Hexavalent chromium removal from municipal wastewater using ferrous salts

Chromium occurs in nature mainly in its trivalent or hexavalent form. Hexavalent chromium Cr(VI) is particularly toxic to humans, animals, and plants. The extensive pollution of groundwaters with Cr(VI) necessitates the complete understanding of natural chromium oxidation and reduction mechanisms, both for assessing the risk of hexavalent chromium formation and for the development of techniques for the reduction and removal of Cr(VI) from contaminated water bodies. In this work, the possibility of hexavalent chromium reduction by discarded or low-cost materials, which contain reducing compounds, is investigated regarding the creation of a compact, pump-and-treat filter for Cr(VI) removal from groundwater.

Removal of hexavalent chromium and metal cations by a selective and novel carbon adsorbent

The removal of hexavalent chromium from aqueous solutions onto modified holly sawdust was studied at varying initial hexavalent chromium concentrations, adsorbent doses, pHs and contact times. The removal of hexavalent chromium from aqueous solutions increased with increasing adsorbent dosage and contact time. The percentage of hexavalent chromium removed from the aqueous solutions decreased with increasing hexavalent chromium concentration and pH of the solution. The kinetics of the adsorption of hexavalent chromium onto modified holly sawdust was analyzed using pseudo first-order and pseudo second-order models. The pseudo second-order model described the kinetics of adsorption of hexavalent chromium. The Langmuir and Freundlich isotherm models were used for modeling of the adsorption equilibrium data. The Langmuir isotherm model well described the equilibrium data for the removal of hexavalent chromium by modified holly sawdust. The obtained maximum adsorption capacity was 18.86 mg/g at pH 7. The results showed that modified holly sawdust can be used as a low cost adsorbent for the treatment of aqueous solutions containing chromium.

Electrokinetic soil remediation by electroosmosis and ionic migration is restricted to soluble substances. At a higher pH, the soil particles sorb more heavy metals, such as cadmium, than at a lower pH and the precipitation reactions of heavy metals are promoted. Both mechanisms make heavy metals immobile, rendering cleanup more difficult and electrokinetic extraction inefficient. Therefore, the acidification of soils is very important to increase the cleanup efficiencies of heavy metals, such as cadmium by electrokinetic method from contaminated soils. Meanwhile, hexavalent chromium is more mobile and easily soluble. Hexavalent chromium has high solubility in water and exists as chromate, monochromate or dichromate anions depending on the pH of the solutions. This hexavalent chromium is the chemical form requiring remediation. In this study, cadmium and hexavalent chromium transport are predicted with a simple numerical method in which electrical flow is coupled with hydraulic flow, and the sorption process in the soils is induced in the model. The effects of some factors such as electrode configuration, electric field strength and electrode spacing for saturated soils are investigated using this numerical model. The results obtained this study are summarized as follows: The two-dimensional electrode configurations containing cathode surrounded by three or more anodes rapidly remove cadmium from soils compared with one-dimensional electrode configuration. Meanwhile, in the removal of hexavalent chromium from soils, the two-dimensional electrode configurations containing anode surrounded by three or more cathodes can be effectively utilized compared with one-dimensional electrode configuration. A larger electrode spacing increases the processing time required, and it was also found that a larger electric field strength reduced the processing time required.

Modeling and evaluation on removal of hexavalent chromium from aqueous systems using fixed bed column

Waste water from hard chrome is considered to be highly toxic due to the presence of chromium ions in hexavalent form and this hexavalent state of chromium is more toxic to animals and humans due to its ability to produce reactive oxygen species in cells. Such heavy metals are considered as carcinogenic to living organisms and hence either reduction of ions to trivalent chromium or removal of ions has to be done before ejecting the waste water into the environment. Many processes for reduction, neutralisation and removal of hexavalent chromium have been investigated and reviewed extensively. In the present review, studies and research carried out for the removal of chromium from waste water of hard chrome plating effluent are summarised. The study was carried out on the aspects such as percentage removal, efficiency and optimum operating conditions in chrome removal and reduction processes.

Chromium has been widely used in various industries like textile, leather, chemical manufacture, metal finishing, paint industry and many other industries. Since hexavalent chromium is a priority toxic, mutagenic and carcinogenic chemical when present in excess, it is very much required to remove chromium from effluents before allowing it to enter any water system or on to land. In the present study, the removal of hexavalent chromium by adsorption on the Custard apple peel powder as adsorbent has been investigated in the batch experiments. The agitation time, the adsorbent size, adsorbent dosage, initial chromium concentration, temperature and the effect of solution pH are studied. The Freundlich model for Cr (VI) adsorption onto Custard apple peel powder is proved to be the best fit followed by Langmuir model and Tempkin model based on high regression coefficient R^2 value. The adsorption behavior is described by a pseudo second order kinetics. The maximum metal uptake is found to be 7.874 mg/g. The morphology on the surface of adsorbents and also the confirmation of chromium binding on adsorbent surface at different stages were obtained by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analysis. The results obtained in this study illustrate that Custard apple peel powder is expected to be an effective and economically viable adsorbent for hexavalent chromium removal from industrial waste water.

ABSTRACTThe performance of horizontal subsurface flow constructed wetlands in the removal of micropollutants from a wastewater treatment plant effluent was evaluated at mesocosm level. Fifteen mesocosms were studied following a modified Latin Square experimental design with six additional points. Three variables at three levels were studied: porous media –PM- (river gravel, fine volcanic gravel and coarse volcanic gravel), macrophyte type –M- (Thypa latiffolia, Phragmites australis, and Cyperus papyrus) and hydraulic retention time –HRT- (1, 3 and 5 days). As response variables the removal percentages of the total organic load of the effluent (BOD5) and the loads of several micropollutants (caffeine, galaxolide, tonalide, alkylphenols and their monoethoxylates and diethoxylates, methyl dihydrojasmonate, sunscreen UV-15 and parsol) were used. The results showed that the systems remove between 70% and 75% of the organic load and that all the micropollutants were degraded at different extents, from 55% to 99%. The HRT was the variable that showed major effects on the treatment process, while M and PM showed no statistically significant differences in the used experimental conditions.

Understanding the sources and controlling processes of various groundwater contaminants and their removal methods is extremely important, as groundwater contamination is intricately linked to human health. Chromium (Cr) is a common groundwater contaminant with both natural and anthropogenic origins. Dissolved Cr exists in hexavalent and trivalent forms – while the former is carcinogenic and more soluble – the latter is a micronutrient at low levels and is less soluble. Therefore, most chromium removal methods rely on reducing the hexavalent chromium to its trivalent state to decrease the Cr-toxicity. In recent years, several experimental methods have been attempted for hexavalent chromium removal from aqueous media/groundwater. This paper reviews the recent findings on Cr removal by important, effective, and widely used methods such as adsorption by nanoscale zero-valent Fe-based and conventional materials, electrocoagulation (EC), and bioremediation. The reaction pathways, mechanisms, and effectiveness of each method are also highlighted. The role of parameters such as solution pH and temperature, initial Cr(VI) concentration, contact time with the reducing agent, adsorbent dose, and the presence of competing ions on Cr removal was evaluated. Many of the methods exhibit high (>90%) Cr removal efficiency; the main challenge would be to apply these methods for large-scale water treatment.

Biomonitoring programs for urinary chromium (Cr) typically attempt to evaluate occupational exposure via the inhalation route. This study investigated whether Cr can be detected in the urine of people following the ingestion of soils that contain relatively high concentrations of chromium in chromite ore processing residue (COPR). To evaluate the reasonableness of using urinary monitoring to assess environmental exposure, six volunteers ingested 400 mg of soil/day (low-dose group), two others ingested 2.0 g of soil/day (high-dose group) for 3 consecutive days, and one person ingested a placebo on each of 3 days. The soil and COPR mixture contained concentrations of total chromium (Cr) and hexavalent chromium [Cr(VI)] of 103 +/- 20 and 9.3 +/- 3.8 mg/kg, respectively. Therefore, the low-dose group ingested 41 micrograms Cr/day [including 3.7 micrograms Cr(VI)] and the high-dose group ingested 206 micrograms Cr/day [including 18.6 micrograms Cr(VI)] on each of 3 consecutive days. All urine samples were collected and analyzed individually for total Cr on the day prior to dosing, during the 3 days of dosing, and up to the first void 48 h after the last dose. No significant increases in urinary Cr excretion were found when background excretion data were compared with data following each of the 3 days of dosing or in daily mean urine concentrations of the high- vs the low-dose groups. It appears that Cr present in a soil and COPR mixture at Cr doses up to 200 micrograms/day is not sufficiently bioavailable for biomonitoring of urine to be informative.(ABSTRACT TRUNCATED AT 250 WORDS)

In high-altitude regions, a diminished atmospheric oxygen content significantly impairs the aeration efficiency of municipal wastewater, posing a challenge to sustainable wastewater management. Consequently, conventional biological wastewater treatment methods necessitate elevated energy consumption in high-altitude areas, rendering them economically and environmentally unsustainable. The simultaneous nitrification and denitrification (SND) process, owing to its minimal oxygen requirements, emerges as a promising and sustainable solution in low-pressure environments. Additionally, owing to the unique lifestyle and natural conditions in plateau regions, the organic loading in municipal wastewater is often low. To comprehensively assess the impact of low pressure and organic loading on the SND process, three laboratory-scale reactors were implemented. This study revealed that low pressure and the introduction of organic matter enhanced both nitrogen removal performance and SND efficiency. The sludge volume index decreased by 93.5%, indicating a substantial improvement in the microbial aggregation ability and the formation of a more favorable SND sludge structure. 16S rRNA sequencing results demonstrated alterations in the microbial community structure due to low pressure and the addition of organic matter, leading to a substantial increase in the abundance of nitrifying and denitrifying bacteria. Furthermore, the prediction results of functional genes indicated the upregulation of genes related to the nitrification and denitrification processes with decreasing pressure and the addition of organic matter. This enhancement underlines the improved microbial nitrogen removal function. This study underscores the positive influence of low pressure and organic loading on the SND system, thereby substantially enhancing the economic and environmental sustainability of the SND process in plateau regions.

Treatment of the textile effluent becomes complex due to the high color, organic load, dissolved solids content, and toxicity of the textile effluents. An optimized pre-ozonation treatment has been proposed, aimed at the maximum color and organics removal leading to possible resource recovery at the field-scale. The standalone ozonation at optimum conditions of pH, salinity, and dye concentration resulted in the color removal and chemical oxygen demand (COD) removal of 96.13% and 67.40%, respectively, for synthetic textile effluent. The percentage removal of individual dyes from synthetic textile effluent was also analyzed. The standalone ozonation treatment of real textile effluent at optimum conditions resulted in 94.6% color removal and 67.4% COD removal. The pre-ozonation treatment followed by a sequential batch reactor proved effective in the removal of color (98.6%) and organic load (91.6%) of the real textile effluent. Pre-ozonation enhanced the biodegradability of the textile effluent, as observed by the 81.8% increase in biodegradability index and 126.2% increase in specific oxygen uptake rate. The proposed industrial textile effluent scheme, when applied to the industry producing 3500 kg fabric per day, presented a potential recovery of 157 m3 effluent and 375 kg salt per day, which can result in the net annual saving of about INR 3.7 million, including additional treatment cost. The proposed treatment scheme significantly contributes to sustainable textile production.

This study developed a composite material (JBC-nZVI) using jujube tree branch biochar (JBC) to support nano zero-valent iron (nZVI) for hexavalent chromium (Cr(VI)) removal from aqueous solutions. The porous structure of JBC improved nZVI dispersion, reducing aggregation and enhancing reactivity and stability. The composite was characterized using SEM-EDS, BET, FT-IR, and particle size distribution analysis. Under optimized conditions (50 mg/L Cr(VI), pH 3, 1 g/L dosage), JBC-nZVI achieved 88.7 % Cr(VI) removal in 120 min. Kinetic studies showed the removal followed a pseudo-second-order model, with chemical adsorption as the main process. JBC-nZVI facilitated electron transfer between Cr(VI) and nZVI, promoting the reduction process. XRD and XPS analyses revealed that Cr(VI) removal involved adsorption, reduction, and coprecipitation. This composite not only provides an efficient method for Cr(VI) remediation but also valorizes jujube tree branch biomass waste, enhancing nZVI stability and offering a novel approach for environmental cleanup.

Robust three-dimensional g-C3N4@cellulose aerogel enhanced by cross-linked polyester fibers for simultaneous removal of hexavalent chromium and antibiotics

Fe0-nanoparticles-chitosan composite beads (CS-NZVI beads) were prepared and used to remove hexavalent chromium (Cr (VI)) from wastewater. Characterization with a scanning electron microscopy (SEM) demonstrated that with the increase of the concentration of CS from 5.0 to 20.0 g/L and NaOH from 0.5 to 2.0 mol/L, the average aperture size of the CS-NZVI beads decreased from 26.8 to 10.6 μm and 42.6 to 0.8 μm, respectively. Batch experiments revealed that the removal of Cr (VI) using CS-NZVI beads was consistent with pseudo first-order reaction kinetics. The rate constant increased with increasing NZVI dosage but decreased with the increase in pH values. The Freundlich isotherm described the adsorption process better, suggesting that the CS-NZVI beads were heterogeneous in the surface properties. At equilibrium qmax was 35.97 mg/g. The thermodynamics study suggested that Cr (VI) removal by CS-NZVI beads was an endothermic and spontaneous process, reflecting good affinity of the sorbent for Cr (VI) ions and increasing randomness at the solid-solution interface during the adsorption process. This result will be very useful to understand the effects of NZVI on heavy metal Cr (VI) removal from wastewater in the successful application.