The adsorption capacity of synthetic NaX zeolite for Pb2+, Cd2+, Cu2+ and Zn2+ in single and multi-component systems were investigated. The effects of electronegativity and hydration energy on the selective adsorption, as well as potential selective adsorption mechanism of the NaX zeolite for Pb2+, Cd2+, Cu2+ and Zn2+ were also discussed. The maximum adsorption capacity order of the heavy metals in the single system was Pb2+ > Cd2+ > Cu2+ > Zn2+, and this could be related to their hydration energy and electronegativity. The values of the separation factors (α) and affinity constant (KEL) in different binary systems indicated that Pb2+ was preferentially adsorbed, and Zn2+ presented the lowest affinity for NaX zeolite. The selective adsorption capacities of the metals were in the order, Pb2+ > Cd2+ ≈ Cu2+ > Zn2+. The trend for the selective adsorption of NaX zeolite in ternary and quaternary systems was consistent with that in the binary systems. Pb2+ and Cu2+ reduced the stability of the Si-O-Al bonds and the double six-membered rings in the NaX framework, due to the high electronegativity of Pb2+ and Cu2+ than that of Al3+. The selective adsorption mechanism of NaX zeolite for the high electronegative metal ions could mainly result from the negatively charged O in the Si-O-Al structure of the NaX zeolite, hence heavy metal ions with high electronegativity display a strong affinity for the electron cloud of the oxygen atoms in the Si-O-Al. This study could evaluate the application and efficiency of zeolite in separating and recovering certain metal ions from industrial wastewater.
Nanoscale zero-valent iron (nZVI) and sulfides have been confirmed to be effective in arsenic sequestration from aqueous solution. In this study, attapulgite supported and sulfide-modified nanoscale zero-valent iron (S-nZVI@ATP) are synthesized to realize the superposition effect of enhanced arsenic sequestration. The results indicated that nZVI clusters were well disaggregated and the BET specific surface area increased from 19.61 m2·g−1 to 46.04 m2·g−1 of S-nZVI@ATP, resulting in an enhanced removal efficiency of arsenic from 51.4% to 65.1% at 20 min. The sulfides in S-nZVI@ATP mainly exists as mackinawite (FeS) and causes the spherical nanoparticles exhibiting a larger average particle size (94.6 nm) compared to bare nZVI (66.0 nm). In addition, S-nZVI@ATP exhibited a prominent ability for arsenic sequestration over a wide pH range of 3.0–6.0. The presence of anions SO42− and Cl− can enhance the arsenic removal whereas HCO3− inhibited it. The arsenic adsorption by S-nZVI@ATP could be explained by the pseudo-second-order kinetic model and the Langmuir model, with the maximum adsorption capacity of 193.8 mg·g−1. The mechanism of As(III) sequestration by S-nZVI@ATP involved multiple processes, mainly including precipitation conversion from FeS to As2S3, surface-complexation adsorption and co-precipitation.
The porous-material loading and noble-metal doping of nanoscale zero-valent iron (nFe) have been widely used as countermeasures to overcome its limitations. However, few studies focused on the experimental identification of the roles of Fe, the carrier and the doped metal in the application of nFe. In this study, the nitroreduction and dechlorination of p-chloronitrobenzene (p-CNB) by attapulgite-supported Fe/Ni nanoparticles (ATP-nFe/Ni) were investigated and the roles of Fe, Ni and attapulgite were examined. The contributions of Ni are alleviating the oxidization of Fe, acting as a catalyst to trigger the conversion of H2 to H*(active hydrogen atom) and promoting electron transfer of Fe. The action mechanisms of Fe in reduction of -NO2 to -NH2 were confirmed to be electron transfer and to produce H2 via corrosion. When H2 is catalyzed to H* by Ni, the production H* leads to the nitroreduction. In additon, H* is also responsible for the dechlorination of p-CNB and its nitro-reduced product, p-chloroaniline. Another corrosion product of Fe, Fe2+, is incapable of acting in the nitroreduction and dechlorination of p-CNB. The roles of attapulgite includes providing an anoxic environment for nFe, decreasing nFe agglomeration and increasing reaction sites. The results indicate that the roles of Fe, Ni and attapulgite in nitroreduction and dechlorination of p-CNB by ATP-nFe/Ni are crucial to the application of iron-based technology.
The rate at which freshwater sources are being contaminated by mining operations in the South-Western part of Ghana is alarming. However, no study has quantified the degree of contamination of the freshwater in such areas thus, leaving a gap in the literature that requires immediate attention. This study assessed the quality of the surface and groundwater in the Tarkwa Nsuaem Municipality. Even though the physical parameters such as pH and electrical conductivity were indicative of safe freshwater, other parameters such as turbidity, total suspended solids (TSS), dissolved oxygen (DO), and heavy metals in the water sources were high; thus, confirming possible leaching, runoff, and dissolution of the hazardous substances employed in the manganese mining operations. The water quality of 82% of the water sources along the Kawere Stream was low (Classes III and IV). Therefore, the local people are at risk of contracting water-related diseases, and health problems associated with the ingestion of Fe, As, and Mn. The findings in this study are important in establishing the rate at which mining operations are reducing the quality of freshwater in developing countries, and potentially affecting human health.
The removal of chlorinated pollutants from water by nanoparticles is a hot topic in the field of environmental engineering. In this work, a novel technique that includes the coupling effect of n-Fe/Ni and its transformation products (FeOOH) on the removal of p-chloronitrobenzene (p-CNB) and its reduction products, p-chloroaniline (p-CAN) and aniline (AN), were investigated. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were employed to characterize the nano-iron before and after the reaction. The results show that Fe0 is mainly oxidized into lath-like lepidocrocite (γ-FeOOH) and needle-like goethite (α-FeOOH) after 8 h of reaction. The coupling removal process and the mechanism are as follows: Fe0 provides electrons to reduce p-CNB to p-CAN and then dechlorinates p-CAN to AN under the catalysis of Ni. Meanwhile, Fe0 is oxidized to FeOOH by the dissolved oxygen and H2O. AN is then adsorbed by FeOOH. Finally, p-CNB, p-CAN, and AN were completely removed from the water. In the pH range between 3 and 7, p-CAN can be completely dechlorinated by n-Fe/Ni within 20 min, while AN can be nearly 100% adsorbed by FeOOH within 36 h. When the temperature ranges from 15 °C to 35 °C, the dechlorination rate of p-CAN and the removal rate of AN are less affected by temperature. This study provides guidance on the thorough remediation of water bodies polluted by chlorinated organics.
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