Factors influencing silver recovery and power generation in bio-electrochemical reactors

Bioelectrochemical technology for recovery of silver from contaminated aqueous solution: a review

A large amount of silver-rich wastewater is generated from different industrial processes. This wastewater is not considered a waste, but a valuable source for recovery due to the precious silver (Ag). Previous studies have used traditional methods such as membrane filtration, electrolysis, chemical precipitation, electrochemical, and cementation for Ag recovery. However, many drawbacks have been reported for these techniques such as high cost, hazardous waste generation, and the needed refinement of recovered products. In this study, a bioelectrochemical system (BES) for Ag recovery from aqueous solution is introduced as an effective and innovative method, as compared with other techniques. Different types of Ag(I)-containing solutions that have been investigated in recent BES studies (e.g., Ag⁺ solution, [Ag(NH₃)₂]⁺, [Ag(S₂O₃)]^-, [Ag(S₂O₃)₂]^3- complexes) are reported. A BES is an anaerobic system consisting of anode and cathode chambers, which are normally separated by an ion-exchange membrane. The electron flow obtained from the anodic biological oxidation of organic matter is used directly for the cathodic electrochemical reduction of Ag(I) ions. The recovered product is Ag electrodeposits, formed at the cathode surface. Several studies have reported high Ag recovery efficiency by using a BES (i.e., > 90%), with high purity of metallic silver, and simultaneous electricity production. Furthermore, a BES can be employed for a wide range of initial Ag(I) concentrations (e.g., 50-3000 mg/L). The advantages of BES technology for Ag recovery are highlighted in this study for further practical applications.

Recovery of resources from industrial wastewater employing electrochemical technologies: status, advancements and perspectives

In the last two decades, water use has increased at twice the rate of population growth. The freshwater resources are getting polluted by contaminants like heavy metals, pesticides, hydrocarbons, organic waste, pathogens, fertilizers, and emerging pollutants. Globally more than 80% of the wastewater is released into the environment without proper treatment. Rapid industrialization has a dramatic effect on developing countries leading to significant losses to economic and health well-being in terms of toxicological impacts on humans and the environment through air, water, and soil pollution. This article provides an overview of physical, chemical, and biological processes to remove wastewater contaminants. A physical and/or chemical technique alone appears ineffective for recovering useful resources from wastewater containing complex components. There is a requirement for more processes or processes combined with membrane and biological processes to enhance operational efficiency and quality. More processes or those that are combined with biological and membrane-based processes are required to enhance operational efficiencies and quality. This paper intends to provide an exhaustive review of electrochemical technologies including microbial electrochemical technologies. It provides comprehensive information for the recovery of metals, nutrients, sulfur, hydrogen, and heat from industrial effluents. This article aims to give detailed information into the advancements in electrochemical processes to energy use, improve restoration performance, and achieve commercialization. It also covers bottlenecks and perspectives of this research area.

Overview of recent developments of resource recovery from wastewater via electrochemistry-based technologies

As the rapid increase of the worldwide population, recovering valuable resources from wastewater have attracted more and more attention by governments and academia. Electrochemical technologies have been extensively investigated over the past three decades to purify wastewater. However, the application of these technologies for resource recovery from wastewater has just attracted limited attention. In this review, the recent (2010-2020) electrochemical technologies for resource recovery from wastewater are summarized and discussed for the first time. Fundamentals of typical electrochemical technologies are firstly summarized and analyzed, followed by the specific examples of electrochemical resource recovery technologies for different purposes. Based on the fundamentals of electrochemical reactions and without the addition of chemical agents, metallic ions, nutrients, sulfur, hydrogen and chemical compounds can be effectively recovered by means of electrochemical reduction, electrochemical induced precipitation, electrochemical stripping, electrochemical oxidation and membrane-based electrochemical processes, etc. Pros and cons of each electrochemical technology in practical applications are discussed and analyzed. Single-step electrochemical process seems ineffectively to recover valuable resources from the wastewater with complicated constituents. Multiple-step processes or integrated with biological and membrane-based technologies are essential to improve the performance and purity of products. Consequently, this review attempts to offer in-depth insights into the developments of next-generation of electrochemical technologies to minimize energy consumption, boost recovery efficiency and realize the commercial application.

Spontaneous reduction of low-potential silver(I) dithiosulfate complex in bioelectrochemical systems for recovery of silver and simultaneous electricity production

Waste fixer solutions generated from photographic processing are silver-rich effluents, in which silver exists in the form of a dithiosulfate complex ([Ag(S₂O₃)₂]^3-), a stable and water-soluble chemical compound. This study investigated the electrochemical reduction of [Ag(S₂O₃)₂]^3- at different initial concentrations for silver recovery, combined with electricity production in a two-chamber bio-electrochemical system using a cation exchange membrane as the separator. During the biological oxidation of acetate to produce electrons in the anode chamber, [Ag(S₂O₃)₂]^3- was reduced spontaneously by acting as an electron acceptor in the cathode chamber, despite its low standard redox potential ([Ag(S₂O₃)₂]^3-/Ag⁰, E ⁰ = 0.016 V). After 48 h in each batch of operation, a Ag recovery efficiency of 81.7-95.2%, with a columbic efficiency of 12.9-21.4% and a maximum power density of 1500-2647 mW/m³, were obtained with an initial [Ag(S₂O₃)₂]^3- concentration of 10-20 mM, respectively. When the initial [Ag(S₂O₃)₂]^3- concentration increased to 30 mM, cell voltage production did not improve significantly, and a small decrease in Ag recovery efficiency to 93.2% was found. After 61 days of operation, the cathode surface was covered by different-sized silver clusters under SEM observation. The results were confirmed by EDX and XRD characterization, in which metallic silver with high purity was detected. SEM-EDX-XRD characterization of the membrane and the measurements in the control reactor confirmed that there was no diffusion of negatively charged [Ag(S₂O₃)₂]^3- complex through the membrane. Thus, this study showed a successful recovery of Ag from the low-potential [Ag(S₂O₃)₂]^3- complex without energy consumption, secondary waste generation, and loss of Ag.

Recent advances in the recovery of metals from waste through biological processes

Wastes containing critical metals are generated in various fields, such as energy and computer manufacturing. Metal-bearing wastes are considered as secondary sources of critical metals. The conventional physicochemical methods of metals recovery are energy-intensive and cause further pollution. Low-cost and eco-friendly technologies including biosorbents, bioelectrochemical systems (BESs), bioleaching, and biomineralization, have become alternatives in the recovery of critical metals. However, a relatively low recovery rate and selectivity severely hinder their large-scale applications. Researchers have expanded their focus to exploit novel strain resources and strategies to improve the biorecovery efficiency. The mechanisms and potential applicability of modified biological techniques for improving the recovery of critical metals need more attention. Hence, this review summarize and compare the strategies that have been developed for critical metals recovery, and provides useful insights for energy-efficient recovery of critical metals in future industrial applications.

Biotechnological strategies for the recovery of valuable and critical raw materials from waste electrical and electronic equipment (WEEE) - A review

Critical raw materials (CRMs) are essential in the development of novel high-tech applications. They are essential in sustainable materials and green technologies, including renewable energy, emissionfree electric vehicles and energy-efficient lighting. However, the sustainable supply of CRMs is a major concern. Recycling end-of-life devices is an integral element of the CRMs supply policy of many countries. Waste electrical and electronic equipment (WEEE) is an important secondary source of CRMs. Currently, pyrometallurgical processes are used to recycle metals from WEEE. These processes are deemed imperfect, energy-intensive and non-selective towards CRMs. Biotechnologies are a promising alternative to the current industrial best available technologies (BAT). In this review, we present the current frontiers in CRMs recovery from WEEE using biotechnology, the biochemical fundamentals of these bio-based technologies and discuss recent research and development (R&D) activities. These technologies encompass biologically induced leaching (bioleaching) from various matrices,biomass-induced sorption (biosorption), and bioelectrochemical systems (BES).

Electrochemical reduction of different Ag(i)-containing solutions in bioelectrochemical systems for recovery of silver and simultaneous power generation

Different Ag(i) solutions can be successfully reduced electrochemically to form Ag⁰ deposits with dendritic and cubic structure at the cathode surface.