Effect of pH to the surface precipitation mechanisms of arsenate and cadmium on TiO2

Support Platform of Functionalized Sustainable Cellulose Self-Entanglement Monolithic Adsorbents for Efficient Adsorption of Cadmium(II) Ions

Serious water contamination induced by massive discharge of cadmium(II) ions is becoming an emergent environmental issue due to high toxicity and bioaccumulation; thus, it is extremely urgent to develop functional materials for effectively treating with Cd2+ from wastewater. Benefiting from abundant binding sites, simple preparation process, and adjustable structure, UiO-66-type metal-organic frameworks (MOFs) had emerged as promising candidates in heavy metal adsorption. Herein, monolithic UiO-66-(COOH)2-functionalized cellulose fiber (UCLF) adsorbents were simply fabricated by incorporating MOFs into cellulose membranes through physical blending and self-entanglement. A two-dimensional structure was facilely constructed by cellulose fibers from sustainable biomass agricultural waste, providing a support platform for the integration of eco-friendly UiO-66-(COOH)2 synthesized with lower temperature and toxicity solvent. Structure characterization and bath experiments were performed to determine operational conditions for the maximization of adsorption capacity, thereby bringing out an excellent adsorption capacity of 96.10 mg/g. UCLF adsorbent holding 10 wt % loadings of UiO-66-(COOH)2 (UCLF-2) exhibited higher adsorption capacity toward Cd2+ as compared to other related adsorbents. Based on kinetics, isotherms, and thermodynamics, the adsorption behavior was spontaneous, exothermic, as well as monolayer chemisorption. Coordination and electrostatic attraction were perhaps mechanisms involved in the adsorption process, deeply unveiled by the effects of adsorbate solution pH and X-ray photoelectron spectroscopy. Moreover, UCLF-2 adsorbent with good mechanical strength offered a structural guarantee for the successful implementation of practical applications. This study manifested the feasibility of UCLF adsorbents used for Cd2+ adsorption and unveiled a novel strategy to shape MOF materials for wastewater decontamination.

Exploring the interference mechanisms of surface and aqueous complexes with groundwater arsenate and arsenite adsorption

Ca2+, Mg2+, and HCO3- are extremely common coexisting ions with arsenic (As) in geogenic As-polluted groundwaters. Although extensive research has improved our knowledge of groundwater As removal techniques and mechanisms, there is still a lack of a definite explanation of the distinct influences of Ca2+ and Mg2+ on As immobilization. Furthermore, the question of whether the occurrence of metal-As aqueous complexes has positive or detrimental effects on As adsorption is still open, which hinders our ability to predict the effectiveness of groundwater As removal. The goal of our present work was to investigate the molecular-level interference mechanisms of Ca2+, Mg2+, and HCO3- on arsenic adsorption with batch/column filtration experiments and spectroscopic techniques. The results showed that the co-presence of Ca2+ and As significantly increased As(V) and As(III) adsorption by 22.1 and 12.2% in batch studies and by 20.1 and 16.7% in column adsorptive filtrations, which could be explained by forming a ternary Ca-As-TiO2 complex. Without the surface complex, Mg2+ only had a slightly positive effect on As removal. Co-existence of Ca2+ and HCO3- prevented the generation this surface complex, which significantly decreased the elimination of As(III). Inversely, the As(V) ternary complex and adsorption were not interfered by HCO3-. Moreover, an aqueous Ca-As(V) complex was detected without surface, which facilitated generation of the surface complex and As(V) adsorption. The results of this work clarified the distinct effects of Ca2+ and Mg2+ and As(V) and As(III) adsorption, which was critical in predicting the As adsorption efficiency in column filtration processes.

A review on multi-synergistic transition metal oxide systems towards arsenic treatment: Near molecular analysis of surface-complexation (synchrotron studies/modeling tools)

The science and interface chemistry between the arsenic (As) anions and the different adsorbent systems have been gaining interest in recent years in environmental remediation applications. Metal-oxides and the corresponding hybrid systems have shown promising performance as novel adsorbents in various treatment technologies. The abundance, surface chemistry, high surface area (active-centres), various synthesis and functionalization methodologies, and good recyclability make these metal oxide-based nanomaterials as potential remediating agents for As oxyanions. This work critically reviews eight different platforms focused on the arsenic contamination issue, where the first classification describes the origin of arsenic contamination and presents geographical and demo-graphical considerations. The following section briefs the state-of-the-art remediation techniques for arsenic treatment with a comparative evaluation. An emphasized discussion has been provided regarding the adsorption and classification of various metal oxide adsorbents. In the next classification, various multi-synergism abilities like Redox activity, Surface functional groups, Surface area/morphology, Heterogeneous catalysis, Reactive oxygen species, Photo-catalytic/electro-catalytic reactions, and Electrosorption are detailed. The classification of various characterization tools for accessing the arsenic remediation qualitatively and quantitatively are given in the fifth chapter. The first-of-its-kind dedicated analysis has been given on the surface complexation aspects of the arsenic speciation onto various metal adsorbent systems using synchrotron results, surface-complexation modeling, and molecular simulation (e.g., DFT) in the sixth chapter. The current sensing applications of these novel nano-material systems for arsenic determination using colorimetric and electrochemical-based analytical tools and a note about the economic parameters, i.e., regeneration aspects of various adsorbent systems/the sustainable applications of the treated sludge materials, are provided in the final sections. This work makes a critical analysis of 'Environmental Nanotechnology' towards 'Arsenic Treatment'.

The fate of co-existent cadmium and arsenic during Fe(II)-induced transformation of As(V)/Cd(II)-bearing ferrihydrite

Ubiquitous co-existence of arsenic (As) and cadmium (Cd) in smelting operations and mine drainage presents a major challenge to the environment. Fe(II)-induced ferrihydrite transformation into secondary, more crystalline minerals often controls the geochemical behavior of associated contaminants including arsenate (As(V)) and Cd(II) in natural and contaminated environments. However, the fate of co-existent As(V) and Cd(II) and the underlying mechanism during this transformation process remain unclear. In this contribution, ferrihydrite containing co-precipitated Cd(II) and As(V) with Fe(II) under diverse pH conditions has been investigated. Results from powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FTIR), and Raman spectra show that the co-existence of As(V) and Cd(II) significantly retards the transformation rates of As(V)/Cd(II)-bearing ferrihydrite to more stable iron oxides and reduces that from the newly formed lepidocrocite to goethite. Compared to Cd(II), the co-existent As(V) has stronger influence on the compositions of the transformation products. Chemical analysis shows that phosphate-unextractable As(V) and 0.4 M HCl unextractable Cd(II) both increase as the reaction proceeds during the recrystallization of As(V)/Cd(II)-bearing ferrihydrite, indicating that both As(V) and Cd(II) partially transform to a more stable phase. The co-existent Cd(II) has negligible effects on the As(V) redistribution, but the co-existent As(V) at high loadings has a significant modification in the distribution of Cd(II) during the transformation, which reduces the liberation of Cd(II) into solution, thus decreasing the mobility of Cd(II). These findings have important implications for understanding the mobility and fate of the co-existent As(V) and Cd(II) under natural anoxic environments, remediating the co-existent contaminants, and predicting the long-term behavior of As(V) and Cd(II) in natural and contaminated environments.

Co-sorption of metal ions and inorganic anions/organic ligands on environmental minerals: A review

Co-sorption of metal ions and anions/ligands at the mineral-water interface plays a critical role in regulating the mobility, transport, fate, and bioavailability of these components in natural environments. This review focuses on co-sorption of metal ions and naturally occurring anions/ligands on environmentally relevant minerals. The underlying mechanisms for their interfacial reactions are summarized and the environmental impacts are discussed. Co-sorption mechanisms of these components depend on a variety of factors, such as the identity and properties of minerals, pH, species and concentration of metal ions and anions/ligands, addition sequence of co-sorbed ions, and reaction time. The simultaneous presence of metal ions and anions/ligands alters the initial sorption behaviors with promotive or competitive effects. Promotive effects are mainly attributed to surface electrostatic interactions, ternary surface complexation, and surface precipitation, especially for the co-sorption systems of metal ions and inorganic anions on minerals. Competitive effects involve potential complexation of metal-anions/ligands in solution or their competition for surface adsorption sites. Organic ligands usually increase metal ion sorption on minerals at low pH via forming ternary surface complexes or surface precipitates, but inhibit metal ion sorption via the formation of aqueous complexes at high pH. The different mechanisms may act simultaneously during metal ion and anion/ligand co-sorption on minerals. Finally, the potential application for remediation of metal-contaminated sites is discussed based on the different co-sorption behaviors. Future challenges and topics are raised for metal-anion/ligand co-sorption research.