Kinetics of Adsorption and Isotherm Models for Sustainable Zeolite Nano Resin for Water Decontamination

Clinoptilolite, a ubiquitous zeolite mineral with aluminosilicates framework intertwined with cations (Na, K, Ca) is highly porous and has shown strong cation exchange affinity towards some analytes. Our group has functionalized clinoptilolite grains with anion exchange polyelectrolyte into novel Zeolite Nano Resin (ZNR) which adsorbs toxic contaminants from drinking water. The targeted contaminants are PFOA and PFOS which are highly stable in nature and proposed as unsafe to environment by USEPA with interim lifetime health advisory level down to 0.004 parts per trillion. This study investigates various parameters such as kinetics of adsorption, thermodynamics, concentration of adsorbate and adsorption capacity of several ZNR materials. After comparing with the first order kinetics and Intraparticle diffusion mechanism, ZNR tends to follow pseudo second order chemisorption model with fast kinetics and maximum mass transfer capacity of 3.40 ± 0.04 mg/g. Adsorption isotherm experiments have been performed with initial concentration ranges from 200-2300 mg/L of fluorescein. The adsorption behavior has been explored with Langmuir and BET adsorption isotherm models fit on adsorption isotherm curves. Strong interaction between ZNR and fluorescein has been seen with Langmuir adsorption constant of 0.28 ± 0.1 L/mg and q_max of 4.28 ± 0.20 mg/g which will be optimized further. The grain size adsorption pattern has been explored by crushing the ZNR grains into smaller dimensions and comparing with full grain size (14 × 40 mm) adsorption isotherm curves. It has been found that the adsorption process is dominant on outside porosity of the framework. However, the elemental analysis by energy dispersive x-ray spectroscopy of half cut grain reveals that fluorescein content was able to reach the inner porous nanochannels of ZNR grains as well. Clinoptilolite undergoes dehydration and rehydration when exposed to water and heat for ZNR aqueous synthesis which was characterized by thermogravimetric analysis along with quantification of polymerization reaction. Kinetics of polymerization of clinoptilolite grains has been examined under 3 different reaction times for optimization. The optimized longest reaction of 118 hours exhibits maximum adsorption capacity. Thermodynamic parameters have been calculated with initial concentration of 1000 mg/L fluorescein at different temperatures. The adsorption capacity found to be maximum at room temperature which shows non-spontaneous process corroborated by positive ΔG as desired under the influence of hot and cold temperature. The high ΔH value also indicates the chemical adsorption where adsorbate molecules are held at the ZNR grain interface by chemical bonds or electrostatic force of attraction. The ΔS is also positive which means that there is randomness at the fluorescein-ZNR interface to reach the adsorption equilibrium. The fluorescein molecules are adsorbing and desorbing simultaneously after certain mass transfer. The equilibrium adsorption constant considered for thermodynamic results is only valid in the adsorption isotherm curve where Langmuir adsorption capacity is equal to the maximum adsorption capacity after the model fitting. Nevertheless, ZNR analyte adsorption seems not limited by only monolayer adsorption as per the isotherm curves. Preliminary studies for the regeneration of ZNR for controlled desorption reveals that approx. 1.5 mg of adsorbed analyte desorbs with just one iteration of 4 mL of 4M NaCl aqueous solution.