Removal of fluoride and arsenic from waters with acidic pH

SANDRA ULACO; NARIO N.; FUENTE SILVIA A; VIDAL EZEQUIEL; GRUNHUT M.; DOMINI CLAUDIA; C. MORGADE

Buenos Aires

Congreso; WCCE11 - 11th WORLD CONGRESS OF CHEMICAL ENGINEERING; 2023

The present work shows partial results of a larger study in development, of the adsorptive and photocatalytic activity of different oxides for sodium fluoride and arsenic oxide in water. Although both elements are a problem in groundwater in various areas of the planet, generally slightly alkaline, they are also a problem in water of industrial origin. Therefore, the behavior at different pH of the oxides studied acquires special relevance.The particular objective of this part of the work is to obtain at low pH (pH=3) the percentage of removal of fluoride and arsenic (III). In all cases, the oxides are below the respective isoelectric points, so they are positively charged. The determinations were made at room temperature (20°C) with a concentration of 1g/L of each oxide. The initial concentration was 5 ppm NaF (Merck, analytical grade) and 200 ppb for As2O3, being the World Health Organization recommendations 1.5 ppm for fluoride and 10 ppb for arsenic. Rutile TiO2, anatase TiO2, MgO and Fe2O3 were studied. The determination of the concentrations for fluoride anion were carried out with a selective electrode Orion pH/ISE meter 710 (Van London Co) while the determination of As(III) was based on the inhibitory effect of this species on the discoloration of a helianthin solution caused by the products of the oxidation-reduction reaction between potassium bromate and hydrochloric acid. Helianthin decolorization was monitored on an Agilent 8453 spectrophotometer at 508 nm [1]. All determinations were performed in triplicate. The results obtained show that rutile TiO2 and MgO are more effective for fluoride removal in the absence of light, 65.5% and 68.6% respectively. While, anatase TiO2 and Fe2O3 for the removal of As(III), 69% and 46% respectively. On the other hand, in the presence of light, rutile TiO2 has an important photocatalytic effect both in the removal of F- and As(III), reaching a removal percentage of 58% and 75.8% respectively. Anatase TiO2 is effective to remove As(III) (73.7%).From the results obtained it can be observed that at pH=3 the best removal is by adsorption, being MgO the best adsorbent followed by rutile TiO2. This conclusion is in agreement with the percentages of ionic character of the metal-F- bonds, which are respectively 86% for Mg-F, 79% for Ti-F and 70% for Fe-F. The case of anatase TiO2 is known to present a particular behavior, since F- binds preferentially to O atoms inside the crystalline structure, which determines a very low adsorptive activity for the anion [2]. As for arsenic, the best removal is produced by photocatalysis with polymorph rutile TiO2. At the pH studied, it is planned to study the competitive removal efficiency for both contaminants with rutile TiO2, which compared to the isolated studies presents the best comparative performance.