Abstract
Rising world population, industrialization, urbanization and global warming have adversely affected the fresh water reserve. Current estimations show a serious drinking water crisis imminent globally. The effects would be more acutely felt in the water stressed regions. One possible solution to the crisis is wastewater reuse. However, wastewater reuse requires technological investments. Resource recovery coupled with wastewater treatment is considered to offset the treatment costs, and leads to sustainability. To strengthen the paradigm of resource recovery, it is imperative to explore resources in the waste streams with market values. Also, the environmental and social aspects must be aligned with the identification of resources suitable for recovery from waste streams. One of the emerging contaminants of concern in the waste streams, particularly in domestic wastewater, reported extensively in the past one decade is silver (Ag). The use of silver (Ag) in consumer products, as an anti-microbial agent, particularly on textiles, has been steadily rising. Ag-leaching during laundry and its subsequent discharge in the environment pose ecotoxicological risks. For an effective waste management, Ag recovery is important, as it mitigates environmental hazards, recycles and reuses Ag, and leads to sustainability. Trace concentration of Ag⁺, high concentration of other competing cations (Na⁺, Ca2⁺, Mg2⁺), and complexity of the detergent matrix makes the separation process challenging. The use of ion-exchange resin with thiol group in its chain offers a potential solution due to its high selectivity for silver. The present study utilizes a thiol-group functionalized ion-exchange resin (Ambersep GT74) in a fixed-bed column to remove Ag from laundry. The regeneration process in the work has been optimized with respect to the solution pH and thiourea concentration (0.5 M thiourea concentration at pH 1). Finally, Ag has been recovered as Ag⁰ nanoparticles and Ag₂S (as nanoparticles and micro-sized powder) from the spent regenerant solution (TU: ~0.5 M, pH: ~1.1 - 1.2) using electrolytic and wet-chemistry techniques respectively. The recovered Ag⁰ nanoparticles are monodisperse, consistently spherical in shape, and have a mean diameter of ~6 nm with standard deviation of the Gaussian fit as ~2.66 nm. The process scheme outlined in the work is in keeping with sustainability ideals, as it recycles and reuses the resin and the regenerant solution (TU solution) and recovers a precious product (Ag⁰ nanoparticles). The role of each detergent component in affecting Ag-speciation and the resin performance has been analyzed. Builders and bleaching agents are reported to negatively impact the resin performance. pH and co-cationic species (Ca²⁺) concentration are reported to be critical parameters for the successful recovery scheme. The resin and the regenerant have been used in multiple cycles (4 times) without compromising on their performance. The study successfully demonstrates a closed-loop sustainable scheme by reusing and recycling all the raw materials to the point of exhaustion with no chemicals/toxic released into the environment.