A critical review on silver nanoparticles: From synthesis and applications to its mitigation through low-cost adsorption by biochar

Highlights

• Silver nanoparticles and ions are toxic to living organisms.

• Adsorption is a sustainable process for silver removal from wastewater.

• Silver adsorption by low-cost biochar is efficient in a wide range of conditions.

• Modification of biochar properties enhance adsorption rate and reusability.

• Biochar-graphene composites demonstrate high silver adsorption.

Abstract

Silver nanoparticles are one of the most beneficial forms of heavy metals in nanotechnology applications. Due to its exceptional antimicrobial properties, low electrical and thermal resistance, and surface plasmon resonance, silver nanoparticles are used in a wide variety of products, including consumer goods, healthcare, catalysts, electronics, and analytical equipment. As the production and applications of silver nanoparticles containing products increase daily, the environmental pollution due to silver nanoparticles release is increasing and affecting especially the aqueous ecosystem. Silver nanoparticles can kill useful bacteria in soil and water, and bioaccumulate in living organisms even at low concentrations from 10⁻² to 10 μg/mL silver can show antibacterial effect. On the other hand, the maximum silver discharge limit into freshwater is 0.1 μg/L and 3.2 μg/L for Australia and the USA, respectively. To reduce its toxic consequences and meet the regulatory guidelines, it is crucial to remove silver nanoparticles from wastewater before it is discharged into other water streams. Several technologies are available to remove silver nanoparticles, but the adsorption process using low-cost adsorbents is a promising alternative to mitigate silver nanoparticle pollution in the bulk stage. As one of the low-cost adsorbents, biochar produced from the biomass waste could be a suitable adsorbent. This review focuses on collating the latest evidence on silver nanoparticle production, applications, environmental consequences, and cost-effective technological approaches for silver removal from wastewater.

Introduction

Silver nanoparticle (Ag NP) is an important innovation in nanotechnology. Unique physicochemical and strong antimicrobial properties make silver nanoparticles suitable for numerous applications (Oćwieja et al., 2015; Syafiuddin et al., 2017; Pulit-Prociak et al., 2015). Particularly biomedicines, medical devices, functional textiles, cosmetics, food packaging, food supplements, odour-resistant items, electronics, household appliances, dental amalgam, water disinfectants, paints, and room spray (Schluesener and Schluesener, 2013; Suresh et al., 2012). Therefore, increasing demand has led to a rise of silver nanoparticles’ production. Worldwide, the total estimated production of silver nanoparticles was about 500 tonnes per year in 2009, while expecting an increase of approximately 900 tonnes by 2025 (Khan et al., 2012; Massarsky et al., 2014; Du et al., 2018; Syafiuddin et al., 2018; Calderón-Jiménez et al., 2017).

Silver ion and metallic silver nanoparticles are both hazardous to living organisms and aquatic ecosystems (Wang et al., 2017a). Ingestion of silver may cause health risks by metabolising and depositing in subcutaneous fat (Zhou et al., 2014; Fewtrell, 2014). Silver ion in the water system is classified as a hazardous material by the World Health Organization (WHO), the U.S. Environmental Protection Agency (USEPA), Australia, and Germany (Zhou et al., 2014; Fewtrell, 2014; NHMRC, 2011). Further, wastewater generating from manufacturing processes and usage of these products are contaminated with silver nanoparticles and/or silver ions (Marambio-Jones and Hoek, 2010; Dumont et al., 2015; Wimmer et al., 2019; Jeon, 2015). Therefore, it is important to remove either form of silver from the wastewater before discharging the effluent into the natural environment (Ghassabzadeh et al., 2010; Wang et al., 2012a).

The concentration of silver nanoparticles or silver compounds in wastewater varies depending on the wastewater generating points. In Malaysia, concentrations of silver nanoparticles found in sewage treatment plant (STP) vary between 0.13 and 20.02 mg/L (Syafiuddin et al., 2018), in Canada up to 1.9 μg/L (Hoque et al., 2012), and in Germany between 0.32 and 3.05 μg/L (Li et al., 2013). Wastewater treatment plant (WWTP) with primary treatment processes, such as screening, precipitation, coagulation, and followed by biological treatment can remove most of the silver nanoparticles and silver compounds (Wang et al., 2012a; Li et al., 2013; Ma et al., 2014). However, wastewater treatment facilities, which have no preliminary or primary treatment process, can be impacted by silver toxicity and may lead to a discharge of contaminated effluents (Zhang et al., 2016a; Alizadeh et al., 2019) as silver ions have a negative influence on bacteria dominant aerobic treatment processes (Khan et al., 2012; Syafiuddin et al., 2019; Wang et al., 2017b; Barker et al., 2018). Even low concentrations of silver (ng/L) are toxic and still representing a real threat to the environment for the long run (Sim et al., 2014; Zhang et al., 2019).

In terms of economic feasibility, easy operation, and sustainability, adsorption is a promising method to remove low concentrations of heavy metals including silver and other precious metals from wastewater (Mohan et al., 2014; Jeon, 2017; Song et al., 2011; Antunes et al., 2017). Adsorption is the adhesion of atoms, ions, or molecules to the surface of a substrate by chemical or physical interactions (Antunes et al., 2017). Silver adsorption study by using a low-cost adsorbent has gained substantial interest in recent years (Zhou et al., 2014; Alandis et al., 2019). Several studies are found on silver removal using different adsorbents, but just a few research has been done so far on the application of biochar to remove silver from an aqueous solution (Antunes et al., 2017; Yao et al., 2015). However, biochar produced from biomass might bring the breakthrough (Chen et al., 2018).

The present review made an effort to analyse the progress on silver nanoparticles application, consequences, and mitigation of its environmental toxicity by adsorption. To our best knowledge, this is the first review on silver nanoparticles from the synthesis processes to their environmental mitigation.