Comparison of Zn-Al and Mg-Al layered double hydroxides for adsorption of perfluorooctanoic acid.

Per-and polyfluoroalkyl substances (PFAS), a large class of synthesized chemicals, are persistent in nature and generally recalcitrant to conventional chemical and biological treatment. Adsorption is considered an economical and practical method for PFAS treatment. Layered double hydroxides (LDHs) represent a promising class of mineral-based adsorbents for PFAS removal because of the highly positive charge of their structural layers. In this research, the performance of two representative LDHs with varied cation compositions, namely Zn-Al and Mg-Al LDHs, were investigated and compared for the removal of perfluorinated carboxylic acids (PFCAs) with an emphasis on perfluorooctanoic acid (PFOA). Zn-Al LDH showed high efficiency for the removal of medium- and long-chain PFCAs (i.e., C ≥ 7), and performed consistently better than Mg-Al LDH. Based on detailed adsorption kinetics and isotherm studies toward PFOA, Zn-Al LDH showed higher adsorption capacity, stronger adsorption affinity, and faster kinetics than Mg-Al LDH. Presence of natural organic matter had minimal impact on PFOA removal by Zn-Al LDH, but sulfate severely inhibited PFOA adsorption. Combined results of aqueous adsorption experiments and sorbent characterization suggested that electrostatic interactions may be the primary mechanism for PFOA adsorption onto LDHs. Our results suggested that cation composition of LDHs can have significant effect on the performance for PFCA removal.

Zirconium-modified natural clays for phosphate removal: Effect of clay minerals.

Excessive amount of phosphate entering water bodies may cause eutrophication and have detrimental effects on ecosystems. Clay-based materials have been drawing attractive attention in mitigating phosphate release to aquatic environment. In this study, we prepared a series of zirconium (Zr)-modified clays to investigate the effect of clay structure and expansion property on phosphate adsorption. Kaolinite, montmorillonite, and vermiculite were selected as three representative natural clays for Zr modification, and the resulting Zr-modified clays were characterized using various techniques that included powder X-ray diffraction, scanning electron microscopy, and zeta potential measurement. Different Zr-modified clays exhibited substantially different phosphate adsorption behaviors, which may be related to the distinct structural and expansion properties of each clay substrate. Particularly, Zr-modified montmorillonite had fastest phosphate adsorption kinetics and highest phosphate adsorption capacity among all Zr-modified clays, which may be attributed to the good expansion property of montmorillonite that favored the uniform intercalation of Zr species, making the adsorption sites easily accessible by phosphate. Furthermore, all Zr-modified clays showed robust performance for phosphate adsorption under various water chemistry conditions. Combined aqueous sorption and solid characterization analyses suggested that formation of inner-sphere surface complexes may be the primary mechanism for phosphate adsorption by Zr-modified clays.

Analysis of operational parameters, reactor kinetics, and floc characterization for the removal of estrogens via electrocoagulation.

Estrogenic compounds can cause human and ecological health issues and have been detected in surface and drinking water. In this research a reactor analysis determined the impact of operational parameters, the best fit kinetic model for the removal of estrone (E1), 17ß-estradiol (E2), estriol (E3), and 17α-ethynylestradiol (EE2) using a bench-top iron electrocoagulation reactor, and characterized the floc generated in-situ. The parameters investigated were current density, conductivity, stir rate, and polarity reversal. Estrogen removal correlated well with an increase in current density, while conductivity did not impact removal but did reduce potentials. High stir rates and frequent polarity reversal demonstrated greater removal. The operating parameters that achieved the greatest estrogen removal were a current density of 16.7 mA cm-2, conductivity of 1000 µS cm-1, stir rate of 500 rpm, and a polarity reversal time of 30 s. These parameters led to average removal efficiencies of 81%, 87%, 85%, and 97% for E1, E2, E3, and EE2, respectively. The removal data for all estrogenic compounds best fit a pseudo-first order relationship with kinetic rate constants of 0.015 min-1 for E1 and E2, 0.016 min-1 for E3 and 0.040 min-1 for EE2. The floc formed in-situ were characterized by determining the crystalline phases with X-ray diffraction, the size and zeta potential, and the shape and major components using scanning electron microscope with energy-dispersive X-ray spectrometer. The iron coagulant generated during electrocoagulation was lepidocrocite with a point of zero charge of 5.67 and an average floc diameter of 2255 nm.

Free standing TiO2 nanotube array electrodes with an ultra-thin Al2O3 barrier layer and TiCl4 surface modification for highly efficient dye sensitized solar cells.

Dye sensitized solar cells were fabricated with free standing TiO2 nanotube (TNT) array films, which were prepared by template assisted atomic layer deposition (ALD) with precise wall thickness control. Efforts to improve the photovoltaic performance were made by using Al2O3 barrier layer coating in conjunction with TiCl4 surface modification. An Al2O3 thin layer was deposited on the TNT electrode by ALD to serve as the charge recombination barrier, but it suffers from the drawback of decreasing the photoelectron injection from dye into TiO2 when the barrier layer became too thick. With the TiCl4 treatment in combination with optimal thickness coating, this problem could be avoided. The co-surface treated electrode presents superior surface property with low recombination rate and good electron transport property. A high conversion efficiency of 8.62% is obtained, which is about 1.8 times that of the device without surface modifications.