•Cr(VI) of 0.025–8.00 mg/L could be determined with the ABTS method.•The absorbance of ABTS+ at 415, 649 or 732 nm can be used for Cr(VI) analysis.•The ABTS method has excellent convenience, flexibility and sensitivity.•The ABTS method has a good anti-interference performance.A new method for Cr(VI) (0.025–8.00 mg/L) determination based on the reaction of Cr(VI) and 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) in aqueous solutions was developed. The colorless ABTS reacted with Cr(VI) under strong acidic conditions ([H+] = 3.0 or 6.0 M) producing a stable colored radical ABTS+, which could be measured spectrophotometrically at 415, 649 or 732 nm. The absorbance increase at these three wavelengths for ABTS+ generation were all linear (less than 1.0% deviation) with respect to the amount of added Cr(VI) and the sensitivity were 1.099 × 105, 3.720 × 104, and 4.150 × 104 M−1 cm−1 of added Cr(VI) at 415, 649 or 732 nm, respectively. Cr(VI) of 0.025 mg/L below the discharge standard of drinking water could be detected with the method quantitatively and 0.002 mg/L Cr(VI) was determined qualitatively. The molar absorptivity of ABTS+ generated was determined to be (3.69 ± 0.01) × 104 M−1 cm−1 at 415 nm and the reaction between Cr(VI) and ABTS had a stoichiometric factor of 1:3 in excess of ABTS. The absorbance of generated ABTS+ was found to be stable in deionized water or wastewater and Cr(VI) spiked in wastewater could be determined accurately. The ABTS method also had a good anti-interference performance against Co(II) ions. Moreover, the ABTS method could be successfully used in the Cr(VI)-S(IV) system for Cr(VI) determination.The graphic abstract showed Cr(VI) determination based on the reaction between Cr(VI) and ABTS.Download high-res image (286KB)Download full-size image

The enhancing effect of bisulfite (HSO3−) on the kinetics of Se(IV) sequestration by zerovalent iron (ZVI) was systematically investigated as a function of headspace volume, HSO3− concentration and initial pH (pHini). To exclude the role of HSO3− as an electrolyte, the kinetics of Se(IV) removal by ZVI with the presence of SO42− was determined as a control. With increasing headspace volume from 0 to 2.0 mL, the rate of Se(IV) removal by ZVI experienced a considerable enhancement whereas the further increase in the headspace volume resulted in a drop in Se(IV) removal rate. Se(IV) was always removed by ZVI with a higher rate in the presence of HSO3− than that in the presence of SO42− at various headspace volumes, which was mainly ascribed to the release of H+ and the depletion of O2 from the oxidation of HSO3− (i.e., 2HSO3− + O2 → 2SO42− + 2H+). Furthermore, HSO3− accelerated the reduction of ferric oxides and hydroxides to a Fe(II)-containing solid intermediate, which was beneficial to the reductive removal of Se(IV). The SEM, Fe K-edge XAFS and Se XANES analysis for Se(IV)-treated ZVI samples confirmed that HSO3− facilitated the transformation of ZVI to iron(oxyhydr)oxides (e.g., magnetite and lepidocrocite) and the reduction of Se(IV) to Se(0) compared to SO42−. The enhancing effect of HSO3− on Se(IV) sequestration varied with the concentration of HSO3− and initial pH, with the greatest effect achieved at 2.0 mM of Se(IV) and pHini 5.0. Since bisulfite is inexpensive and its final product is sulfate, a common anion existing in water, taking advantage of bisulfite to enhance the ZVI's reactivity under limited oxygenated conditions is a promising method.

•MIL-53(Al) and mesostructured MIL-53(Al) were used as adsorbents to remove BPA.•Adsorption capacities of both adsorbents are considerably satisfying.•pH and temperature have influence on the adsorption process.•Breathing of the MIL-53 structure was inferred to the absorption mechanism.•π–π Bonds and hydrogen bonding were proposed for the adsorption mechanism.In this work, metal–organic framework MIL-53(Al){Al(OH)[O2C-C6H4-CO2]} and MIL-53(Al)-F127{Al(OH)[O2C-C6H4-CO2]} were synthesized and used as sorbents to remove bisphenol A (BPA) from aqueous system. The sorption kinetics data of BPA were found to be in agreement with the pseudo-second-order model. The equilibrium sorption amounts of BPA on MIL-53(Al) and MIL-53(Al)-F127 reached 329.2 ± 16.5 and 472.7 ± 23.6 mg g−1, respectively, far more than that of commercial activated carbons (ranging from 129.6 to 263.1 mg g−1). Both MIL-53(Al) and MIL-53(Al)-F127 could remove BPA fast from aqueous solutions, and the required contact time to reach equilibrium was approximately 90 min for MIL-53(Al) and 30 min for MIL-53(Al)-F127, respectively. The optimum pH levels for the removal of BPA using MIL-53 (Al) and MIL-53(Al)-F127 were 4 and 6 separately. The optimum temperature for the sorption behavior of BPA on the two sorbents was 20 °C. The results performed show that the resulting products, as one kind of MOFs, can be regarded as a new class of sorbents for water treatment and could find great applications in the fields of environmental water pollution control and resources reuse.

Coagulation is one of the most commonly used technologies for arsenic removal from water and wastewater. Jar tests were carried out to determine the influence of pH, initial As/Fe molar ratio, equilibrium As concentration and co-occurring solutes on the crossover pH, where As(V) removal was equivalent to As(III) by FeCl3 coagulation. As(V) was more effectively removed than As(III) at lower pH values in FeCl3 coagulation process but the opposite was true at higher pH level. Increasing As/Fe ratio from 0.12 to 0.50 progressively lowered the crossover pH from 8.5 to 7.4. The arsenic isotherms revealed that As(V) removal was favored at low pH and low equilibrium concentration yet the opposite was true for As(III). The co-existing solutes had different influences on the crossover pH, depending on their ability to compete for sorption sites and to hinder or facilitate the aggregation of ferric hydroxide flocs. The presence of sulfate broadened the pH range where As(III) removal was better than As(V) removal. However, As(V) removal was always superior to As(III) removal in the presence of Ca2+ and no crossover pH was observed. The results from this study revealed that arsenic speciation, arsenic loading, and pH should be considered when predicting and managing arsenic removal by coagulation.Graphical abstractHighlights► As(V) removal was more favored than As(III) at low pH and equilibrium concentration. ► Increasing As/Fe ratio from 0.12 to 0.50 progressively lowered the crossover pH from 8.5 to 7.4. ► The co-existing solutes had different influences on the crossover pH.

Weak magnetic field (WMF) was employed to improve the removal of Cr(VI) by zero-valent iron (ZVI) for the first time. The removal rate of Cr(VI) was elevated by a factor of 1.12–5.89 due to the application of a WMF, and the WMF-induced improvement was more remarkable at higher Cr(VI) concentration and higher pH. Fe2 + was not detected until Cr(VI) was exhausted, and there was a positive correlation between the WMF-induced promotion factor of Cr(VI) removal rate and that of Fe2 + release rate in the absence of Cr(VI) at pH 4.0–5.5. These phenomena imply that ZVI corrosion with Fe2 + release was the limiting step in the process of Cr(VI) removal. The superimposed WMF had negligible influence on the apparent activation energy of Cr(VI) removal by ZVI, indicating that WMF accelerated Cr(VI) removal by ZVI but did not change the mechanism. The passive layer formed with WMF was much more porous than without WMF, thereby facilitating mass transport. Therefore, WMF could accelerate ZVI corrosion and alleviate the detrimental effects of the passive layer, resulting in more rapid removal of Cr(VI) by ZVI. Exploiting the magnetic memory of ZVI, a two-stage process consisting of a small reactor with WMF for ZVI magnetization and a large reactor for removing contaminants by magnetized ZVI can be employed as a new method of ZVI-mediated remediation.Download full-size image