Anionic sorption onto modified natural zeolites using chemical activation

Use of (modified) natural adsorbents for arsenic remediation: A review

Arsenic (As) is a ubiquitous element found in the atmosphere, soils and rocks, natural waters and organisms. It is one of the most toxic elements and has been classified as a human carcinogen (group I). Arsenic contamination in the groundwater has been observed in >70 countries, like Bangladesh, India, West Bengal, Myanmar, Pakistan, Vietnam, Nepal, Cambodia, United States and China. About 200 million people are being exposed to excessive As through consumption of contaminated drinking water. Therefore, developing affordable and efficient techniques to remove As from drinking water is critical to protect human health. The currently available technologies include coagulation-flocculation, adsorption, ion exchange, electrochemical conversion and membrane technologies. However, most of the aforementioned treatment techniques require high initial and maintenance costs, and skilled manpower on top of that. Nowadays, adsorption has been accepted as a suitable removal technology, particularly for developing regions, because of its simple operation, potential for regeneration, and little toxic sludge generation. Processes based on the use of natural, locally available adsorbents are considered to be more accessible for developing countries, have a lower investment cost and a lower environmental impact (CO₂ emission). To increase their performance, these materials may be chemically modified. Hence, this review paper presents progress of adsorption technologies for remediation of As contaminated water using chemically modified natural materials.

Pretreatment effects on the sorption of Cr(VI) onto surfactant-modified zeolite: Mechanism analysis

Adsorption of Cr(VI) onto different pretreated zeolites modified with cetylpyridinium chloride (CPC) is investigated using batch studies, Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) measurements and scanning electron microscopy (SEM). The results indicate that acidification after alkalization is the most effective pretreatment method, and only alkalization would significantly reduce the sorption capability. This behavior is due to the precipitates, such as CaCO3 and MgCO3, generated after alkalization on zeolite surface interfere the formation of the CPC bilayer, which provides active sites for sorbing Cr(VI). The schematic of the adsorption mechanism is presented. The results indicate that developing a better understanding of the influence of different pretreatments is quite helpful and suggest that the content of Ca/Mg/Fe could be a good indication of the effectiveness of preprocessing.

Tailored zeolites for the removal of metal oxyanions: overcoming intrinsic limitations of zeolites

This review aims to present a global view of the efforts conducted to convert zeolites into efficient supports for the removal of heavy metal oxyanions. Despite lacking affinity for these species, due to inherent charge repulsion between zeolite framework and anionic species, zeolites have still received considerable attention from the scientific community, since their versatility allowed tailoring them to answer specific requirements. Different processes for the removal and recovery of toxic metals based on zeolites have been presented. These processes resort to modification of the zeolite surface to allow direct adsorption of oxyanions, or by combination with reducing agents for oxyanions that allow ion-exchange with the converted species by the zeolite itself. In order to testify zeolite versatility, as well as covering the wide array of physicochemical constraints that oxyanions offer, chromium and arsenic oxyanions were selected as model compounds for a review of treatment/remediation strategies, based on zeolite modification.

Attenuation of trace elements in coal fly ash leachates by surfactant-modified zeolite

Potential leaching of trace elements from older, unlined fly ash disposal facilities is a serious threat to groundwater and surface water contamination. Therefore, effective methods for containing the pollutant elements within the unlined coal combustion products (CCPs) disposal facilities are required to minimize any potential impact of leachate emanating from such facilities into the nearby environment. Because surfactant-modified zeolite (SMZ) has the potential to sequester both cationic and anionic trace elements from aqueous solutions, bench-scale batch and column experiments were performed to test its ability to remediate trace elements in leachates generated from both alkaline and acidic fly ash samples. Fly ash leachate treatment results showed the potential application of SMZ as an effective permeable reactive barrier (PRB) material to control the dispersion of heavy metals and metalloids from ash disposal sites. Quantitative comparison of the elemental composition of SMZ-treated and untreated leachates indicated that SMZ was effective in decreasing the concentrations of trace elements in fly ash leachates. Similarly, SMZ treatment column experiments showed the delayed peak leaching events and overall reductions in leachate concentrations of trace elements. The effectiveness of SMZ column treatments, however, decreased with time potentially due to the saturation of sorption sites.

Removal of Cr(VI) and As(V) from aqueous solutions by HDTMA-modified zeolite Y

The synthesized zeolite NaY from rice husk ash (RHA) and the commercial zeolite NaY both modified with surfactants in amounts equal to 50%, 100% and 200% of their external cation exchange capacity (ECEC) were used to remove chromate and arsenate anions from aqueous solutions. While the unmodified zeolite Y had little or no affinity for the Cr(VI) and As(V) anionic species, the surfactant-modified zeolite Y (SMZY) showed significant ability to remove of these anions from the aqueous solutions. The highest chromates and arsenates adsorption efficiency was observed from solutions of pH values 3 and 8, respectively because of the dominance of the univalent species of both anions. The adsorption equilibrium data were best fitted with the Langmuir isotherm model with the highest removal capacities observed for the SMZY initially prepared considering the hexadecyltrimethyl ammonium (HDTMA) amount equal to the 100% of the ECEC of zeolite Y. Synthesized SMZY remove Cr(VI) and As(V) more than the corresponding commercial one due to its lower silica to alumina ratio. Thus, the HDTMA-covered modified zeolite Y synthesized using RHA can be used to remove Cr(VI) and As(V) from water.

Adsorption of As(V) on surfactant-modified natural zeolites

Natural mordenite (NM), natural clinoptilolite (NC), HDTMA-modified natural mordenite (SMNM) and HDTMA-modified natural clinoptilolite (SMNC) have been proposed for the removal of As(V) from aqueous solution (HDTMA=hexadecyltrimethylammonium bromide). Influence of time on arsenic sorption efficiency of different sorbents reveals that NM, NC, SMNM and SMNC require about 20, 10, 110 and 20h, respectively to reach at state of equilibrium. Pseudo-first-order model was applied to evaluate the As(V) sorption kinetics on SMNM and SMNC within the reaction time of 0.5h. The pseudo-first-order rate constants, k are 1.06 and 0.52h(-1) for 1 and 0.5g of SMNM, respectively. The observed k values 1.28 and 0.70h(-1) for 1 and 0.5g of SMNC, respectively are slightly high compared to SMNM. Surfactant surface coverage plays an important role and a significant increase in arsenate sorption capacity could be achieved as the HDTMA loading level on zeolite exceeds monolayer coverage. At a surfactant partial bilayer coverage, As(V) sorption capacity of 97.33 and 45.33mmolkg(-1) derived from Langmuir isotherm for SMNM and SMNC, respectively are significantly high compared to 17.33 and 9.33mmolkg(-1) corresponding to NM and NC. The As(V) uptake was also quantitatively evaluated using the Freundlich and Dubinin-Kaganer-Radushkevich (DKR) isotherm models. Both SMNM and SMNC removed arsenic effectively over the initial pH range 6-10. Desorption performance of SMNM and SMNC were 66.41% and 70.04%, respectively on 0.1M NaOH regeneration solution.