Treatment of a platinum leachate by membrane distillation: Mechanism of combined silica scaling and organic fouling for distinct system performance decline
Graphical abstract
Introduction
Platinum belongs to the platinum group metals (PGMs) including platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), iridium (Ir) and osmium (Os), which are wildly applied in various industrial fields in products such as catalysts, electronic devices, space materials, and biomedical device due to their distinct physicochemical properties (Ding et al., 2019; Dong et al., 2015; Hong et al., 2019; Komendova, 2019; Maes et al., 2016; Malekian et al., 2019; Truong et al., 2018; Yuan et al., 2020). Among the PGMs, Pt together with Pd and Rh are the key active components of auto catalytic converters (Kim et al., 2016; Reddy et al., 2010; Saguru et al., 2018; Trinh et al., 2019; Wei et al., 2019). Moreover, Pt metal and PtO2 are also widely used as catalysts in a variety of reactions in the chemical and petroleum fields (Chiang et al., 2011; Greggio et al., 2008; Sun and Lee, 2015). Statistical report showed that about 57 % of Pt (95 tons) was used in automotive catalyst in 2018 (Komendova, 2019). The demand of Pt is increasing worldwide due to its versatile applications in various industrial fields. However, the natural reserves of Pt limited and distributed and are mainly found in South Africa, Russia, Canada and North America (Rao and Reddi, 2000; Zhang et al., 2017). Additionally, the concentration of Pt is extremely low, ranging from 2 to 10 ppm (g/t), and generally associated with base metal sulfide minerals (Jha et al., 2013; Reith et al., 2014). On the contrary, considerable amounts of spent catalysts containing Pt and other precious metals derived from the automotive, chemical and petroleum industries are produced. More importantly, the concentration of Pt in the spent catalysts is about a thousand times than that in the general Pt ores (Tyson and Bautista, 1987). Given the cost of Pt production from limited natural ores is high, it is significant to reclaim it from secondary resources for sustainable development.
Hydrometallurgical method with several advantages, such as low capital cost, low temperature condition and high metal recovery, was widely employed in recovery of precious metals from secondary sources (Cui and Zhang, 2008; Ghosh et al., 2015). In the hydrometallurgical process, Pt in the solid waste was leached out in the form of its soluble complexes (e.g. [PtCl6]2−). The common reagent used for Pt leaching is hydrochloric acid (HCl) medium combined with various oxidizing agents (Lee et al., 2010; Marinho et al., 2010, 2011; Nguyen et al., 2016; Nikoloski et al., 2015; Paiva et al., 2017; Trinh et al., 2019; Truong et al., 2018). It was reported that over 95 % of Pt was leached out in the form of [PtCl6]2− in HCl solution system (Kim et al., 2016). For recovery of Pt from spent catalysts, because cordierite as substrate is often used in catalysts production (Wei et al., 2019), Pt usually coexist with heavy metals such as aluminum (Al), iron (Fe), magnesium (Mg), cerium (Ce) and zirconium (Zr) as well as nonmetallic silicon (Si). Therefore, the platinum leachate (PL) is always highly acidic and complex, including organic oxidizing agents, heavy metal ions and silica, which should be properly treated after extraction of Pt before being discharged into aqueous environment.
Compared with traditional leachate such as landfill leachate with high concentration of chemical oxygen demand, nitrogen and phosphorus (Luo et al., 2020), the common biological treatment methods based on active sludge are not suitable for treatment of PL with strong acidity and high salinity (Table 1). Membrane distillation (MD) as one of the advanced membrane technologies has been widely used in treatment of industrial waste streams (Cai and Guo, 2020; Criscuoli and Figoli, 2019; Du et al., 2018; Guo et al., 2020; Li et al., 2019, 2018; Noor et al., 2019; Tun et al., 2005; Zhang et al., 2018). Not only high-quality water with almost 100 % rejection of metal ions can be achieved, but also the volume of waste streams can be reduced to achieve near zero waste stream discharge. In our previous studies, direct contact membrane distillation (DCMD) as one type of MD technologies has been adopted to treat high-salinity waste streams with different characteristics from recovery of precious metals (Chen et al., 2020a, b). Although high quality water and hydrochloric acid were reclaimed, silica scaling was still present in DCMD process, resulting in significant system performance decline. However, it is noteworthy that the interaction between silica and organic compounds such as proteins (bovine serum albumin and lysozyme), alginic acid and aromatic compounds in membrane process often occurred, resulting in water flux decline in membrane process (Higgin et al., 2010; Quay et al., 2018). On the other hand, because the kind and concentration of organic compounds besides silica in leaching solution are different for recovery of various metals, various degree of combined silica scaling and organic fouling may occur in DCMD process. Therefore, it is necessary to evaluate the degree of the combined silica scaling and organic fouling during MD process and reveal the mechanism of their respective contribution to system performance decline.
In this work, the aim is to explore DCMD process to treat a high acidic PL to reclaim water and reduce volume of the waste stream. DCMD performance was evaluated in detail during treatment of PL after pretreatment under various initial pH. More importantly, mechanisms of DCMD performance decline in different degrees induced by combined silica scaling and organic fouling was clarified through scaled membrane autopsy by field-emission scanning electron microscopy and X-ray photoelectron spectroscopy as well as the identification of silica oligomers and molecular weight distribution of organic compounds by mass spectroscopy.
Section snippets
Source and characteristics of platinum leachate
The PL derived from recovery of Pt in spent catalysts was kindly provided by Sino-Platinum metals resources (Yimen) Co., Ltd. Water samples were kept in the 25 L plastic bucket and stored a laboratory at room temperature. The main water quality of the waste stream was listed in Table 1.
DCMD test system
The lab-scale DCMD setup assembled for filtration of PL from Pt recovery is illustrated in Fig. 1. Briefly, a transparent module made of Perspex with a channel depth of 0.4 cm, length of 10 cm and width of 30 cm
Silica and metal ions removal
As shown in Fig. 2 A, the concentration of the total dissolved silica slightly declined from 15.81 mg/L to 11.08 mg/L as initial pH of PL increased from 1 to 3, then sharply dropped to 0.39 mg/L and 0.13 mg/L as initial pH of PL increased to 5 and 7, respectively. The change trend of Al3+ concentration of was in line with that of silica. The concentration of Al3+ slightly decreased from original 11,938 mg/L to 11,227 mg/L as initial pH of PL increased from 1 to 3, then rapidly decreased to 5
Conclusion
In this work, DCMD process was selected to treat a real PL from Pt recovery. The relation between silica removal and initial pH of PL was investigated. Moreover, influence of PL as feed with various initial pH on DCMD performance was systematically evaluated. The main conclusions drawn from this work are as following:
- f06c;
The concentration of the total dissolved silica in PL was effectively reduced through adsorption and co-precipitation with the in-situ formed Al(OH)3. The highest removal rate 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.
Acknowledgements
The authors would like to thank the partial financial support from Natural Science Foundation of Shanghai (No.20ZR1400100), Fundamental Research Funds for the Central Universities (No.2232018D3-09), National Natural Science Foundation of China (No.21507142) and National Key Research Development Program of China (No.2019YFC0408304).
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