The presence of iron in groundwater, even at low concentrations, results in many problems regarding the drinking water. Iron increases the growth of chlorine-resistant microorganisms in drinking water distribution system, leading to an increase in disinfection cost, in addition to problems regarding changes in odor and taste of water. In order to remove iron from water, diverse techniques are being used including oxidation and filtration, absorption or catalytic bed filtration, ion exchange, softening, biofiltration and adsorption. One of the cheapest methods to remove iron from drinking water is adsorption by cheap minerals such as glauconite. In this study, drinking water iron removal by two mineral absorbent glauconites from Sarakhs and Maraveh Tapeh has been investigated.
Material and methods:
The natural glauconites were collected from glauconitic sandstones in Sarakhs (Neyzar formation) and Maraveh Tapeh (Aitamir formation) and were processed in the laboratory and graded in particle diameter 0.5-1.0 mm. The specific surface area and volume of the cavities of the two specimens were determined by BET analysis by nitrogen absorption method. In order to study the kinetic and equilibrium behavior of the adsorption process, iron adsorption kinetic and adsorption isotherm curves on glauconite absorbents have been determined through experimental tests. These tests were done at three pH levels (5, 7, and 9). The aqueous solutions containing 5 mg/l of iron in a volumetric flask were exposed to different amounts of the absorbent at a constant temperature (20 °C).
Results and discussion:
BET analysis as a nitrogen absorption method revealed the specific surfaces of the two glauconite samples from Sarakhs and Maraveh Tapeh as 0.999 and 2.833 m2/g, respectively. The pore volume of Sarakhs and Maraveh Tapeh glauconites were measured as 0.006 and 0.0123 cm3/g, respectively and the average pore diameter were determined 24.07 and 17.31 nm, respectively. The results indicated that as the pH increased, the iron adsorption capacity and absorption rate by the glauconite from Sarakhs and Maraveh Tapeh increased significantly. Comparing the iron adsorption of glauconites revealed that the extracted glauconite from Maraveh Tapeh had more iron adsorption capacity than that of Sarakhs, corresponding to the higher specific surface area of this absorbent. At pH 5, 7, and 9, the ultimate absorption capacity of glauconite from Maraveh Tapeh was 17.3, 11.7 and 13.9 % higher than that of Sarakhs. The kinetic model regression indicated that Hu et al. and Ritchie's models have absolutely similar behavior in describing the iron adsorption kinetics curves on glauconite absorbents. Eventually, it can be stated that the process of iron adsorption by glauconite follows the second order kinetics. The best isotherm model to describe the iron adsorption equilibrium data on glauconite are the models developed by Temkin and Davoudinejad. Accordance with Davoudinejad's model demonstrates the presence of monolayer adsorption along with heterogeneous adsorbent surface and steric hindrances for absorption. Complying with Temkin’s model indicates that absorption enthalpy is a linear function of absorbent surface loading.
Glauconite mineral absorbents extracted from Sarakhs and Maraveh Tapeh performed better than natural Zeolite and Kaolin and had similar performance to Manganese zeolite, Pyrolusite, and Pumice for iron absorption from the water. Regarding their abundance in Iran, they can be used as an affordable method to solve the problem of the presence of iron in drinking water in Iran.