ARSENIC REMOVAL USING IRON-MODIFIED ZEOLITES

Abstract

We determined the ability of two iron-and-zeolite materials to remove arsenic from groundwater. The first material is composed of iron, surfactant-modified zeolite, and hard silicate foam, and is known as Surfactant-Modified Zeolite/ Zero-Valent Iron (SMZ/ZVI). The second material is a new formulation that consists of iron-modified zeolite (IMZ). We prepared eight different prototypes of IMZ for arsenic removal and eventually used one for further testing. The iron content, surface area, and arsenic adsorption capacity were analyzed for each fully prepared material. Lab-based batch experiments were performed on each material to estimate the adsorption capacity. Additionally, the effect of pH on adsorption and the rate of adsorption were determined for the SMZ/ZVI material and one of the newly prepared IMZ. All batch tests were performed using synthetic water based on the chemistry of Socorro Springs water from Socorro, NM. Batch isotherms were performed on each IMZ material using arsenic concentrations that ranged from 10 – 200 mg/L. Batch experiments were accompanied by small-scale column experiments in the lab, and in the case of SMZ/ZVI, a field column test was performed at a wellhead. All of the water used in the column experiments was sourced from the Socorro Springs near the city of Socorro, NM (containing 41-45 μg/L arsenate, pH = 8.1, and TDS = 356 μS/cm), and was transported to the lab for column testing. Influent and effluent arsenic concentrations, column flowrates, and total volume were measured in each column study. The measured parameters were used to calculate the volume of water treated below EPA arsenic concentration specifications. All arsenic concentrations were obtained using a hydride generation method coupled with Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES). The batch experiments showed that the SMZ/ZVI has a maximum arsenic adsorption capacity as calculated by the Langmuir isotherm of 512 mg/kg. The pH study showed that maximum arsenic adsorption occurs at pH 6.5 and that sorption decreases below or above this pH value. The kinetic studies showed that 95% adsorption is achieved in less than 2 minutes at low concentrations (66 μg/L) and within 20 minutes at high arsenic concentrations (0.78 mg/L). A Toxicity Characteristic Leaching Procedure (TCLP) analysis was performed and showed that the spent media loaded with 25 mg/kg arsenic can be safely disposed in a landfill. The field column experiments showed lower column adsorption than did batch experiments. Lab-based column studies showed that with long column residence times (40 minutes), SMZ/ZVI could treat about 200 pore volumes of the Socorro Springs water below an arsenic concentration of 10 μg/L . With water adjusted to pH 6.6, SMZ/ZVI treated about 1,000 pore volumes of the Socorro Springs water to reduce arsenic concentrations 10 μg/L. The arsenic adsorption capacity varied greatly between the different types of newly created IMZ and was heavily dependent on the procedures used for iron application. The most successful technique that resulted in good iron coating coverage, robustness, and a high arsenic adsorption capacity was a multi-step process that coated the iron on the zeolite in several smaller amounts rather than one large loading. The material chosen is labeled IMZ-8 and contained 28,400 mg of iron per kg material. Compared with the raw zeolite that had a surface are of 13.8 m2/g, the surface area of IMZ-8 increased about 150% to 21.4 m2/g. Using the Langmuir isotherm, IMZ-8 had a maximum arsenic adsorption capacity of 654 mg/ kg and a Langmuir coefficient of 4.65 L/mg. The IMZ-8 material was further tested by looking at the effects of pH on adsorption and by performing column breakthrough experiments with media regeneration at the end of each breakthrough cycle. The material was found to have maximum arsenic adsorption between pH 2.0 and 7.0. For column experiments, the IMZ-8 material was loaded into a glass column 10.5 cm in length and 2.5 cm in diameter. Using Socorro Springs water as the influent, the column effluent arsenic concentration remained below 10 μg/L for 800 pore volumes. The IMZ-8 column was successfully regenerated 3 times with no loss of adsorption capacity using 1 L of 0.25 M NaOH and 6 L of reverse osmosis water. The IMZ-8 material may be a viable alternative for municipal water treatment due to the low cost of zeolite, the high arsenic adsorption capacity, and the easily regenerated media. The estimated cost of treating the local water with IMZ-8 was about $0.50 per 1,000 L. For both materials there was a lack of correlation between estimated adsorption capacity from batch experiments and the measured capacity during column studies. The principal reason is believed to be the availability of adsorption sites. The production of fine iron particles during batch experiment shaking may have led to an overestimation of removal capacity. Adsorptive competition with high concentrations of silica (25 mg/L) as well as low intraparticle diffusion rates using SMZ/ZVI are believed to have led to low adsorption in those column experiments.