Recovery of precious metals from processed wastewater: conventional techniques nexus advanced and pragmatic alternatives

The increase in population coupled with rapid economic growth experienced all over the world due to industrialization has placed an ever-increasing demand on the need for both clean and safe water (Ebenstein 2012). Water is an important natural resource for the sustainability of life, especially potable water (Adeeyo et al. 2020). However, its unavailability in sufficient quantity and quality is a global concern, though more intense in developing countries (Edokpayi et al. 2014). SDG goal 6 anticipates increasing access to clean and safe water for all, globally, in the year 2030. Clean and safe drinking water is essential for the protection of public health, the environment, and the economy of any nation.

Among the wide spectrum of water contaminants, processed wastewater from industrial activities especially in the mining sector has tremendously contributed to environmental pollution and various health-impaired effects in man (Wang et al. 2008; Odiyo et al. 2012). These harmful impacts stem from the significant presence of mined metals in the discharged water. Furthermore, the recent emphasis on the toxicological impacts of processed wastewater resulting from various precious metal mining has raised a global concern (Birungi et al. 2020). Generally, the emphasis and demand for these metals are based on their economic worth and high-valued products (Table 1) that can be realized from them (Chen et al. 2021). For instance, materials such as the platinum group metals and gold are highly sought in dental, electronics, automobiles, and the jewellery industries (Takahashi et al. 2007; Abisheva et al. 2011; Umeda et al. 2011). These metals (gold, silver, platinum, palladium, iridium, ruthenium, and rhodium) are often called rare earth metals, having premium economic and industrial demands yet scarce and unevenly distributed in the earth's crust (He & Kappler 2017). Furthermore, this group of metals is ranked as one of the most critical elements with a diminishing supply due to the cost of mining and appreciable loss through processed wastewater, leading to deleterious environmental impacts on discharge (Birungi et al. 2020).

The reported losses are basically associated with mining, extraction, purification, and processing (Mannina et al. 2020). The disposal of waste or processed water from the aforementioned processes and metallurgical activities, therefore, constitutes the major source for the discharge of valuable metals into the environment (Baysal et al. 2013). Large volumes of processed wastewater with substantial amounts of high-value metals have been reportedly discharged indiscriminately into the environment (Khaliq et al. 2014; Kaya 2016). These processed wastewaters cause contamination of different environmental media (Bambas-Nolen et al. 2018). Hence, valued metals, in themselves, have become pollutants to the environment even as demands for such fast-growing economies are very high. The use of water in industrial mining covers processes from extraction to the transportation of minerals with a subsequent release of significant amounts of the metals as waste to the environment (Van Berkel 2007; García et al. 2014). Soil, surface water, and groundwater have been highly polluted by various mining activities due to tailing leaks and disposal into water systems (Akcil & Koldas 2006; Gunson et al. 2012; Li et al. 2014). The release of precious metals as solid or liquid waste unrecovered is accompanied by huge environmental impacts, but not economical, and poses financial and strong penalties burdens on industries (Ravindra et al. 2004; Wei et al. 2016). In addition, the mining and processing of precious and heavy metals have subjected human health and the ecosystem to unwarranted risks (Eisler 2004; Tabari et al. 2008). Some critical health conditions, above the normal safety concentrations, have been attributed to the accumulation of metals in human bodies. Usually, they accumulate in food chains due to their solubility in aqueous media and their ability to be absorbed by microorganisms (Tabari et al. 2008).

To meet up with the growing demand and to minimize the loss during the processing of precious/rare earth metals, it necessitates the need for alternative technologies associated with pragmatic recovery approaches. The pragmatic approach describes the practical emerging technology such as bio-recovery, mechanochemical technology, ionic liquid technology, and nano-biotechnology (Ramachandran et al. 2016). The main criteria for the evaluation of these alternative technologies are their significant recovery performance and the capacity to annul secondary pollutants, which is a major drawback in conventional techniques (Shin et al. 2015; Awasthi et al. 2016; Palomo-Briones et al. 2016). The recovery of precious metals is, therefore, of great interest vis-a-vis existing recovery technologies for the sequestration of precious metals from processed wastewater. Some reported techniques used in the recovery of valuable metals include chelation, ion exchange, chemical precipitation, solvent extraction leaching, adsorption, and biosorption methods (Golunski et al. 2002; Schreier & Edtmaier 2003; Da¸browski et al. 2004; Wang et al. 2007; Birinci et al. 2009; Nikoloski & Ang 2014; Firmansyah et al. 2018; Lopes Colpani et al. 2019). This documentation enumerates cogent toxicological reports, detection and analytical techniques, and different processes of recovery methods for metals, particularly, precious metals. Conventional and recent alternative approaches for the sequestration of valuable metals from processed wastewater are discussed. A brief analysis of previous studies focused on precious metal recoveries is reported with recent limitations. Implication for theory and practice is highlighted.

Metals that have percolated into the environment and human system have posed a great threat in recent times. Valuable metals have been reported to have toxic effects on aquatic organisms, particularly, when they reach the aqueous environment in their organic forms (Shimada et al. 2010). Their volatility and ability to be transported through long ranges have equally increased their presence in the environment (Laliberte 2015). Precious metals generally pose harmful effects on plants and animals at a concentration above their toxicity limits, whereas there are a few that can greatly disturb the balance of the ecosystem and the human body even at very low concentrations. The presence of valuable metals unabated in the environment can result in serious health challenges ranging from mild to chronic ones (Abdul-Wahab & Marikar 2012; Laliberte 2015). A review of different literature has established various toxicological effects of Au, Ag, and the platinum group metals including bone marrow suppression and necrosis, cranial neuropathy, nephrotoxicity, cancer, haemorrhage pulmonary injury, blood dyscrasias among others (García et al. 2014). Concerns have been raised on the toxicological impact of untreated metal-laden effluents and the need for an effective recovery procedure was proposed as a very critical topic (Benavente et al. 2011; Mondal & Sharma 2016).

Analytical techniques such as Atomic Adsorption Spectrometry (AAS), Anodic Stripping Voltammetry (ASV), Laser-Induced Breakdown Spectroscopy (LIBS), Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) can be employed in detecting various valuable metals. In the AAS technique, different atomization of samples gives rise to different procedures for analyzing elements. For example, in Flame Atomic Adsorption Spectrometry (FAAS), air or nitrous oxide/acetylene flame can be used with respect to the thermal stability of the metal for analysis (Komendova 2020). Metals such as zinc, tin, silver, rhodium, platinum, nickel, lead, gold, cobalt, calcium, and antimony among others, can be determined using air/acetylene flame while elements like vanadium, silicon, rhenium, barium, and aluminium among others, can be determined using nitrous oxide/ acetylene flame (Rice et al. 2012). FAAS is limited as certain elements require higher temperatures to atomize than is obtainable for this technique.

Another technique used in the detection of precious metals is ICP-OES which uses plasma for the excitation of atoms. Atomization temperature can reach up to 10,000 °C, which gives this technique an advantage over AAS (Zeeshan & Shehzadi 2019). Metals can be determined even at low concentrations in this technique and several elements can be analyzed simultaneously in a few seconds. Other recent techniques that have been employed include LIBS and ASV, ICP-MS which are characterized by better detection limits ranging in part per trillion (ppt) (Nebeker & Hiskey 2012). The major limitation attached to these techniques is the cost associated while running them.

There are several traditional and emerging approaches employed to recover commonly used precious metals such as gold, silver, platinum, palladium, and indium. These traditional approaches are categorized as physical or mechanical, pyrometallurgical, and hydrometallurgical (Zhao et al. 2004; Parajuli et al. 2006). Advanced and more recent precious metal recovery processes include mechanochemical technologies, solvato-metallurgy (use of ionic liquid), and electrochemical technologies from which bio-electrochemical technology stems and biotechnology or bio-recovery (Binnemans & Jones 2017). A critical and extensive study on the recovery of precious metals from processed wastewater will proffer insights and solutions that will remove contaminants and recover valuable products from processed wastewater (Zhang et al. 2010; Benit & Roslin 2015).

This separation and recovery technology is described as being foundational to other conventional techniques (Yoo et al. 2009). It involves the mechanical breaking and crushing of precious metals in the processed wastewater followed by separation (Table 2). This process follows three basic steps, which are crushing, corona electrostatic separation, and metallic material recovery as described by Huang et al. (2009) and Zeng et al. (2014). An appreciable benefit of the process is reduced secondary pollution. However, this procedure is not sufficient to facilitate the recovery of a significant amount of metal scraps and tailings in effluents. Hence, the process is only used as a pre-treatment process while other recovery processes are incorporated (Chancerel et al. 2009; Marra et al. 2018).

Hydrometallurgy possesses advantages such as reduced power consumption and lower cost of pre-treatment. Hydrometallurgy systems are relatively small in capacities coupled with a controlled recovery process when compared to pyrometallurgy (Ranjbar et al. 2014). Hydrometallurgy is concerned with the use of aqueous solutions known as leaching reagents (cyanide, thiourea, thiosulphate, acids) to isolate, purify, and recover precious metals from its attendant effluents (Tuncuk et al. 2012). There is a wide spectrum of leaching reagents used for this process as described in Table 3. Table 4 describes the chemistry involved vis-a-vis the leaching solvent selected for the leaching process. The choice of solvent used by these techniques categorizes the method into either green or conventional (Kumari et al. 2015). Cyanide leaching may be used for gold recovery; however, cyanide is a very toxic compound that requires intensive care and treatment during usage and disposal (Konyratbekova et al. 2015). In this process, methods such as adsorption using activated carbon, cementation, solvent extraction, and ion exchange may be used as recovery technology.

Table 4

Chemistry of leaching in pyrometallurgy

S/N .	Type of leaching .	Leaching reagent(s) .	Leaching reaction .
1.	Acid leaching	Nitric Acid (HNO3)
		Aqua Regia (HNO3:HCl)
		H2SO4: H2O2
2	Cyanide leaching	NaCN
3	Thiourea leaching	CS(NH2)2
4	Thiosulphate
5.	Halide leaching	Bromine, chlorine, iodine
6	Organosulphur leaching	Organosulphur compounds

S/N .	Type of leaching .	Leaching reagent(s) .	Leaching reaction .
1.	Acid leaching	Nitric Acid (HNO3)
		Aqua Regia (HNO3:HCl)
		H2SO4: H2O2
2	Cyanide leaching	NaCN
3	Thiourea leaching	CS(NH2)2
4	Thiosulphate
5.	Halide leaching	Bromine, chlorine, iodine
6	Organosulphur leaching	Organosulphur compounds

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Despite the industrial application of the solvent extractions, the toxicological impacts of these solvents, cost, and lack of adequate recovery of precious metal tailings from the processed wastewater of mining have led to search of other improved technologies.

Also, the optimization of the performance and kinetics of the process depends on parameters such as the mass transfer rate, the film thickness, the reaction between counterions and fixed groups, and sorption conditions.

There are justifications and a quest for highly selective and efficient emerging processes to effectively clean up and recover precious metals from processed wastewater other than the known traditional methods. These emerging processes include mechanochemical technologies, the use of ionic liquid (Solvato-metallurgy) (Binnemans & Jones 2017), and others as discussed in the following.

Although, the challenges in the use of the bacterial strains are the limiting factor of the cyanide lixiviant and the difficulty in getting CV due to its growth in stringent living conditions (Asere et al. 2019); however, recent studies have pointed out the use of a new mode of bacterial strain for the leaching of these precious metals, one of which is Pseudomonas chlororaphis (PC), which produces CN⁻ for the leaching process (Mincke et al. 2019). This new bacterial strain is easily obtainable and can be employed for a continuous industrial system. However, C. violaceum (CV), still has higher strength for the generation of CN⁻ when compared with the P. chlororaphis (PC). This justifies the low recovery yield of 8.2, 12.1, 52.3% for Au, Ag, and Cu, respectively (Nebeker & Hiskey 2012).

Other examples involve the sulphate-reducing bacteria reduction of platinum(II) to platinum (Riddin et al. 2009; Homchuen et al. 2016). These bacteria include Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans, and Leptospirillum ferrooxidans. Among the fungi microbial class, a notable microorganisms used in the bio-recovery of precious metals are Penicillium sp. and Aspergillus niger. The use of fungi is an example of eukaryotes engaged in leaching technology for recovering metals of concern from their wastes (Ha et al. 2010). Soluble precious metals can be recovered with biosorption and reduction (Colica et al. 2012; Maes et al. 2016). Trichoderma harzianum was reported for the efficient removal of silver from metal-polluted waste-rock tips or processed wastewater (Cecchi et al. 2017). In addition to this, Stenotrophomonas sp. a magnetotactic microbe removes gold(III) from waste effluents to reduce the gold(III) to gold and deposit nanocrystals particles of gold on the surface of the cell (Song et al. 2008).

Hydrogen gas is released from the cathode, while the generation of the coagulants metal ions happens at the anode (Azimi et al. 2017). Generally, the merits of electrochemical technology include proper controlled process and speed, reduced use of chemicals, improved selectivity, and absence of sludge. The process involves high starting and operational costs since it involves appreciable energy supply and facility demand (Gunatilake 2015).

Mechanochemical technology involves a chemical reaction that is initiated via mechanical energy in the extraction or recovery of metals from processed wastewater. This technology is carried out by integrating milling and leaching operations into a single step (Nasser & Mingelgrin 2012; Mengmeng et al. 2017). In a broader sense, the study of physicochemical transformation brought about by mechanical and chemical action is known as mechano-chemistry (ball milling). Mechanical forces exert energy on metals during ball milling, which turns the metals separated from processed wastewater (by physical/ mechanical method) into powder of appreciable surface area and reduced size. The surface area of these reduced metal particles increases to facilitate reaction efficiency (Nasser & Mingelgrin 2012). The distinct difference existing between this technology and thermochemical technology is that mechanochemical technology uses mechanical energy and not thermal energy. Hence, the reaction proceeds without harsh conditions such as increased pressure and temperature (James et al. 2013). Baláž (2008) categorized this emerging technology into dry and wet milling. In dry milling, the two processes of milling and leaching are separated distinctly while in wet milling, the processes are performed concurrently together. The recovery of indium (Id) which has its renowned use in indium-tin-oxide (ITO) thin films, mainly in liquid crystal display (LCD) has been investigated based on its increasing scarcity and demand (Hasegawa et al. 2018). Investigation on the recovery of this precious metal via mechanochemical technology was carried out by Janiszewska et al. (2019), who recovered indium by grinding the representative sample of indium oxide with tin doping. After including alumina powder (α-⁠), the result of the leaching yield exceeded 80%.

The use of sorbents in the adsorption of metals from processed wastewater is a commonly used process due to its clean and fast operation, high productivity, simplicity, and reduced cost-effectiveness. Accessibility and new research into diverse adsorbents has resulted in the high-quality treatment of effluent (Adeeyo et al. 2019) with sorbents of various types.

Different adsorption procedures have been used over the years for various adsorption purpose with such adsorbent originating from mineral, organic, or biological material. Common adsorbents are activated carbon, zeolite, clay, and silica beads (Ghoul et al. 2003). However, the usual economic origin of adsorbents is commonly industrial by-products or agricultural wastes (Netpradit et al. 2003). Also, Song et al. (2008) reported that adsorbents from polymer and biomass are equally used in the sequestration of metals. For instance, Chand et al. (2009) reported the use of barley straw and rice husk at optimized conditions as adsorbents in precious metal recovery. Also, the efficiency of activated carbon as sorbent in the extraction of platinum, gold, and palladium has been studied by Mpinga et al. (2014) and Quinet et al. (2005) with a report of favourable adsorption capacity, adsorption rate as well as its abrasion resistance. The frequent study of activated carbon as an adsorbent in the recovery process is due to the appreciable properties of extended surface area, porous structures, high adsorption capacity as well as an appropriate functional group that helps its surface reactivity (Chand et al. 2009). Mosai et al. (2019) reported the recovery of platinum from an aqueous solution using hydrazine-functionalized zeolite. However, a bentonite clay adsorbent was studied by Mosai et al. (2019) with a record of high recovery potential for numerous metal ions. Polysaccharides including chitin, starch, cyclodextrin, and chitosan have also been explored as adsorbents and they are cheap and effective (Varma et al. 2004). The structure, physicochemical characteristics, chemical stability, high reactivity, and selectivity of polysaccharides give high preference towards aromatic compounds and metals (Crini 2005). Chitosan is greatly used as an adsorbent and its main advantage in the adsorption of precious metals is due to its amino acids that are easily protonated in acid media. Other effective sorbents are tannins which are commonly used for redox capabilities (Ma et al. 2006).

A magnetite adsorbent was also used for recovering gold, platinum, and palladium (Tsyganova et al. 2013). However, separating magnetic adsorbent should be considerably checked at the post-precious metal recovery phase (Kraus et al. 2009). While numerous materials are described as capable of recovery of precious metals from processed wastewater, conventional methods using these materials usually results in operational issue, especially when using sorbents of the nanometer size range (Donia et al. 2007). A summary of selected sorbents investigated for precious metal recoveries is presented in Table 6.

The emphasis of nanosorbents over conventional sorbents is due to appreciable features such as availability of more active sites and ability to form composite with improved morphological attributes, adsorption capacity, durability, reusability, surface area, crystallite size and distribution, dispersibility, as well as mechanical and thermal stability (Oyetade et al. 2022). Predominantly, higher surface-to-volume ratio improves the reactivity of nanomaterials with environmental contaminants (Mincke et al. 2019). Other advantages of nano-sorbents include short intra-particle diffusion distance, tunable surface properties and easy reuse (Rickerby & Morrison 2007). Carbon nanotubes, zeolites, and dendrimers have been reported in a few studies for exceptional adsorption properties (Rickerby & Morrison 2007; Theron et al. 2008).

Metallic oxide nanomaterials of titanium, zinc, manganese, and iron have been applied for the adsorption of metals in processed wastewater (Amin et al. 2014). Unique characteristics of high surface area, reactivity and strong sorption make them suitable adsorbents (Savage & Diallo 2005; Kamali et al. 2019). TiO₂ nanoparticle was reported for adsorption in metal recovery (Kim et al. 2020). Metal-based nanomaterials were better than activated carbon. Iron-oxide nanomaterials are of low cost for recovering metals from solution (Salipira et al. 2007). Carbon-based nanomaterials have been used for the treatment of processed wastewater due to ease of chemical or physical modification and broad affinity for inorganic and organic pollutants (Mayo et al. 2007). Application of magnetic material as adsorbent (Kraus et al. 2009) in the recovery of platinum (Oliveira et al. 2004), iron-carbon composite in the recovery of gold (Banimahd Keivani et al. 2010) and modified chitosan magnetite resin in the recovery of silver (Homchuen et al. 2016) have been documented. Silica-based nanomaterials are used in the removal of metals because of their non-toxicity and excellent surface characteristics (Tewari et al. 2005). Many advantages of magnesium oxide nanoparticles as adsorbents for metals are listed in literature and include a high adsorption strength, low cost, non-toxicity, availability, as well as an environmentally friendly character (Dresselhaus & Terrones 2013). Biosorption is a promising technology because it is inexpensive and can use inactive or dead microbial cells which are available in large quantities. Biomass properties affect the binding and concentration of metal ions (Bessong et al. 2009) and the mechanism of sorption for the metal uptake depends on the biomass type (Mahmoud et al. 2016). Also, using biomass is advantageous in that it can be exposed to an environment of high toxicity and bio-augmentation is not required for the process (Maes et al. 2016). Biosorption can also be used in situ and requires little or no industrial operation when properly designed (Janiszewska et al. 2019). Biosorbents including fungi, plant biomass, algae, bacteria, and yeast have been used in the recovery of gold (Chand et al. 2009), silver (Homchuen et al. 2016), and platinum group of metals (Banimahd Keivani et al. 2010).

Traditional techniques have been broadly applied for the recovery of gold, silver, platinum, palladium, iridium, ruthenium, and rhodium; palladium (28.3%), gold (26.7%), silver (20.0%), and platinum (16.77%) were the most recovered under these processes. With regards to the more advanced and recent techniques, gold metal has been mostly recovered (46.2%), followed by palladium (23.1%), platinum (19.2%), and silver (11.5%). The recovery of ruthenium, iridium, and rhodium are poorly reported and there is still a research gap within the recovery of precious metals using the more recent advanced techniques (Figure 5).

Methods such as hydrometallurgical processes, ion exchange, solvent extraction (Cai et al. 2017), as well as pyrometallurgical methods have been used in precious metal recovery. The methods when engaged in industrial cycling, are expensive, time-consuming, and generate large waste. Some of the metal recovery methods are less efficient in the recovery of precious metals, toxic, and cause secondary pollution (Jacobsen 2005; Cai et al. 2017). Though recent techniques for activated carbon preparation have made it cheap, it is still relatively costly, requiring high temperatures and entails special kilns usage to reactivate the carbon (Colica et al. 2012). When using magnetic particles, the disadvantages include little selectivity towards the metal ions of choice in complex matrices and instability of the metal particles in strongly acidic solutions (Jacobsen 2005). In the Merrill Crowe method, the pregnant solution needs clarification and deoxygenation based on free cyanide concentration and pH. The use of ion-exchange resins is also costly and has a low loading capacity and the resin must be regenerated in an acid medium. The use of solvent extraction in this field of study is also comparatively costly when compared to other procedures (Anbia & Mehrizi 2016). These limitations are part of what research must further focus on regarding precious metal recovery from processed wastewater.

The practical implication of bio- and nano-sorbents as envisaged in this study would be in cost reduction, selectivity and improved yield, environmental friendliness and application in very toxic waste. Biosorption has been reported as a cost-effective recovery process, leading to a lesser cost for the product (Das 2010; Rana et al. 2020). The structure, selectivity and high reactivity of the procedure may be adapted for specific and optimal metal recovery which also implies an improved yield of the process (Crini 2005; Ma et al. 2006; Adeeyo et al. 2019). When considering nano-sorbents, easy re-use may result in long-term cost reduction (Rickerby & Morrison 2007). Nanosorbents synthesized through the green routes come with the merits of eco-friendliness and less toxicity following sustainable development goals. These procedures also have the merits of the possibility of being applied in highly toxic waste for the recovery of useful materials where other processes may fail. When used in situ and properly designed, these techniques require little or no industrial operations. The development of facile, reliable and green chemistry procedures for nanosorbent production is recommended for future studies as the production of some of these nanosorbents materials requires the use of toxic and highly reactive reducing agents which in turn elicit undesirable effect on the environment. The use of treatment train strategy is encouraged for future studies as no single recovery technique can be efficient for complete recovery of valuable metals from processed wastewaters.

This study has put forward recovery procedures implying cost reduction, improved yield and eco-friendliness which are basic factors in appraising the effectiveness of different procedures in industrial applications. The performance of various conventional treatments was analytically compared with more recent, advanced, and pragmatic alternatives. The study identified the simplicity of most conventional techniques. However, they are associated with high-water sludge content and generation of secondary pollutants coupled with its low selectivity for highly valued metals in the presence of competing ions, and high operational cost. Although has an advantage of simplicity, especially, the identified limitations of the conventional techniques are the predominant strength of the pragmatic approach for the recovery of these metals. The flexibility, cost-effectiveness and selectivity of the pragmatic approach (especially the use of bio- and nano-materials) account for the high recovery performance for metals such as gold, platinum and palladium as compared to the traditional technologies. These appreciable advantages serve the SDG goals such as: no poverty (1), good health and well-being (3), clean water (6), affordable energy (7), climate action (13), life below water (14), life on land (15) which will consequently lead to decent work and economic growth (8) and sustainable cities and community (11), and the recovery factor ratio.

A.O.A., J.O.O., O.S.B., J.N.E., J.A.O., and R.M. conceptualized the study, R.O.A., A.O.A., and O.S.A. worked on the methodology, software and formal analysis, and data curation, O.S.B., J.N.E., R.M., and J.O.O. supplied resources, R.O.A., A.O.A., O.S.A., and O.S.B. developed the original draft, A.O.A., O.S.B., O.S.A., R.O.A., J.A.O., J.O.O., R.M., and J.N.E. reviewed and edited the draft, J.N.E., O.S.B., R.M., and J.O.O. supervised, R.M. and J.N.E. are involved in project administration and funding acquisition. All authors have read and agreed to the published version of the manuscript. Please turn to the CRediT taxonomy for the term explanation. Authorship must be limited to those who have contributed substantially to the work reported.

A.O.A., R.M., J.N.E., and O.S.B. acknowledge the contributions of Water Research Commission (WRC), National Research Foundation (NRF). and Third World Academy of Science (TWAS).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.