Recovery of gold via adsorption-incineration techniques using banana peel and its derivatives: Selectivity and mechanisms

Highlights

• Banana peel and derivatives showed great potentials for gold recovery.

• Lipid elimination boosted the adsorption capacity and selectivity.

• Lipid-free biomass exhibited pH-dependent nanoparticle formation property.

• High-purity metallic gold (>99%) was obtained via adsorption-incineration.

Abstract

In this study, banana peel (BP) and its derivatives after sequential extraction of biochemical components were evaluated for selective recovery of gold. In-depth instrumental characterizations including XPS, FTIR, XRD and HR-TEM were performed to understand the adsorption mechanisms. The biomass after lipid extraction, BP-L, demonstrated very good affinity and selectivity towards gold. In multi-metal systems containing 100 mg/L of Pt(IV), Au(III), Pd(II), Zn(II), Co(II), Ni(II) and Li(I), the selectivity coefficient increased from 978.45 in BP to 2034.70 in BP-L. Moreover, the equilibrium gold uptake was improved and reached 475.48 ± 3.08 mg/g owing to reduction-coupled adsorption mechanisms. The BP-L also showed improved gold nanoparticle formation properties that were pH-dependent. In a strategic adsorption-combined incineration process, metallic gold reaching 99.96% in purity was obtained. The BP and its derivative, BP-L have thus shown potentials for multiple applications in the areas of precious metal recovery and nanoscience.

Introduction

The growing importance of precious metals, PMs (e.g., platinum, palladium, gold, etc.) for applications in different scientific fields, such as the medical and high-tech industries has sparked an overwhelming market demands and persistent price shoot ups over the years (Kotte and Yun, 2014, Lee et al., 2010, Lin et al., 2015, Wei et al., 2016b). Owing to their industrial exploitations, it is believed that the wastewaters ejected from such industries contain guaranteed amounts of PMs (Hagelüken and Corti, 2010, Wei et al., 2016b, Zheng and Wang, 2013b). The contents of PMs in electronic scraps (e-wastes) are much higher than their contents in ores (Chand et al., 2009, Ogata and Nakano, 2005, Park et al., 2012). For instance, it has been reported that in typical e-wastes from mobile phone handsets, computer circuit boards, and all printed circuit boards, there are about 300–350, 200–250 and 450 g/t of gold present, respectively (Hagelüken and Corti, 2010, Ogata and Nakano, 2005, Park et al., 2012, Zheng and Wang, 2013b). However, the concentrations are much lower in gold ores, i.e., ~5–30 g/t (Hagelüken and Corti, 2010). Considering the wide application of gold (but low abundance in nature) and environmental pollution caused by e-wastes, a recycling approach will lead to reducing the environmental toxicity, whiles maximizing alternative routes for making gold available for the many industries that need it. It is thus essential to treat PM solutions to retrieve the gold contained in them (Changmei et al., 2011, Kotte and Yun, 2014).

Conventional lixiviation involving the use of aqua regia produces solutions that contain chloro-complexes of PMs as well as base metals (Chand et al., 2009). Thus, the most important criterion for targeting gold recovery is to selectively separate it from the bulk solution. To design an effective system for gold recovery, it is important to understand the gold speciation and binding mechanisms. Many studies have explored suitable and economical adsorbents with abilities to selectively capture gold. Adsorption by multiple mechanisms such as electrostatic interaction, ion exchange, adsorption-coupled reduction, chelation, coordination, and/or complexation, is one of the evolving techniques (Chand et al., 2009, Kim et al., 2015, Lin et al., 2015). Microbial/agricultural waste biomasses, organic/inorganic materials, and natural/synthetic polymeric materials have all been investigated for adsorptive gold recovery applications (Das, 2010, Kim et al., 2015, Kwak et al., 2013). Among these materials, agricultural biomasses are the cheapest, most abundant and easily accessible. Moreover, they are readily biodegradable, which makes their post-adsorption handling and disposal much easier than synthetic materials.

Banana is an edible fruit which normally grows in the tropical regions and belongs to the Musaceae family of plants (Worldatlas, 2017), and the banana peel (BP) is one of such agricultural waste materials with less recycling efforts so far (Bediako et al., 2019). The main BP components, i.e., lignocellulose, protein, lipid and polysaccharides, bear carboxyl (COOH), hydroxyl (OH), amine (NH₂) and carbonyl (CO) functional groups which can bind ionic species (Memon et al., 2008, Oyewo et al., 2016). Owing to its rich nutritional and mineral contents, the BP was used as livestock food (Onwuka et al., 1997). Although composed of multiple functional groups, only few attempts were made for its application in water purification (Oyewo et al., 2016). Furthermore, there is scanty literature on its application in PMs recovery, despite previous studies had hinted prospects for application of its extracts in synthesizing metal nanoparticles (Bankar et al., 2010a, Bankar et al., 2010b, Bankar et al., 2010c, Zheng and Wang, 2013a).

Our recent study showed that through sequential extraction of some biochemical components, affinity of the BP could be tuned towards target metals and compounds (Bediako et al., 2019). That is, building on the previous study, the objectives of the present study were to investigate the potential application of the BP biomass and its fractionated derivatives, as bio-adsorbents for selective adsorption and subsequent recovery of gold from acidic media using adsorption-incineration techniques. The selective gold adsorption behaviors were studied in ternary PMs solutions and in multi-metal solutions containing PMs in competitions with other base metal ions. The adsorption mechanisms were studied via instrumental characterization, i.e., EDX, FTIR, XRD, XPS and HR-TEM. Consequently, strategic process involving a two-step incineration of the gold-loaded biomass led to obtaining high-purity metallic gold of over 99.9% purity.