Removal of hexavalent chromium from contaminated waters by ultrasound-assisted aqueous solution ball milling

Synergy of Adsorption and Photocatalytic Reduction for Efficient Removal of Cr(VI) with Polyvinylidene Fluoride@Polyvinyl Alcohol-FeC2O4/Bi2.15WO6

Although the photocatalytic reduction of Cr(VI) to Cr(III) by traditional powder photocatalysts is a promising method, the difficulty and poor recovery of photocatalysts from water hinder their wide practical applications. Herein, we present that FeC2O4/Bi2.15WO6 (FeC2O4/BWO) composites were tightly bonded to modified polyvinylidene fluoride (PVDF) membranes by chemical grafting with the aid of polyvinyl alcohol (PVA) to form photocatalytic composite membranes (PVDF@PVA-FeC2O4/BWO). The contact angle of PVDF@PVA-FeC2O4/BWO (0.06 wt % of FeC2O4/BWO) is 48.0°, which is much lower than that of the pure PVDF membrane (80.5°). Meanwhile, the permeate flux of 61.43 g m-2 h-1 and water flux of 250.60 L m-2 h-1 were observed for PVDF@PVA-FeC2O4/BWO composite membranes. The tensile strength of composite membranes reached 48.84 MPa, which was 9.8 times higher than that of PVDF membrane. It was found that the PVDF@PVA-FeC2O4/BWO membrane exhibited excellent photocatalytic Cr(VI) reduction performance under both simulated and real sunlight irradiation. The adsorption for Cr(VI) by PVDF@PVA-FeC2O4/BWO can reach 47.6% in the dark process within 30 min, and the removal percentage of Cr(VI) could reach 100% with a rate constant k value of 0.2651 min-1 after 10 min of light exposure, indicating a synergistic effect of adsorption and photoreduction for Cr(VI) removal by the composite membrane. The PVDF@PVA-FeC2O4/BWO membrane had good stability and reusability after seven consecutive cycles. Most importantly, the influences of foreign ions on Cr(VI) reduction were investigated to mimic real sewage, which revealed that no obvious adverse effects can be found with the presence of common foreign ions in sewage. The photocatalytic membrane material developed in this study provides a new idea for treating Cr(VI)-containing wastewater and has a more significant application prospect.

Bio-fabrication of porous magnetic Chitosan/Fe3O4 nanocomposite using Azolla pinnata for removal of chromium - Parametric effects, surface characterization and kinetics

In this research, a novel porous nanocomposite, namely Chitosan-iron-oxide @ Azolla pinnata nanocomposite, has been synthesized by co-precipitation and hydrothermal method. The effect of process parameter on adsorption process was investigated. Batch removal of chromium (Cr) was optimized with respect to solution pH, batch stirring time, sorbent dose, initial chromium concentration and temperature. The maximum removal efficiency was found to be 98.58%. The Fourier transform infrared (FTIR) spectroscopy and scanning electron microscope (SEM) analysis of the nano composite confirmed the presence of characteristic functional groups and porous structure of synthesized nanocomposite. The adsorption data fitted well with Langmuir adsorption isotherm (R² = 0.996) confirming mono layer sorption and the maximum uptake was found to be 294.12 mg/g. The adsorption was found to follow pseudo second order model (R² = 0.997). Thermodynamic studies revealed that adsorption is endothermic and spontaneous. Reusability studies have confirmed that removal efficiency attained was 85% after completion of five adsorption-desorption cycles. Electrostatic attraction, ion exchange, coordination bonding and reduction are the major mechanisms responsible for removal of chromium. Surface modification of Azolla pinnata with chitosan and iron oxide improved the ability of Azolla in the adsorption of chromium from aqueous media. The combined effects of facile synthesis, improved adsorption features and easier magnetic separation promotes Chitosan-iron-oxide @ Azolla pinnata nanocomposite as a novel adsorbent.

Removal of hexavalent chromium from water using polyaniline/ wood sawdust/ poly ethylene glycol composite: an experimental study

Polyaniline/ Sawdust /Poly Ethylene Glycol/ (PANi/SD/PEG) composite synthesized chemically is used as an adsorbent to remove hexavalent chromium from water. Adsorption experiments have been done in batch and continuous (column) mode. Some parameter such as pH, contact time, PANi/SD/PEG dose, isotherms in batch mode and pH, column bed depth and fluid flow rate in column mode were investigated. Result shows that PANi/SD/PEG has a good performance to remove hexavalent chromium ion from aqueous media. By presence of PEG, prepared composite has been homogenized and further absorption has been occurred. The best adsorption occurs under pH 2 and optimum contact time for removal of hexavalent chromium ion in batch experiment was about 30 min. Adsorption of Cr (VI) by PANi/SD/PEG fitted well in Langmuir isotherms. Maximum adsorption of hexavalent chromium was calculated 3.2 (mg/g). In column experiments, pH and column bed depth were found to be more prominent than fluid flow rate. Though, about 22% of Cr (VI) can be recovered using 0.1 M NaOH in the batch system, the recovered Cr (VI) in column system was less than 7.9%.

Ultrasound assisted batch operation for the adsorption of hexavalent chromium onto engineered nanobiocomposite

The study demonstrates the comparative retention characteristics of chromium from aqueous environment on iron(III) cellulose nanocomposite bead under conventional (C-ads) and ultrasound assisted (U-ads) batch adsorption operation. Optimization of process parameters was achieved varying pH, contact/sonication time, initial Cr(VI) concentration, adsorbent dose at fixed solution temperature. Equilibrium was achieved within 5 h and 30 min of contact time in C-ads and U-ads respectively. Langmuir isotherm is found suitable in C-ads while Temkin in U-ads. Both the operation is aided by change in negative free energy, positive enthalpy and entropy. Ultrasound assisted adsorption is much favorable in terms of efficiency, feasibility, spontaneity and randomness. Both the operations follow more the second order than the first order kinetic model. The pore diffusion prevails more in case of U-ads while the surface adsorption in C-ads.

Preparation, characterization and performance of an electrospun carbon nanofiber mat applied in hexavalent chromium removal from aqueous solution

Hexavalent chromium, Cr(VI), a highly toxic oxyanion known as a carcinogen and mutagen, is an issue of concern due to its adverse impact on human health. Therefore, development of effective technologies and/or materials for Cr(VI) removal from water has been of great interest for researchers. In this study, an electrospun carbon nanofiber (CNF) mat was prepared via electrospinning polyacrylonitrile (PAN), followed by thermal pre-oxidation and carbonization. Scanning electron microscopy (SEM) observation showed that the fiber diameter of the CNF with carbonization temperature of 950°C (CNF₉₅₀) was about 266 nm. Potentiometric titration analysis demonstrated that the point of zero charge pH (pH_pzc) of CNF₉₅₀ was around 7.93. CNF₉₅₀ demonstrated high adsorption capacity and fast adsorption kinetics for Cr(VI) at pH < 3. Langmuir isotherm calculations showed that the maximum adsorption capacity of Cr(VI) on CNF₉₅₀ was 118.8 mg/g at pH 2. The adsorption isotherm of Cr(VI) on CNF₉₅₀ was well described by the Redlich-Peterson model, revealing that Cr(VI) adsorption was the result of a combination of monolayer and multilayer adsorption, depending on the initial Cr(VI) concentration. Solution pH greatly affected Cr(VI) adsorption onto CNF₉₅₀ due to the electrostatic interaction, and the adsorption capacity was relatively high when pH was below 3. X-ray photoelectron spectroscopy (XPS) analysis revealed that the removal of Cr(VI) might be the result of a combination of redox reaction and electrostatic adsorption. The adsorption-saturated CNF₉₅₀ could be regenerated by NaOH solution. This study extends the potential applicability of electrospun CNF mats for Cr(VI)-contaminated water purification.

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 (Cr_T) 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 Cr_T 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.

Sonochemical reduction of Cr(VI) in air in the presence of organic additives: What are the involved mechanistic pathways?

The sonochemical (850 kHz) reduction of Cr(VI) (0.3 mM, pH 2, reactor open to air) was analyzed in the presence of different additives. The effects on Cr(VI) reduction efficiency of added formic acid (FA, 10 mM), citric acid (Cit, 2 mM), ethylenediaminetetraacetic acid (EDTA, 1 mM), methanol (MeOH, 0.1 M), ethanol (EtOH, 0.1 M), 2-propanol (2-PrOH, 0.1 M), tert-butanol (t-BuOH, 0.1 M), phenol (PhOH, 2 mM) and sodium lauryl sulfate (SLS, 1 mM) have been evaluated in comparison with the system in the absence of additives. Complete Cr(VI) reduction was obtained only when using EDTA (at 120 min) and Cit (at 180 min). Cr(III) complexes with these compounds or with their degradation products were detected as final products. For EDTA, Cit, t-BuOH, FA and SLS, the Cr(VI) decay could be adjusted to a zero-order kinetics; in the cases of MeOH, EtOH and 2-PrOH, there was a deviation from the zero-order kinetics. The Cr(VI) conversion increased in the order SLS (very low) < no additive ≅ MeOH ≅ EtOH ≅ 2-PrOH < FA < t-BuOH < PhOH < Cit < EDTA. The role of EDTA and Cit in stabilizing intermediate Cr(V) peroxo compounds and enhancing their direct transformation into different Cr(III) species is considered a major factor in the acceleration of Cr(VI) reduction processes. Mechanistic pathways are proposed.