Adsorption of gold from waste mobile phones by biochar and activated carbon in gold iodized solution
Introduction
In recent years, waste mobile phones have attracted much attention due to the large quantity of them (Tan et al., 2017), high level of pollution caused by them (Wu et al., 2008, Maragkos et al., 2013), and high resource value contained in this type of waste (MPPI, 2009). Moreover, the content of rare and precious metals in waste printed circuit boards (WPCBs), an important component of mobile phones, is several times that of natural minerals (Deng et al., 2017, Xu et al., 2016), making waste mobile phones a hot topic in the field of resource recycling.
In the past, the treatment of WPCBs has primarily been conducted by private vendors, and high-pollution processes, such as pickling and incineration, have been adopted for the treatment of WPCBs, with little concern for the environmental costs associated with these processes (Xu et al., 2017, Xing et al., 2009, Wong et al., 2007). With the increasing emphasis on environmental protection in China, it is no longer feasible to continue these processes without regard for the level of pollution they generate. Currently, there are few technologies that are practical for industrial production. The discovery of green and efficient resource technologies is a research focus in WPCB recycling.
WPCB resource recovery technologies primarily include hydrometallurgical, bio-hydrometallurgical, pyrometallurgical, and physical–mechanical methods (Li et al., 2007, Lee and Pandey, 2012, Park and Fray, 2009, Lu and Xu, 2016). Among these methods, hydrometallurgy is generally considered to be an efficient, stable, and promising method for the treatment of WPCBs (Sum, 1991, Birloaga et al., 2014, Fleming et al., 2011). The process of hydrometallurgy can be divided into two stages: leaching and recovery. The former uses a lixiviant to oxidize the metal contained in WPCBs into ions, which are then dissolved in a solution. The latter makes the metal ions in the leaching solution become a solid metal that can be processed and utilized.
The techniques of the recovery phase are the focus of this study. Currently, the methods associated with rare and precious metals such as Au, Ag, Pt, and Pd, in WPCBs primarily include adsorption, ion exchange, displacement, electrodeposition, and solvent extraction, among others (Lu and Xu, 2016; Li et al., 2018; Gurung et al., 2013, Zhang and Dreisinger, 2004, Reyes-Cruz et al., 2002). Most treatment methods are expensive, complex, and lead to secondary pollution. The replacement method, although fast and simple, requires complex processes associated with the metal composition of the WPCB leaching liquid due to the large number of compounds needed, the high cost, and the introduction of a large number of new metal ions (Arima et al., 2002). Electrodeposition, which is widely used in many industrial fields, does not require the addition of other metals or chemical reagents, has a high recovery rate, and leads to high product purity. Furthermore, this process is simple, and the recovery cost is low. However, this method has higher requirements for the purity of the solution and is not suitable for recycling of WPCBs that have a complex composition and low metal concentrations (Lekka et al., 2015).
It is critical to find an environmentally and economically feasible method for the treatment of WMPCBs. The WMPCB-treated leaching solution can be regarded as an acidic wastewater that is rich in a variety of metals. The adsorption method is popular for the treatment of wastewater contaminated with toxic metals due to its low cost and ease of operation (Anastopoulos et al., 2017, Bhatnagar and Anastopoulos, 2016). Compared with other adsorption materials (e.g., resin, molecular sieves, silica gel), activated carbon has the advantages of acid and alkali resistance, high temperature resistance, abundant surface functional groups, high mechanical strength, renewable utilization, and stable chemical properties (Bryson, 1995, Khosravi et al., 2017, Vences-Alvarez et al., 2017). With such advantages, activated carbon adsorption has been successfully applied in gold recovery from gold-thiourea and gold-iodide complex solutions (Teirlinck and Petersen, 1996, Zhang et al., 2004). Currently, advances in research on activated carbon have primarily focused on its modification, such as modification of the surface structure properties, chemical characterization, and electrochemical characterization, which are used to enlarge the adsorption capacity and selectivity.
With the advancement of activated carbon adsorption technology, the research focus has shifted to biochar adsorption, which is cheaper and has the option of a more extensive range of raw materials that can be used. Biochar is derived from biomass that has undergone thermochemical conversion in an oxygen-limited environment (IBI, 2012). Kołodynska (Kołodyńska et al., 2017) compared the adsorption of Cu(II), Zn(II), Cd(II), Co(II), and Pb(II) to activated carbon Purolite AC 20 and biochar and demonstrated that biochar could adsorb heavy metals better than activated carbon. The raw materials used to produce biochar are often abundant and cheap, providing an incentive for use as biochar adsorbents. Examples of raw materials that can be used to produce biochar include crop residues and forestry waste, animal manure, food processing waste, paper mill waste, municipal solid waste, and sewage sludge (Ahmad et al., 2013). The recovery of gold from WPCBs is a promising method to convert biomass waste into biochar adsorbent because (i) biochar adsorption technology has the advantages of low cost, capability for repeated use, and simple operations, which can solve existing problems associated with recycling WPCBs. In addition, (ii) waste is used to treat waste, thus achieving a “win–win” situation in terms of environmental protection and a sustainable utilization of resources.
In this study, activated carbon and laboratory-generated biochar are used to treat a solution extracted by iodization and used to adsorb gold. The operating conditions of the activated carbon and biochar adsorption are then compared and optimized.
Section snippets
Materials and apparatus
The waste mobile phone circuit boards (WMPCBs) used in the experiments were provided by the TCL-AOBO Environmental Protection and Development Company. Nitric acid (HNO3) was used during a pre-treatment process to leach the base metals from the WPCBs. Analytical grade potassium hydroxide (KOH) and hydrogen iodide (HI) were used to adjust the solution pH. Other reagents used, including iodine (I2), potassium iodide (KI), and hydrogen peroxide (H2O2), were also of analytical grade, and all the
Gold adsorption on the activated carbon
The adsorption experiments based on the results of the iodine–iodide leaching experiments were conducted under optimized conditions. The leaching solution was filtered and blended to ensure a consistent gold concentration. The activated carbon particle size, activated carbon dosage, solution pH, and reaction time were investigated to optimize adsorption. The influence of the solid–liquid ratio, solution pH, and contact time are discussed in the following sections.
Conclusions
1. The aim of this research was focused on alleviating the problems that exist in technologies for WMPCBs, including their expensive nature, complex processes, and their contribution to secondary pollution, by using waste treatment. In this study, the biomass waste was converted into biochar adsorbent to recover the gold present in the WMPCBs.
2. The adsorption of activated carbon and laboratory-generated biochar with gold in a leaching solution using an iodine–iodide system and the effect of
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was supported by the Tianjin Science and Technology committee [grant number 18YFZCSF00500] and National Key R&D Program in China [grant number 2018YFC1902303]. We thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.
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