ARSENIC REMOVAL IN GROUNDWATER USING ENGINEERED MAGHEMITE (γ-Fe2O3) NANOPARTICLES

Abstract

Arsenic has a global concern as a groundwater contaminant due to its potential for fatal health consequences. The long term consumption of arsenic contaminated water and food results in damage to the liver, kidney and gall bladder. Both developed and developing countries are under a potential threat of groundwater arsenic contamination. To combat the problems related to its contamination, WHO and USEPA have set the standard limit of 10 μg L-1 for drinking water since the year of 2006. Its removal employing nanoadsorbents has gained considerable attention through both ex-situ and in-situ techniques among the scientific groups since last two and half decades. Moreover, nanotechnology based water treatment systems in general are also finding favour due to better resource and energy efficiency. Several metallic nanoadsorbents including oxides of iron, aluminium, cerium, copper, zirconium, titanium alongwith their functionalized nanostructures have been reportedly developed for arsenic removal. In recent years, metallic iron/iron oxides based nanoadsorbents have been widely explored due to their significant affinity towards arsenic and easy availability. Among various polymorphs, nZVI (nano-scale zero-valent iron) has been extensively studied in literature both at the laboratory and pilot scale studies. These nanoparticles, however, are unstable in natural environmental conditions and prone to get oxidized into iron oxides/hydroxides, a fact which limits their application for pilot scale studies effectively. The aim of present study is to develop the maghemite (γ-Fe2O3) nanoparticles and their nanohybrids while also exploring the use of industry waste as one of a precursor material. The γ- Fe2O3 is FDA approved iron oxide phase and considered to have high magnetization, biocompatible and non-toxic nature. At nanoscale, this phase is considered as the most stable polymorph of ironIII oxides. Further, the as-synthesized nanoparticles have been explored for arsenic removal as adsorbents, anticipating their application as ex-situ and/or in-situ remediation materials. To achieve the goal in the present study, several tasks have been performed. The entire study has been divided into 7 chapters. Chapter 1 presents the introduction of the research work. The research goal, objectives and hypothesis have been stated alongwith the justification for conducting the research. Strategic approach highlights the tasks identified in different domains extending from synthesis, characterisation to batch and continuous remediation experiments. vi Chapter 2 highlights the worldwide scenario of arsenic contamination and the mechanisms of its release in groundwater. An overview of the employed treatment technologies adopted at small and large scales have been presented. The issues and scope for increasing the efficacy of already existing arsenic removal units based on adsorption technology have been examined. Further gaps pertaining to the development of nanoadsorbents and their application in batch/continuous scale studies alongwith the fabrication of columns are discussed. Possible scope of in-situ arsenic sequestration and need of upgradation of the technologies in this regard have also been included in this section. Chapter 3 presents the development of methodology for synthesis of bare maghemite (BIO) nanoparticles and functionalized (BIO-DW) nanostructures using chemical approach. Their physio-chemical characterization has been examined for electronic spectroscopy (UV-Visible, XRD, Raman, FTIR and XPS), magnetic characteristics (VSM and SQUID), surface (FE-SEM, HR-TEM, zeta-potential) and physical (DTA-TGA, BET surface area) properties. A fundamental approach in this task has been to develop a method to maintain the yield of nanostructured adsorbents by conserving the maghemite phase of ironIII oxides during the formation of nanohybrids. The phase identification has been confirmed using the analysis such as X-ray diffraction (XRD), Raman and X-ray photoelectron (XPS) spectroscopy. In nanohybrids, the results indicate a decrease of 13.2 % in the average particle size (nm) and an increase of 39.6 % in the surface area have been observed which are beneficial characteristics in providing more reactive sites to arsenic species for electrostatic interactions. An increase in the colloidal stability in BIODW nanoparticles as compared to BIO nanoparticles have been observed through zeta potential measurements, which are determined to be -21.0 and -27.3 mV at pH 7, respectively. Demonstration of super-paramagnetic behaviour is quite apparent with the magnetic moment (emu g-1) value of 72.7 and 67.5 observed for bare and functionalized nanoparticles, respectively. Chapter 4 presents the batch experiments to analyse the sorption kinetics and removal characteristics of targeted arsenic specie (+5) as well as in multi-constituent matrix (associated with other constituents simulating real life scenario) for both as-synthesized nanoadsorbents. The adsorbate-adsorbent reactions are illustrated through various models viz. empirical, chemical and surface complexation models. Optimization of the contact time for AsV removal has been worked out simultaneously maximizing adsorption capacity and minimizing amount of dose for the near neutral pH conditions. This study utilizes the CCD (central composite design) to employ RSM (response surface methodology) for optimization procedure. The modeling of the removal kinetics vii for the adsorbent dose concentration ranging from 0.15 – 0.45 g L-1 follows pseudo-second-order kinetics and intra-particles diffusion models. Further, the equilibrium isotherms models have been examined for concentrations of AsV (5 - 125 mg L-1) and dose (0.30 and 0.40 g L-1) at the physical variables representing for the treatment of groundwater. The Langmuir constant (unit less), Tempkin constant (J mol-1) and mean free energy (KJ mol-1) from these examinations have been found to be 0.034 - 0.393, 18.036 - 30.775 and 0.707- 1.0, respectively. These studies indicated that the removal process involves both partial physisorption as well as partial chemisorption. Chapter 5 presents the batch experiments which were extended to examine the removal capabilities of as-synthesized nanoparticles for the real world applications by formulating synthetic water at laboratory scale representing the concentration of elements equivalent to those of samples collected and analyzed from arsenic effected Ballia district, Uttra-Pradesh, India. Taguchi’s design of experimental methodology has been explored to evaluate the possible effects of process parameters such as initial arsenic concentration, total dissolved solids (TDS), shaking speed, temperature, pH, dose and contact time. Out of these, two-parametric interactions (arsenic concentration x TDS, arsenic concentration x shaking speed, TDS x shaking speed) have also been investigated to explore their effects on AsV removal. The geochemical code Visual MINTEQ has been utilized for surface complexation modeling (SCMs) to understand the adsorption mechanism. The charge distribution multi-sites complexation (CD-MUSIC) model alongwith 2pk-Three-Plane- Model (TPM) and Diffuse Layer Model (DLM) have been examined for this purpose. Further, the ANN model is trained to evaluate the Taguchi’s outcomes using MATLAB neural network tool. Chapter 6 presents the details of the experiments conducted on the fixed bed 1-dimensional column(s) and sand-tank model to explore the arsenic removal under the dynamic flow environment similar to real world scenario. Design and fabrication of columns have been carried out by considering the factors such as height of reactive zone, column with constant porosity and their correlation with the required mass of adsorbents. Analytical grade sand soil of particles size ranging from 0.05 - 0.5 μm has been used as a supporting material alongwith as-synthesized nanoparticles in all the experimental runs. The Bed-Depth-Service-Time (BDST) model has been employed to determine the breakthrough (Tb) and exhaustion time (Te) at different bed height and flow rate values. A percent increase in breakthrough and exhaustion time (min) with the values of 22.9 % and 10.3 % have been observed for functionalized nanostructures as compared to bare maghemite nanoparticles. Further, the breakthrough curve data have also been fitted using viii Thomas, Yoon-Nelson and Adam-Bohart models to understand the removal characteristics under continuous mode of experiments. Further, to overcome the limitations of ex-situ treatment technologies such as high operational cost and generation of toxic sludge, a laboratory scale 3-D sand tank (60 cm x 30 cm x 50 cm) model has been developed to explore the possible factors that may affect the in-situ addition of developed nanoparticles and visualising injection of nanomaterials into the aquifer systems contaminated with arsenic. The efficacy assessment for in-situ employment of developed nanoparticles representing Direct Injection mode of application has been investigated through sand-tank experiments for AsIII removal. The COMSOL Multiphysics software (subsurface flow module) has also been employed to simulate fluid flow below ground during the assessment of arsenic sequestration. Tracer experiments have been performed using molar solution of sodium chloride. The possibilities of recontamination have also been investigated after the arsenic sequestration under variable flow rate. Finally, Chapter 7 summarizes the research work and presents the conclusions drawn based on the research outcomes. It also presents the possible future perspectives of studies based on investigations carried out in the thesis.