Adsorption of hexavalent chromium from water using manganese-aluminum coated sand: Kinetics, equilibrium, effect of pH and ionic strength

Polymerized PEI-modified lignin polyphenolic materials by acid hydrolysis-phase separation for removal of Cr (VI) from industrial wastewater

Cr (VI) accumulates in an aqueous environment and exhibits huge harm to human health and the ecological system. Developed lignin biomass materials are complicated to prepare and have limited properties, and advances in lignin phenolic modification are lacking. Herein, an aminated poplar lignin-pyrogallol (PLP-PEI) with a simple design and adjustable phenolic hydroxyl content was developed using the acid hydrolysis-phase separation (AH-PS) method, and modified by the atom transfer radical polymerization (ATRP) strategy. Through diverse characterization analysis, the structural changes of PLP-PEI in the step-by-step synthesis process were monitored. An effective biomass capture system (Bio-Cap) was shown via systematically investigating the adsorption behaviors of Cr (VI) on PLP-PEI under various environmental conditions. Benefiting from introducing phenolic hydroxyl and amino groups, PLP-PEI demonstrated efficient adsorption capacity (598.80 mg/g for Cr (VI)). Additionally, the material also exhibited advantages, including monomeric chemisorption properties, strong reduction capability, and stable regeneration properties. Multiple driving forces were involved in the capture and removal process of Cr (VI), including complexation and electrostatic interaction. The low-cost natural biomass resources supported the industrial-scale synthesis and practical application of advanced aminated lignin polyphenol material, which showed outstanding advantages and enormous potential in the field of water environmental restoration.

Performance of a modified and intermittently operated slow sand filter with two different mediums in removing turbidity, ammonia, and phosphate with varying acclimatization periods

The present study investigated the utilization of blood clam shells as a potential substitute for conventional media, as well as the influence of the acclimation time on the efficacy of an intermittent slow sand filter (ISSF) in the treatment of real domestic wastewater. ISSF was operated with 16 h on and 8 h off, focusing on the parameters of turbidity, ammonia, and phosphate. Two media combinations (only blood clam shells [CC] and sand + blood clam shells [SC]) were operated under two different acclimatization periods (14 and 28 d). Results showed that SC medium exhibited significantly higher removal of turbidity (p < 0.05) as compared to CC medium (45.99 ± 26.84 % vs. 3.79 ± 9.35 %), while CC exhibited slightly higher (p > 0.05) removal of ammonia (23.12 ± 20.2 % vs. 16.77 ± 16.8 %) and phosphate (18.03 ± 11.96 % vs 13.48 ± 12 %). Comparing the acclimatization periods, the 28 d of acclimatization period showed higher overall performances than the 14 d. Further optimizations need to be conducted to obtain an effluent value below the national permissible limit, since the ammonia and phosphate parameters are still slightly higher. SEM analysis confirmed the formation of biofilm on both mediums after 28 d of acclimatization; with further analysis of schmutzdecke formation need to be carried out to enrich the results.

Genesis of high hexavalent chromium groundwater in deep aquifers from loess plateau of Northern Shaanxi, China

Hexavalent chromium (Cr(VI)) groundwater usually exists in shallow aquifers related to ultramafic and serpentine formations, but knowledge of the genesis of dissolved Cr(VI) in deep sandstone aquifers is limited. Both groundwater and aquifer sediments were taken from the Jingbian County in the Loess Plateau of Northwestern Shaanxi to investigate distribution and genesis mechanism of high Cr(VI) groundwater in deep sandstone aquifers. Results showed that the Cr concentrations (median 142 μg/L) in groundwater from deep aquifers (>100 m) were relatively high, while the Cr concentrations in shallow groundwater were low (median 33.8 μg/L). Dissolved Cr mainly existed in the species of Cr(VI) (average, 90%). Deep groundwater with higher Cr(VI) concentrations generally had higher pH, Eh, and DO than shallow groundwater, indicating that the high Cr(VI) groundwater existed in relatively oxic environment. Cretaceous sandstones in deep aquifers had anomalously high contents of total Cr (average 115 mg/kg), where Cr was mainly present in silicates-bound form, and secondly in strongly adsorbed form. There were positive correlations between Mn and Cr in the unweathered silicate-bound form and adsorbed form, which were conducive to Cr(III) oxidation into Cr(VI) in an alkaline-oxic environment. The different ionic ratios (i.e. (Ca²⁺ + Mg²⁺)/(HCO₃^- + SO₄^2-)) also supported silicate weathering as the dominant rock-water interactions in the deep groundwater, which enhanced the release of the unweathered silicate-bound Cr. Relatively high pH and ionic strength mobilized the adsorbed Cr(VI) into groundwater. This investigation emphasizes the geological origin of high Cr(VI) groundwater in deep sandstone aquifers containing Mn oxides, which deserves more concerns for the purpose of drinking water supply.

Optimizing Cr(VI) adsorption parameters on magnetite (Fe3O4) and manganese doped magnetite (MnxFe(3-x)O4) nanoparticles

Magnetite as an adsorbent is efficient since iron oxides have high affinities for heavy metal pollutants and are environmentally friendly. Manganese oxides provide catalytic properties which are desirable during the remediation of multi valent pollutants. Magnetite (Fe3O4) and manganese doped magnetite (MnxFe(3-x)O4) nanoparticles were synthesized and characterized to determine the manganese doping effects on magnetite’s crystal and surface properties. Fe3O4 and MnxFe(3-x)O4 showed similarities in crystal morphology indicating that manganese doping did not alter the nature of Fe3O4 nanoparticles. Manganese doping improved magnetite’s thermal properties as well as its surface area providing improved adsorption characteristics. The as-synthesized particles were applied in the optimization of hexavalent chromium adsorption. Adsorption proceeded under similar conditions for both adsorbents indicating their structural similarities. Higher efficiencies were observed on the doped adsorbent due to increased surface area and the presence of additional functional groups. Solution pH significantly affected the adsorption process aiding in the reduction of Cr(VI) ions to the less toxic Cr(III) species. The adsorption distribution coefficient KD indicated that manganese doping significantly improved magnetite’s affinity for hexavalent chromium. Adsorption and reduction were determined to responsible for pollutant reduction in solution at optimal conditions of pH 2, 5 g/L and 100 mg/L for adsorbent mass and solution concentration.