Adsorption of arsenic onto films based on chitosan and chitosan/nano-iron oxide.

Films made from neat chitosan and chitosan with magnetic nanoparticles (MNPs) were tested as adsorbents of arsenate ions. Sorption equilibrium and sorption kinetics studies are reported, including different models applied to enlighten experimental observations and predict results. The sorption of As (V) was reasonably explained using Freundlich isotherm for neat chitosan film although it was better represented by Langmuir equation for the composite sample. The experimental kinetics results showed that the adsorption of arsenate ions is very fast during the first minutes and then the composite seems to reach saturation, while a slow desorption in the chitosan film was observed and acceptably fitted with a pseudo first order reversible model. The adsorbent containing MNPs presented higher adsorption capacity, which was associated to the additional adsorbent capacity provided by the MNPs and its much more irregular surface area that leads to an enhanced adsorption surface. For instance, at 10 mg/L equilibrium concentration, which corresponds to an initial concentration of As (V) much higher than the normal concentration of arsenate in natural water, chitosan-MNP sample exhibits a removal capacity of 10.4 mg/g that is more than six times higher than the 1.6 mg/g shown by the chitosan film.

Effect of UV radiation on the structure of graphene oxide in water and its impact on cytotoxicity and As(III) adsorption.

Graphene oxide (GO) is widely used in different applications, however once released into the environment it can change its structure and affect the transport of important contaminants such as arsenic. In this work we show that UV radiation, even in the range of 28-74 µW/cm2 of irradiance up to 120 h of exposure, can induce important changes in the structure of graphene oxide, by eliminating -OH and CO functional groups. This reduction affected the stability of graphene oxide in water by decreasing its zeta potential from -41 to -37 mV at pH=7 with the increase of the exposure time. Our results showed that after 24 and 120 h of UV exposure, As(III) adsorption capacity decreased from 5 mg/g to 4.7 and 3.8 mg/g, respectively, suggesting a lower capacity to transport contaminants with time. Computer modelling showed that even a degraded GO structure can have an interaction energy of 223.84 kJ/mol with H3AsO3. Furthermore, we observed that the cytotoxicity of graphene oxide changed after being irradiated at 74 µW/cm2 for 120 h, showing 20% more cell viability compared to as-produced GO. Our results stress the importance of considering the microstructural and compositional changes that GO undergoes even under low irradiance and short periods, when studying its fate and behavior in the environment and possible applications in water treatment.

As(III) Removal from Aqueous Solution by Calcium Titanate Nanoparticles Prepared by the Sol Gel Method.

Arsenic (As) contamination of water is a serious problem in developing countries. In water streams, arsenic can be as As(V) and As(III), the latter being the most toxic species. In this work, an innovative adsorbent based on CaTiO3 nanoparticles (CTO) was prepared by the sol-gel technique for the removal of As(III) from aqueous solution. X-ray diffraction of the CTO nanoparticles powders confirmed the CTO phase. Transmission electron microscopy observations indicated an average particle size of 27 nm, while energy dispersive X-ray spectroscopy analysis showed the presence of Ca, Ti, and O in the expected stoichiometric amounts. The surface specific area measured by Brunauer, Emmett, and Teller (BET) isotherm was 43.9 m2/g, whereas the isoelectric point determined by Zeta Potential measurements was at pH 3.5. Batch adsorption experiments were used to study the effect of pH on the equilibrium adsorption of As(III), using an arsenite solution with 15 mg/L as initial concentration. The highest removal was achieved at pH 3, reaching an efficiency of up to 73%, determined by X-ray fluorescence from the residual As(III) in the solution. Time dependent adsorption experiments at different pHs exhibited a pseudo-second order kinetics with an equilibrium adsorption capacity of 11.12 mg/g at pH 3. Moreover, CTO nanoparticles were regenerated and evaluated for four cycles, decreasing their arsenic removal efficiency by 10% without affecting their chemical structure. X-ray photoelectron spectroscopy analysis of the CTO surface after removal experiments, showed that arsenic was present as As(III) and partially oxidized to As(V).

Overview of As(V) adsorption on Zr-functionalized activated carbon for aqueous streams remediation.

The present work introduces a simple methodology of carbon modification with zirconium, using an organic complexing ligand, as efficient media for selective As(V) removal. It is hypothesized that the incorporation of Zr-nanoparticles improves the attraction of anionic species such as arsenates (HAsO42-/H2AsO4-) making the material highly selective. The effects of pH (3-11) and temperature (15, 25 and 35 °C) were studied. Furthermore, potentiometric titrations, the effect of competing anions, thermodynamics, and adsorption kinetics were evaluated in order to clarify the rate-controlling process and the adsorption mechanism for arsenic removal. Results demonstrated that OH and COOH groups play an important role during the arsenic adsorption process; a small amount of Zr(IV) species (0.77%) increased the adsorption capacity of activated carbon in about a 43%. Thermodynamic analysis showed the spontaneous exothermic nature of the adsorption process was favored at lower temperatures. The presence of anions, such as chloride, sulfate, carbonate, nitrate and phosphate, did not affect the adsorption capacity, while kinetic studies demonstrated that the arsenic adsorption process in Zr-modified activated carbon is not exclusively controlled by intraparticle diffusion.