Adsorption of Arsenate and Arsenite by Iron-Treated Activated Carbon and Zeolites: Effects of pH, Temperature, and Ionic Strength

Naturally occurring arsenic contaminates groundwater in many countries, including the United States, at levels greater than 10 μ g l^{− 1}, the current WHO guideline value, increasing the risk of skin, lung, bladder, and kidney cancer in millions of people. Arsenic toxicity is dependent on its chemical form; arsenite is more toxic due to its higher affinity for protein than arsenate. This study supports worldwide research efforts to obtain drinking water with arsenic levels below 10 μ g l^{− 1}. Batch adsorption kinetic and isotherm studies were conducted to compare and evaluate iron-treated adsorbents for arsenate and arsenite removal from aqueous media. Two iron treatments were investigated as well as the effects of varied pH, temperature, and ionic strength increases on adsorption effectiveness. Adsorbent materials such as activated carbon and naturally occurring zeolites (clinoptilolite and chabazite) were selected because of their relative low cost and because the zeolites are potential point-of-use materials for mitigating arsenic contaminated groundwater. Molecular sieves, Faujasite (13X) and Linde type A (5A) were selected because they provide a basis for comparison with previous studies and represent well-characterized materials. Iron-treated activated carbon and chabazite showed the most promise as low-cost arsenic adsorbents; activated carbon removed approximately 60% of arsenate and arsenite while chabazite removed approximately 50% of arsenate and 30% of arsenite. Modeling arsenate and arsenite adsorption by these adsorbents using the Langmuir and Freundlich isotherm expressions determined the adsorbents' capacity for arsenic removal from aqueous media. Arsenate removal by iron-treated activated carbon and clinoptilolite best fit the Langmuir model. Arsenate removal by iron-treated chabazite and arsenite removal by activated carbon, chabazite, and clinoptilolite best fit the Freundlich model. Applications of iron-modified activated carbon for effective arsenate removal would require pH values between 7 and 11, chabazite between 4 and 5, and clinoptilolite between 3 and 7. Arsenite removal by iron-modified activated carbon would require pH values between 7 and 11, chabazite between 7 and 10, and clinoptilolite between 4 and 11. Increasing temperature improved adsorption performance for activated carbon and the zeolites. Increasing ionic strength improved performance of iron-treated activated carbon and zeolites.

Keywords:

• Adsorbents
• Arsenate
• Arsenite
• Activated carbon
• Zeolites
• Chabazite
• Clinoptilolite.