Adsorption of cadmium on clay-organic associations in different pH solutions: The effect of amphoteric organic matter.

Clay minerals are important soil components and usually coexist with organic matter, forming mineral-organic associations (MOAs), which control the speciation, mobility, and bioavailability of heavy metals. However, the adsorption mechanism of cadmium (Cd) by MOAs is still unclear, especially for the associations of amphotericorganic matter and clay minerals. In this study, 12-aminododecanoic acid (ALA) and montmorillonite (Mt) were chosen to prepare MOAs via intercalation (Mt-ALA composite) and physical mixing (Mt-ALA mixture). Batch experiments were conducted to investigate the adsorption mechanism of Cd(II) by MOAs under different pH values and initial Cd(II) concentrations. The results showed that the Cd(II) adsorption capacities followed as Mt > Mt-ALA mixture > Mt-ALA composite under acidic conditions, Mt-ALA mixture > Mt > Mt-ALA composite under neutral conditions, and Mt-ALA mixture > Mt-ALA composite > Mt under alkaline conditions, suggesting the adsorption behaviors of Cd(II) by MOAs were primarily constrained by the speciation of ALA and solution pH. Under acidic conditions, cationic HALA+ could intercalate into the interlayer of Mt and occupy the adsorption sites, reducing the adsorption capacity of Cd(II). As pH increased to neutral, HALA+ decreased and changed to a zwitterionic state, which caused ALA to release out from the interlayer of Mt-ALA composite or not easily enter into Mt-ALA mixture and promoted Cd(II) adsorption. Under alkaline conditions, the increase of anion ALA- would cause ALA to be mainly adsorbed on the surface of Mt and chelate with Cd(II), enhancing the adsorption of Cd(II). Further analysis by Fourier transform infrared and X-ray photoelectron spectroscopy indicated that the carboxyl and amino groups of ALA both participated in the adsorption of Cd(II). These findings could extend the knowledge on the mobility and fate of Cd in clay-based soils and be used as a basis for understanding the biogeochemical behavior of Cd in the environment.

A critical review of characteristics of domestic wastewater and key treatment techniques in Chinese villages.

As of 2022, China's rural sewage treatment rate is only approximately 31 %. Rapid rural development has led to higher demand. However, China's rural areas are complex and face many problems, such as uneven economic development, population distribution, and water availability. Long-lasting and low-cost wastewater treatment measures are needed for application in rural areas. The quantity and quality of rural domestic wastewater in China were characterized first. Next, the hot topic of domestic wastewater in Chinese villages was confirmed via bibliometric analysis using CiteSpace, and the treatment technologies for rural domestic wastewater were compared. Specifically, the technical status and challenges of the most common technology in rural domestic wastewater treatment, constructed wetlands, were summarized.

Humic acid controls cadmium stabilization during Fe(II)-induced lepidocrocite transformation.

Abiotic reduction of iron (oxyhydr)oxides by aqueous Fe(II) is one of the key processes affecting the Fe cycle in soil. Lepidocrocite (Lep) occurs naturally in anaerobic, clayey, non-calcareous soils in cooler and temperate regions; however, little is known about the impacts of co-precipitated humic acid (HA) on Fe(II)-induced Lep transformation and its consequences for heavy metal immobilization. In this study, the Fe(II)-induced phase transformation of Lep-HA co-precipitates was analyzed as a function of the C/Fe ratio, and its implications for subsequent Cd(II) concentration dynamic in dissolved and solid form was further investigated. The results revealed that secondary Fe(II)-bearing magnetite commonly formed during the Fe(II)-induced transformation of Lep, which further changed the mobility and distribution of Cd(II). The co-precipitated HA resulted in a decrease in the Fe solid phase transformation as the C/Fe ratios increased. Magnetite was found to be a secondary mineral in the 0.3C/Fe ratio Lep-HA co-precipitate, while only Lep was observed at a C/Fe ratio of 1.2 using X-ray diffraction (XRD) and Mössbauer spectroscopy. Based on XRD, scanning electron microscopy (SEM), Mössbauer, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) results, newly formed magnetite may immobilize Cd(II) through surface complexes, incorporation, or structural substitution. The presence of HA was beneficial for binding Cd(II) and affected the mineralogical transformation of Lep into magnetite, which further induced the distribution of Cd(II) into the newly formed secondary minerals. These results provide insights into the behavior of Cd(II) in response to reaction between humic matter and iron (oxyhydr)oxides in anaerobic environments.

Direct Atomic-Scale Insight into the Precipitation Formation at the Lanthanum Hydroxide Nanoparticle/Solution Interface.

Understanding precipitation formation at lanthanum hydroxide (La(OH)3) nanoparticle-solution interfaces plays a crucial role in catalysis, adsorption, and electrochemical energy storage applications. Liquid-phase transmission electron microscopy enables powerful visualization with high resolution. However, direct atomic-scale imaging of the interfacial metal (hydro)oxide nanostructure in solutions has been a major challenge due to their beam-driven dissolution. Combining focused ion beam and aberration-corrected high-angle annular dark-field scanning transmission electron microscopy, we present an atomic-scale study of precipitation formation at La(OH)3 nanoparticle interfaces after reaction with phosphate. The structure transformation is observed to occur at high- and low-crystalline La(OH)3 nanoparticle surfaces. Low-crystalline La(OH)3 mostly transformed and high-crystalline ones partly converted to LaPO4 precipitations on the outer surface. The long-term structure evolution shows the low transformation of high-crystalline La(OH)3 nanoparticles to LaPO4 precipitation. Because precipitation at solid-solution interfaces is common in nature and industry, these results could provide valuable references for their atomic-scale observation.

Effects of Fe(II)-induced transformation of scorodite on arsenic solubility.

Scorodite (FeAsO4·2H2O) is a pivotal secondary ferric arsenate that immobilizes most of arsenic (As) in acidic As-contaminated environments, but secondary As pollution may occur during dissolution of scorodite in environments involving redox changes. Reductive dissolution of scorodite by coexisting dissolved Fe2+ (Fe(II)aq) under anaerobic conditions and its effects on the behavior of As have yet to be examined. Here, this study monitored the changes in mineralogy, solubility and speciation of As during scorodite transformation induced by Fe(II) under anaerobic conditions at pH 7.0 and discussed the underlying mechanisms. Mössbauer and X-ray diffraction (XRD) analysis showed the formation of parasymplesite and ferrihydrite-like species during scorodite transformation, which was highly controlled by Fe(II)aq concentrations. 1 mM Fe(II)aq enhanced As mobilization into the solution, whereas As was repartitioned to the PO43--extractable and HCl-extractable phases with 5 and 10 mM Fe(II). The neo-formed parasymplesite and ferrihydrite-like species immobilized dissolved As(V) through adsorption and incorporation. Additionally, As(V) reduction occurred during Fe(II)-induced scorodite transformation. Our results provide new insights into the stability and risk of scorodite in anaerobic environments as well as the geochemical behavior of As in response to Fe cycling.