeled by applying a three-factor central composite experimental design (CCD). To reveal as many mechanisms as possible for CrT removal, other verification experiments were performed. The CrT removal efficiency decreased, which can be explained by the following three reasons: dissolution of Cr(Ⅲ), competition for adsorption between Ca2+ and Cr(Ⅲ) at different coagulation times, and formation of a solubility complex with Cr(Ⅲ) due to the surplus SO32- in solution. The increasing CrT removal efficiency can be explained by the following two reasons: dissolved Ca2+ from CaO can neutralize CrO2- that is produced by the dissolution of Cr(OH)3 in alkaline solution and can broaden the optimal final pH range of coagulation. Ca2+ could also strengthen the CrT removal through adsorption bridging and co-precipitation with CaO as the core of flocs.

Pilot-scale fluoride-containing wastewater treatment by the ballasted flocculation process.

A pilot-scale ballasted flocculation system was used to remove fluoride from one type of industrial wastewater. The system included the formation of calcium fluoride (CaF2) using calcium hydroxide followed by coagulation sedimentation. Calcium fluoride was recycled as nuclei for enhancing CaF2 precipitation and as a ballasting agent for improving fluoride removal and flocculation efficiency. Factors affecting fluoride and turbidity removal efficiencies, including pH in the CaF2-reacting tank and coagulation-mixing tank, sludge recycling ratio, and dosages of FeCl3 and polyacrylamide (PAM), were investigated in the pilot-scale system. The recycled CaF2 precipitates improved CaF2 formation kinetics, enhanced fluoride removal and flocculation performance. Under the optimized condition, the ballast flocculation process reduced fluoride concentration from 288.9 to 10.67 mg/L and the turbidity from 129.6 NTU to below 2.5 NTU.

Characteristics and disinfection by-product formation potential of dissolved organic matter in reservoir water in cold area.

The severe cold in winter with harsh natural conditions in Northeastern China seriously affect the water quality of the reservoir, showing the increased content and more complex types of organic matter, which brings severe challenges to the control of disinfection by-products (DBPs) in drinking water treatment with reservoir water as the water source. In this study, the fractions of dissolved organic matter (DOM) in source water at before ice formation period (P1), ice-age period (P2), and ice begin to melt period (P3) were separated by membrane separation technology. Subsequently, the contributions of DOM fractions with different molecular weights (MW) to DOC, UV254, and SUVA254, and their disinfection by-product formation potential (DBPFP) were evaluated. Although DOM with high MW (5-10 kDa) contributed the most to dissolved organic carbon (DOC) and UV254, but the contribution of DOM with low MW (0-1 kDa) to DBPs formation could not be ignored, especially during ice-age period. There was no significant difference in the total numbers of DOM formula belonged to low MW fraction at these three periods, mainly including lignin, followed by N-containing saturated compounds and tannins. Additionally, redundancy analysis revealed that DOC and UV254 as the predictors had good correlation with DBPFP, while SUVA254 could not be used as a single indicator to predict the generation potential of DBPs, and could be used as the prediction factors together with AImodwa parameter closely related to DBPFP. The study provided key information for controlling the DBPs formation of DOM in water, especially in the ice-age period, and provided the theoretical basis for water plant production.

Formation pathway of disinfection by-products of lignin monomers in raw water during disinfection.

In this study, the dissolved organic matter (DOM) profiles of water samples from a water source in northeastern China were analyzed by high-resolution mass spectrometry (HRMS), and its changes after chlorination were investigated. The results showed that lignin substances accounted for a significant proportion in DOM and chlorinated products and were the main precursors of disinfection by-products (DBPs). During disinfection, macromolecular DOM was transformed into small molecules, and lignin substances have the most obvious and complex changes in reaction. Two lignin monomers 4-propylphenol (4PP) and 4-propylguaiacol (4PG) were used as model compounds to study their reaction kinetics and degradation pathways during disinfection. The degradation of both lignin monomers conformed to pseudo-first-order reaction kinetics, and the reaction rate constant of 4PG was higher than that of 4PP. The effects of chlorine dosage, pH and temperature on the degradation reaction kinetics of two lignin monomers were investigated. The degradation rates of 4PP and 4PG increased with increasing chlorine dosage, pH and temperature. The two monomers showed similar properties in the chlorination degradation process, and generated multiple intermediates, which were mainly transformed into small molecules by chlorine electrophilic substitution and nucleophilic substitution, and further generated DBPs.

N-nitrosodimethylamine formation during oxidation of N,N-dimethylhydrazine compounds by peroxymonosulfate: Kinetics, reactive species, mechanism and influencing factors.

This study found that peroxymonosulfate (PMS) oxidation without activation has the potential to generate a suspected human carcinogen, N-nitrosodimethylamine (NDMA), in water containing N,N-dimethylhydrazine compounds. Considerable amounts of NDMA formed from three compounds by PMS oxidation were observed. 1,1,1',1'-Tetramethyl-4,4'-(methylene-di-p-phenylene) disemicarbazide (TMDS), which is an industrial antiyellowing agent and light stabilizer, was used as a representative to elucidate the kinetics, transformation products, mechanism and NDMA formation pathways of PMS oxidation. TMDS degradation and NDMA formation involved direct PMS oxidation and singlet oxygen (1O2) oxidation. The oxidation by PMS/1O2 was pH-dependent, which was related to the pH-dependent characteristics of the reactive oxygen species and intermediates. The degradation mechanism of TMDS mainly included the side chain cleavage, dealkylation, and O-addition. NDMA was generated from TMDS mainly via O-addition and 1,1-dimethylhydrazine (UDMH) generation. The cleavage of amide nitrogen in O-addition products and primary amine nitrogen in UDMH are likely the key steps in NDMA generation. The results emphasized that the formation of harmful by-products should be taken into account when assessing the feasibility of PMS oxidation.

Regrowth potential of chlorine-resistant bacteria in drinking water under chloramination.

The regrowth of chlorine-resistant bacteria in drinking water can deteriorate water quality. The study evaluated the relationship between organic carbon and the regrowth potential of chlorine-resistant bacteria remaining in chloraminated water samples. The results showed that the community structure of bacteria changed with the increase of chloramine dosage. The order in which organic carbon utilized by bacteria was affected by the composition of bacterial community. The biodegradable dissolved organic carbon (BDOC), assimilable organic carbon (AOC), bacterial regrowth potential (BRP) and total cell concentration (TCC) in cultivated water sample after disinfection with 1.8 mg/L chloramine increased form 0.22 mg/L, 33.68 µg/L, 2.70 × 105 cells/mL and 3.48 × 104 cells/mL before cultivation to 1.20 mg/L, 193.90 µg/L, 4.74 × 105 cells/mL and 1.46 × 105 cells/mL, respectively. The increase of TCC did not result in the decrease of BDOC, AOC and BRP in the cultivated water samples. The results showed that other biodegradable organic carbon in chloraminated water samples assimilated by residual chlorine-resistant bacteria besides AOC, BDOC, and organic carbon assimilated by indigenous bacteria. AOC, BDOC, and BRP indicators used to characterize the biostability of drinking water were not enough to accurately assess the regrowth potential of chlorine-resistant bacteria remaining in drinking water. It is suggested to supplement the index of TCC in cultivated water samples, which might be able to more accurately evaluate the regrowth potential of chlorine-resistant bacteria remaining in drinking water.

Activation of peroxymonosulfate by palygorskite-mediated cobalt-copper-ferrite nanoparticles for bisphenol S degradation: Influencing factors, pathways and toxicity evaluation.

Peroxymonosulfate (PMS)-based advanced oxidation process is considered a potential technology for water treatment. Here, palygorskite (PAL)-mediated cobalt-copper-ferrite nanoparticles (16%-CoCu0.4Fe1·6O4@PAL, donated as 16%-CCFO@PAL) were employed for PMS activation to remove bisphenol S (BPS). BPS degradation was greater than 99% under the optimal conditions within 25 min, on which the effects of various influencing factors were explored. The adsorption dissociation energy of PMS over 16%-CCFO@PAL was -6.27 eV, which was lower than that of the Cu-free catalyst (-6.15 eV), demonstrating the excellent catalytic ability of 16%-CCFO@PAL. The efficient catalytic ability of 16%-CCFO@PAL was also verified in real water samples. The oxidation intermediates were identified and their generations were systematically analyzed by DFT calculations. The possible degradation pathways of BPS were proposed and the toxicity of products was predicted. BPS affected the normal development of zebrafish embryos and the levels of sex hormone in adult male zebrafish, and was harmful to the tissues, such as testis, liver, and intestine of zebrafish. The 16%-CCFO@PAL/PMS process can effectively reduce the toxicity of BPS-polluted water. This study paves the way for the real application of 16%-CCFO@PAL/PMS oxidation process and provides a new perspective for the evaluation of water toxicity.

Non-radical dominated degradation of bisphenol S by peroxymonosulfate activation under high salinity condition: Overlooked HOCl, formation of intermediates, and toxicity assessment.

Extensive studies revealed that Cl- could inhibit the removal of targeted pollutants under low Cl- conditions in the peroxymonosulfate (PMS) system. However, the enhanced effect of Cl- has always been overlooked under high Cl- conditions. Here, we find that high concentration of Cl- played a critical role in bisphenol S (BPS) degradation by activating PMS using 16%-CoFe2O4@PAL (16%-CFO@PAL). The removal of BPS was sharply enhanced after introducing 0.5 and 1.0 M Cl-, and the corresponding kobs increased to 0.922 min-1 and 1.103 min-1, which was 6-fold and 7-fold higher than the control (0.144 min-1), respectively. HOCl was demonstrated as the dominant species for removing BPS in 16%-CFO@PAL/PMS system under high Cl- circumstances. The typical chlorinated BPS intermediates were identified, which showed higher eco-toxicity than BPS. The chlorinated byproducts along with their toxicity could be effectively eliminated after 30 min. The possible formation mechanism of chlorinated products was further revealed by theoretical calculations. Toxicity assessment experiments showed that BPS significantly affected hormone levels of zebrafish and showed toxicity on the testis and liver of zebrafish, which could be reduced using 16%-CFO@PAL/PMS system. This study attracts attention to the overlooked HOCl in PMS-based processes under high salinity conditions.

Powdered activated carbon doping improves the mechanical and adsorption properties of cementitious microfiltration membrane.

Cementitious membrane (CM) is a promising microfiltration membrane with low cost for raw materials and low energy consumption of non-sintering fabrication process. A novel carbon-cementitious microfiltration membrane (CCM) was fabricated with powdered activated carbon (PAC) as an additive based on CM, to solve the low mechanical strength of CM during multiple practical uses. While maintaining adequate pure water flux and porosity, the mechanical strength of the membrane was greatly improved to ensure the stability of the membrane in the filtration process. The bending strength of the CCM was 2-3 times higher than that of CM. 10 wt% CCM has the smallest critical pore size and optimal permeability, which was chosen to be the optimal PAC doping ratio. The X-ray diffraction and FT-IR results indicated that the addition of PAC did not change the mineral composition of cement hydration products, and the appropriate amount of PAC acted as a nucleation site and accelerated hydration. The effect of size effect on bending strength was more obvious with the decrease of membrane thickness. In the membrane adsorption experiments of benzophenone-4, nitrobenzene and p-chloronitrobenzene, the CCM exhibited prominent adsorption properties than CM. These results broaden the application scope of microfiltration membranes in water treatment process.

Preparation of novel N-doped biochar and its high adsorption capacity for atrazine based on π-π electron donor-acceptor interaction.

Novel nitrogen (N)-doped cellulose biochar (NC1000-10) with large adsorption capacity (103.59 mg g-1) for atrazine (ATZ) was synthesized through the one-pot method. It has the best adsorption efficiency than N-doped biochars prepared from hemicellulose and lignin. The adsorption behaviors of ATZ by N-doped biochars with different N doping ratios (NC1000-5, NC1000-10, NC1000-20 and NC1000-30) were significantly different, which was attributed to the difference of sp2 conjugate C (ID/IG = 0.99-1.18) and doped heteroatom N (pyridinic N, pyrrolic N and graphitic N). Adsorption performance of ATZ on NC1000-10 conformed to the pseudo-second-order kinetic and Langmuir adsorption isotherm model. Thermodynamic calculations showed that adsorption performance was favorable. Besides, wide pH adaptability (pH = 2-10), good resistance to ionic strength and excellent recycling efficiency make it have extensive practical application potential. Further material characterizations and the density functional theory (DFT) calculations indicated that good adsorption performance of NC1000-10 for ATZ mainly depended on chemisorption, and π-π electron donor-acceptor (EDA) interaction contributed the most due to high graphitization degree. Specifically, pyridinic N and graphitic N further promoted adsorption performance by hydrophobic effect and π-π EDA interaction between ATZ and NC1000-10, respectively. Pyrrolic N and other surface functional groups (-COOH, -OH) facilitated the hydrogen bond effect.

EEM-PARAFAC characterization of dissolved organic matter and its relationship with disinfection by-products formation potential in drinking water sources of northeastern China.

Dissolved organic matter (DOM) is the precursor of disinfection by-products (DBPs) which is widely found in the aquatic environment. The analysis of DOM in raw water is helpful to evaluate the formation potentials of DBPs. However, there is relatively little research on the DOM identification of raw water in northern China. In this study, the variation in DOM in M reservoir water in one year by fluorescence excitation-emission matrix-parallel factor analysis (EEM-PARAFAC) was investigated to evaluate the DBP formation potential (DBPFP). The results suggested that five components, namely, two humic-like substances (C2, C3), two fulvic-like substances (C1, C4) and one protein-like substance (C5), were identified in the DOM of M reservoir water. The content of DOM in autumn and winter was higher than that in spring and summer. The source of DOM in the water body of M reservoir was mainly from terrestrial source, but less from aquatic source. The source, types and humification degree of DOM affect the formation of DBPs. The formation potential of DBPs had the following order: trihalomethanes (THMs) > dichloroacetic acid (TCAA) > trichloroacetic acid (DCAA) > chloral hydrate (CH). The formation potentials of THM and TCAA were strongly correlated with C2 (rTHM = 0.805, rTCAA = 0.857). The formation potential of CH has a good correlation with C1 (r = 0.722). The formation of DCAA has a good correlation with C4 (r = 0.787). DOM and DBPFP were negatively correlated with the biological index (BIX) and fluorescence index (FI) of the raw water, and positively correlated with the humification index (HIX).

Selective and efficient removal of Hg (II) from aqueous media by a low-cost dendrimer-grafted polyacrylonitrile fiber: Performance and mechanism.

Polyacrylonitrile fiber was successfully modified with triazine-based dendrimer via grafting method as a promising adsorbent for removal of mercury species from aqueous media. The prepared adsorbent was characterized by elemental analysis, scanning electron microscope, Fourier transform infrared spectroscopy, porous structure analysis and X-ray photoelectron spectroscopy, providing the evidence of successful fabrication. The adsorption conditions were found via varying pH, dosage, coexisting substances, contact time, temperature and concentration. Adsorption performance, described better by the pseudo-second-order kinetics with intraparticle diffusion as rate controlling step and Langmuir isotherm model, indicated a chemisorption process with the maximum Langmuir adsorption amount of 227.64 mg g-1 for mercury ions. Thermodynamically, adsorption of mercury ions was spontaneous and endothermic. Desorption and regeneration experiments demonstrated that it could be reused in five successive adsorption cycles without significant loss of its original performance. Experimental data and density functional theory calculation disclosed the coordination geometries and chelating mechanism between the adsorbent and mercury ions. The proposed study would provide a new prospect for the purification of mercury in aqueous system by functionalizing commercial polyacrylonitrile fiber with dendrimers.

Catalytic Efficiency of Carbon-Cementitious Microfiltration Membrane on the Ozonation-Based Oxidation of Small Molecule Organic Compounds and Its Alkaline Buffering Effect in Aqueous Solution.

In this study, powdered activated carbon (PAC) was added to replace the silica in a cementitious microfiltration membrane (CM) to solve the problems of the low mechanical strength and short lifetime of CMs. The carbon-cementitious microfiltration membrane (CCM) was fabricated by the dry pressing method and cured at room temperature. The bending strength of CCM was 12.69 MPa, which was about three times more than that of CM. The average pore size was 0.129 µm, and was reduced by about 80% compared to that of CM. The addition of PAC did not reduce the degradation efficiency of membrane catalytic ozonation. Because of the strong alkaline buffering ability of CCM, the CCM-ozone coupling process could eliminate the effect of the pH value of the solution. The strong alkaline environment inside the membrane pores effectively accelerated the ozone decomposition and produced oxidizing radicals, which accelerated the reaction rate and improved the utilization rate of ozone. The CCM-catalytic ozonation reaction of organic compounds occurred within the pores and membrane surface, resulting in the pH of the solution belonging to the neutral range. The addition of PAC accelerated the mass transfer and made the pollutants and oxidant react in the membrane pores and on the membrane surface. The reuse experiments of the CCM-ozone coupling process for removing nitrobenzene demonstrated that CCM has good catalytic activity and reuse stability. It broadens the application scope of CCM in the field of drinking water and provides theoretical support for the practical application of CCM.

Improvement of the fabricated and application of aluminosilicate-based microfiltration membrane.

Aluminosilicate composite materials are characterized by their low cost, nontoxicity and facilely shaped. Membrane prepared using aluminosilicate composites have the following disadvantages: large mean pore size and low mechanical strength. To address these limitations, flat microfiltration membranes were fabricated using SiO2 powder and aluminosilicate composite as raw materials. The membrane performance was optimized by regulating the particle size of SiO2, the ratio of SiO2 to aluminosilicate composite (s/a), and the type of chemical admixture. The X-ray diffraction results indicated that the crystalline SiO2 particles were favorable for the preparation of membranes with higher bending strengths. The decreasing particle sizes of SiO2 (1.33-0.15 µm) decreased the pore size distribution. The bending strength of the membrane reduced with an increase in s/a, while was effectively enhanced by adding dissolved Na2SiO3. The optimized inorganic microfiltration membrane could also catalyze ozone to remove 100% of benzophenone-4 with an initial concentration of 10 mg L-1 within 15 min, and TOC removal by 52.67%. This paper presents a revised method for preparing an inorganic microfiltration membrane, which is an increasingly promising material for water treatment because of its low cost, low energy consumption, and high catalytic performance.

UV/ peroxymonosulfate process for degradation of chloral hydrate: Pathway and the role of radicals.

In this study, kinetics, influencing factors and potential mechanisms involved in the degradation of chloral hydrate (CH) by UV/peroxymonosulfate (PMS) process were demonstrated. The degradation rate of CH could reach 89.6% by UV254/PMS process, significantly exceeding UV300/PMS (0.7%), UV350/PMS (6.3%), UV254 direct photolysis (9.0%) and PMS alone (0.0%) processes. CH degradation in UV254/PMS system followed pseudo first-order degradation kinetics with an apparent rate constant of 0.186 min-1, which was suppressed by Cl- and HCO3-. The optimal pH for CH degradation was around 5.0. Direct mineralization accounted for the CH degradation in UV/PMS system. Interestingly, the addition of PMS at the neutral condition before UV irradiation transferred CH into trichloroacetic acid (TCAA). The transformation efficiency of CH into TCAA at 10 min was enhanced from 2.17%-40.38% with the elevation of initial pH from 7.0-8.0. The subsequent exposure of UV lamps ceased the transformation of CH into TCAA and facilitated the direct mineralization of CH, but it did not work in the refractory TCAA degradation. Finally, it was revealed that HO predominantly participated CH degradation in UV/PMS process, while O2- was responsible for the transformation of CH into TCAA by addition of PMS before UV irradiation.

Selective adsorption and enhanced photodegradation of diclofenac in water by molecularly imprinted TiO2.

In the paper, molecularly imprinted TiO2 was prepared by surface molecularly imprinted technology and liquid phase deposition method for preferential removal of persistent toxic pollutants from complex environmental water. Diclofenac was selected as the template molecule and target for photodegradation study. The characterization results of SEM, TEM, FTIR and XRD showed that the TiO2 film with imprinted diclofenac was successfully synthesized on the surface of TiO2 particles. Meanwhile, the adsorption and photodegradation experiments also indicated that the molecularly imprinted TiO2 had larger adsorption capacity, better selectivity and higher photodegradation performance for diclofenac than non-imprinted TiO2. The primary active species and degradation pathways during photodegradation process were also elucidated according to radical capture experiments and UPLC-MS-TOF technology. The prepared molecularly imprinted TiO2 has the advantages of efficient removal ability, high stability and environmental protection, so it has a wide application value in water treatment and water environmental restoration, especially when involved persistent toxic pollutants.

Variations of disinfection byproduct precursors through conventional drinking water treatment processes and a real-time monitoring method.

In this investigation, raw water (RW), settled water (SW), and filtered water (FW) collected from a drinking water treatment plant were fractionated into 24 natural organic matter (NOM) fractions with varying molecular weights and hydrophobicity. The yields of disinfection byproducts (DBPs) obtained during the chlorination of the NOM fractions were explored. Results revealed that the 0-1 kDa, 5-10 kDa, and hydrophobic DBP precursors dominated RW. Hydrophobic fractions cannot be effectively removed, which contributed to the high DBP precursors remaining in the FW. The optional optical parameters, including UVA (UV340, UV360, and UV380), UVB (UV280, UV300, and UV310), and UVC (UV254, UV260, and UV272), were analyzed to determine the DBP yields during chlorination of different NOM fractions. Results revealed that UVC could be applied to indicate the regulated DBP yields of the humified precursors. Contrary to the generally accepted view, for biologically derived precursors, their regulated DBPs and dichloroacetonitrile correlated better with UVA (e.g. UV340). Moreover, PARAFAC analysis was applied to decompose an array of 24 EEM spectra. Good linear correlations were found between the PARAFAC components and most DBP yields. Furthermore, four fluorescence parameters were proposed via a modified fluorescence picking method, which can serve as excellent surrogates of PARAFAC components. These fluorescence parameters were found to be effective in indicating most DBP yields. Finally, the fluorescence intensity at excitation wavelength/emission wavelength = 310/416 nm was found to be a promising built-in parameter for the real-time monitoring of DBP precursors, regardless of the humification degree of the precursors.

Formation and interdependence of disinfection byproducts during chlorination of natural organic matter in a conventional drinking water treatment plant.

It is crucial to explore the source, formation process and interdependence of disinfection byproducts (DBPs) to reduce their risk on public health. In this investigation, a source water was chlorinated to evaluate the initial formation rates and the maximum yields of trichloromethane (TCM), dichloroacetic acid (DCAA), and trichloroacetic acid (TCAA) based on a hyperbola model. The results showed that TCM achieved the highest initial formation rate and maximum theoretical concentration compared with DCAA and TCAA. The TCM yield can be used to forecast the yields of DCAA and TCAA throughout the whole reaction process, and the yields of chloral hydrate (CH), dichloroacetonitrile (DCAN) and 1,1,1-trichloropropanone (1,1,1-TCP) within the initial reaction stage. Besides, the raw water, settled water and filtered water collected from a drinking water treatment plant were divided into five fractions, respectively, by ultrafiltration membranes to evaluate their DBP formation after chlorination. Compared with the medium molecular weight species, high and low molecular weight organic matters exhibited relatively high specific regulated and unregulated DBP yields (expressed as µg/mg C), respectively. Humic acid-like compositions predominantly contributed to regulated DBP yields, while soluble microbial by-product-like compounds preferentially generated DCAN. The correlation study revealed that the TCM could also serve as an indicator for the measured DBPs from chlorination of sample fractions with different molecular weight. Finally, it was found that the theoretical cytotoxicity was enhanced during chlorination of filtered water compared with chlorination of settled water.

Cr(VI) removal using different reducing agents combined with fly ash leachate: A comparative study of their efficiency and potential mechanisms.

Remediation of high concentrations of Cr(VI) in wastewater involves its chemical reduction to Cr(III), a product with low toxicity that can be easily removed. To date, NaBH4 has rarely been used to reduce Cr(VI). This article reports a comparative study of Cr(VI) removal by NaBH4 and five sulfur-based reducing agents (FeSO4, Na2S2O5, NaHSO3, Na2S2O3, and Na2SO3). The potential mechanisms of Cr(VI) removal by these six reducing agents with and without fly ash leachate (FAL) are also discussed. The results revealed that the reduction and subsequent removal of Cr(VI) are influenced by the hydrolysis and ionization of the reducing agents in solution. Thus, the reduction reaction was significantly enhanced when Na2S2O5 and NaHSO3 were added in excess of 600 mg L-1. Combined with FAL, smaller amounts of NaBH4 were required to reduce Cr(VI) to Cr(III) at pH 3.0 compared to those with the other reducing agents. NaBH4 combined with FAL at a dose of 100 mg L-1 afforded a total Cr (CrT) removal of 96.32% within 20 min, a value much higher than that obtained with the other reducing agents. The catalytic mechanism of NaBH4 for such a FAL-catalyzed Cr(VI) reduction system is similar to that of acid catalysis via the hydrolysis of the Fe(III) and Al(III) species in FAL. Improvement of the CrT removal was also observed via Cr(VI) entrapment in the structure of Fe(III) and Al(III) metal hydroxides. These results indicate that relatively low loadings of NaBH4 combined with FAL show great promise for Cr(VI) pollution remediation.