Nanoscale Zeolite from Low-Cost Resources: Synthesis, Optimization and Application in Wastewater Treatment

Abstract

Versatile green material, nanoscale zeolites (X, Y and ZSM-5) have been synthesized from low-cost local resources-coal fly ash (CFA) as well as rice husk ash (RHA). Both the solid waste, one from power-plant industry and another agricultural industry, are mainly consisting of amorphous silica-alumina with trace impurities and therefore, can be converted to crystalline nanoscale zeolites such as X, Y and ZSM-5 by alkali-activation. The current study aimed at synthesis of nanozeolites from those two resources by both conventional hydrothermal technique as well as hybrid hydrothermal technique aided by microwave and ultrasound and then the utilization of both in remediation of toxic heavy metals and dyes, which are often present in wastewater from different industries. Sonication-assisted technique was found to be the best among all methods in terms of shorter processing time, better crystallinity, adsorption capacity and surface area of the zeolite produced. The synthesized nanoscale porous zeolites have been characterized by various techniques such as XRD, XRF, FESEM, FTIR, TEM, DSC-TG and BET etc. While CFA-derived nanozeolite X with surface area of 648.42 m2/g, average pore size of 9.048 Å and average crystal size of 22.55 nm was obtained by sonication-assisted hybrid hydrothermal method, RHA-derived zeolite Y with surface area of 444.09 m2/g, average pore size of 3.29 nm, and crystal size of 47 nm was obtained by three step route method. Both CFA-derived and RHA-derived nanozeolite were found to have adsorption capacity of ~150 mg/g and more and successfully removed different toxic metals (Pb, Ni, Cd, As etc.), hardness producing metals (Ca, Mg) as well as different toxic dyes (Indigo carmine, methylene blue, crystal violet etc.) with more than 99% efficiency which is far better than both the raw materials as well as commercially available zeolites. Synthetic conditions for synthesis of nanozeolite X from fly ash have been optimized and a detailed characterization of the product has been done in this study. Total processing time to synthesize nanozeolite X for sonication-assisted method (19.20 h) was found to be much less than conventional hydrothermal method (115 h). Detailed kinetic and isotherm studies have also been done in all cases.

Statistical Design of Experiment was used to optimize the synthesis of zeolite materials and adsorption of dyes and metals onto those produced. The optimized CV removal of 99.52% onto nZX was observed. Adsorption of CV dye onto nZX was found to follow Langmuir isotherm and a pseudo-second order model described it perfectly. Regenerated nZX was successfully utilized for CV dye removal for upto five consecutive cycles with more than 90% dye removal. The study can therefore be utilized in removing toxic dyes like CV from real wastewater with high efficiency.
Sono-assisted adsorptive elimination of dyes from industrial effluent was proved to be an fast, efficient and clean technique. Highly pure nanozeolite X (nFAZX) was synthesized from waste coal post-combustion fly ash (CFA) by pre-fusion hydrothermal method. Rapid sono-assisted adsorptive elimination of methylene blue (MB) dye from water solution was studies in presence nFAZX , CFA and commercial zeolite X (CZX). The influence of adsorption variables, for example solution pH (3-11), adsorbent dose (0.05-0.25 g/100 ml), sonication time (2.50-12.50 min), and initial dye concentration (12.50-62.50 mg.L), on the % elimination of MB was explored by response surface methodology (RSM). Maximum MB removal (64.52 % by CFA, 99.30% by nFAZX, and 96.57% by CZX) was obtained at pH 8.25, 0.164 g/100 ml of adsorbent, 9.92 min sonication time, and 25 mg/L MB concentration. The adsorption isotherms and kinetic models were best fitted with Langmuir isotherm and pseudo second order model respectively for all three adsorbents. Also, the maximum Langmuir adsorption capacity of CFA, nFAZX and CZX for MB were 78.22 mg/g, 345.36 mg/g, and 250.41 mg/g respectively for 0.160 g/100 ml adsorbents. The experimentation demonstrated a route of development of a cheap eco-friendly sorbent nFAZX as well as a green, rapid and highly efficient protocol of treating synthetic dye.

Nano ZSM-5 zeolite was synthesized from RHA by two-step process. As-synthesized nZSM-5 was utilized for elimination of toxic CV dye from aqueous solution by conventional and ultrasonic assisted adsorption method. While, rapid ultrasound-aided adsorption process was found to remove over 99% of the dye, slow conventional method merely removed 63.8%. The performance of the synthesized Y and Fe-modified zeolite Y was compared with commercial zeolite Y in effective adsorption of As (V) under ultrasonication. The results demonstrated that IMZY can be used as an effective adsorbent for As (V) in water. The maximum As (V) removal of 40.31%, 99.9%, and 41.24% was achieved with NaY, IMZY, and CZY respectively. The results are encouraging and the study can be utilized to up-scale the process to produce advanced materials like zeolite from low-cost waste resources in a cost-effective and eco-friendly way so that the synthesized nanoscale material can be used in treating real wastewater.