Eco-friendly geo-nanocomposites for achieving aqueous sustainability

Khandelwal, Nitin Kumar (2022) Eco-friendly geo-nanocomposites for achieving aqueous sustainability. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Clean water, the elixir of life, is of tremendous importance in achieving environmental sustainability and the balanced functioning of our ecosystem. Water or “aqueous sustainability” stands over three central pillars i.e., resiliency, efficiency, and quality. Sustainably addressing water means providing safe, easily accessible, and reliable water, sanitation, and pollutionprotected pathways, including resiliency in extreme climatic stresses such as water scarcity and floods. Coupled with population growth, several anthropogenic activities and environmental catastrophes have contributed to an alarming increase in the concentration of toxic pollutants in water bodies. Therefore, achieving “water quality” among other parameters of aqueous sustainability, has become a major societal and scientific challenge. Diversified physiochemical conditions of water matrices, ranging from mining drainage to seawater, are the critical challenge in designing adsorbents for contaminants removal and generating clean drinkable water. By utilizing several structural and surface properties of geo-adsorbents and nanomaterials, this thesis focuses on designing and exploring various geo-nanocomposites for the treatment of four different contaminant systems, i.e. (i) oxi-anions (here- chromium), (ii) cationic heavy metals and radionuclides, (iii) multi-ionic metallic species and (iv) organic dyes. In the end, thesis also explores the environmental sustainability of the utilized adsorbents before and after the removal of contaminants, to comment on their applicability and safe disposal. Different synthesized nanocomposites were characterized thoroughly for their morphology, composition, crystallinity, surface area, charge and functionality before exploring their applications for targeted contaminant removal. To test the efficiency and environmental applicability of the nanocomposite, detailed sorption studies were performed that involved batch and continuous filtration, the impact of environmental parameters, and removal in complex and real water matrices. In addition, changes in surface properties of composites were assessed thoroughly to comment on probable contaminant removal mechanisms. Among various chromium removal mechanisms addressed through different synthesized geonanocomposites, electrostatic attraction and ion exchange failed (q< 2mg/g). At the same time, chemical complexation and redox-transformation found efficient (q> 30mg/g) in treating chromite ore processing residue contaminated groundwaters that were collected from Khan- Chandpur village, Kanpur, Uttar Pradesh, India. Agro-waste-derived biochar-based redoxactive iron nanocomposite showed enhanced continuous filtration capacities for COPRcontaminated water, reaching as high as 3500 L/Kg. alkali-earth metal phosphates such as Hydroxyapatite and Farringtonite were synthesized in the nano-size range due to their high cation exchange capacities. Corresponding to the high mobility of magnesium ions, tri-magnesium phosphate or nano-Farringtonite showed ultrahigh removal of cations (Sr²⁺, Cd²⁺, and Zn²⁺) compared to hydroxyapatite with continuous filtration capacities reaching more than 2,00,000 L/Kg. Detailed EXAFS analysis was performed to obtain mechanistic insights on cations incorporation and removal mechanisms.Water bodies contaminated with multi-metallic species require special consideration in material design for their remediation. Redox-sensitive nanoparticles can have interfacial electron transfer, leading to redox transformation of contaminants and their co-precipitation. Therefore, various iron-based redox nanocomposites were synthesized to address continuous co-contaminant filtration. Metal cations (Ni²⁺ and Cd²⁺) and oxy-anions (CrO₄²⁻ and AsO₂₋) were targeted in mono and multi-metal contaminated solutions. Zerovalent iron nanoparticles (nZVI) showed drastically different growth behavior on swelling bentonite (B) and nonswelling kaolinite (K) clays, which led to net negative zeta potential on B-nZVI and positive on K-nZVI. This variation in zeta potential resulted in preferential higher sorption of cationic species on B-nZVI and anionic species on K-nZVI. While both composites had >90% removal efficiency in multi-metallic solutions, B-nZVI and K-nZVI lowered the toxic cations and oxyanions concentration below WHO permissible limits. Redox-sensitive nanoparticles (RSNPs) such as nZVI and their composites face a critical limitation of instant open-air oxidation before interaction with contaminants in water bodies. Here, biochar generated from almond shells and antioxidants extracted from almond skins were used to provide combined surface support and antioxidant capping to support and preserve nZVI particles. This new strategy resulted in efficient multi-metals separation from polluted river and groundwater with at least 2-fold increase in earlier reported sorption capacities and continuous filtration capacities of >1,50,000 L/Kg were achieved. The impact of long-term open-air exposure on the reactivity of RSNPs was so far unknown and is crucial to understand for their commercialization. In addition, various charcoal-clay-based nZVI composites were explored for the removal of organic dyes. 300 days of open-air exposure was provided to these composites to compare their reactivity for dyes before and after aging. Results showed that composites containing a higher amount of charcoal (>50%) were more efficient in preserving nZVI particles and showed higher removal of organic dyes. sorbed geo-nanocomposites were evaluated for their disposal possibilities in soils and landfills. Results suggest that composites having higher sorption capacities (>100 mg/g) and lower soil dilution requirements (< 500 Kg/Kg) should be the focus of future materials for environmental remediation. Therefore, research work conducted in this thesis targets the water quality parameter of water sustainability and clean water goal (SDG-06) of defined sustainable development goals. The thesis describes the combined role of the knowledge in geochemistry, material science, and environmental engineering in achieving super-efficient geo-nanocomposites to remove various contaminants, generate clean drinkable water, and achieve aqueous sustainability.

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