Chapter 10

The modern inorganic fluorochemical industry

Fluorotelomer carboxylic acids (FTCAs) represent an important group of per- and polyfluoroalkyl substances (PFAS) given their high toxicity, bioaccumulation potential, and frequent detection in landfill leachates and PFAS-impacted sites. In this study, we assessed the biodegradability of 6:2 FTCA and 5:3 FTCA by activated sludges from four municipal wastewater treatment plants (WWTPs) in the New York Metropolitan area. Coupling with 6:2 FTCA removal, significant fluoride release (0.56∼1.83 F-/molecule) was evident in sludge treatments during 7 days of incubation. Less-fluorinated transformation products (TPs) were formed, including 6:2 fluorotelomer unsaturated carboxylic acid (6:2 FTUCA), perfluorohexanoic acid (PFHxA), perfluoropentanoic acid (PFPeA), and perfluorobutanoic acid (PFBA). In contrast, little fluoride (0.01∼0.09 F-/molecule) was detected in 5:3 FTCA-dosed microcosms, though 25∼68% of initially dosed 5:3 FTCA was biologically removed. This implies the dominance of “non-fluoride-releasing pathways” that may contribute to the formation of CoA adducts or other conjugates over 5:3 FTCA biotransformation. The discovery of defluorinated 5:3 FTUCA revealed the possibility of microbial attacks of the C-F bond at the γ carbon to initiate the transformation. Microbial community analysis revealed the possible involvement of 9 genera, such as Hyphomicrobium and Dechloromonas, in aerobic FTCA biotransformation. This study unraveled that biotransformation pathways of 6:2 and 5:3 FTCAs can be divergent, resulting in biodefluorination at distinctive degrees. Further research is underscored to uncover the nontarget TPs and investigate the involved biotransformation and biodefluorination mechanisms and molecular basis.

In-situ remediating phosphogypsum (PG) for cemented paste backfill (CPB) in the contaminated site is economic management for promoting sustainable developments in the phosphate industry. This study concerns the combined use of NaOH pretreatment and ground-granulated blast furnace slag (GGBFS) additives to promote the solidification/stabilization of PG with a lower carbon footprint pathway. According to physico-chemical analyses, the NaOH pretreatment effectively removed approximately 95% of F within the PG, which may originally be present as sparingly soluble fluorides or coexisting with silicates. The micro mineralogical characterization illustrates that the pretreatment can accelerate the early age hydration, with more hydration products observed, including calcium silicate hydrates and ettringite, effective F and P retention candidates. Whereas the incorporation of GGBFS plays an essential role in promoting the generation of additional cement hydrates at the following stages. The macro mechanical performance analysis indicates that the mixtures of pretreated-PG-OPC-GGBFS exhibit an excellent mechanical performance satisfying the design criteria. Subsequent elemental mapping and toxicity characteristic leaching procedures demonstrate that this combined approach has a competitive F and P immobilization ability compared to the typical OPC binder and individual GGBFS addition. The newly formed phases effectively controlled the concentration of F and P through adsorption, incorporation, or encapsulation. Objectively, the proposed methodology can be a promising candidate pathway for extrapolating the in-situ immobilization of PG. This study opens up new perspectives for synergetically recycling PG and GGBFS in a profitable and low carbon footprint way.

Lithium (Li), as a new energy metal, is becoming a “hot” topic in both academia and industry due to the rapid vehicle electrification and grid storage. Although brines have been the major Li sources, Li-bearing minerals, owing to the wider distribution and more rapid pathway to market, have also attracted much attention in recent years, with a number of new industrial projects launched and various novel methods proposed. The present study provides a start-of-the-art review of Li recovery from different mineral resources (i.e. excluding brines), and gives perspectives and outlook towards various recovery methods. In this study, the major mineral deposits of Li are summarised and illustrated, which shows its high abundance and wide distribution around the global. Various methods of Li recovery reported so far are then summarised with flowsheets and discussed by different type of minerals, covering spodumene, lepidolite, zinnwaldite, amblygonite and clays. It is predicted that spodumene will continue being dominantly used as Li source over other minerals with sulfuric acid (H₂SO₄) roasting as a major method of processing. However, other novel methods including direct processing of natural spodumene and the process that favours the direct production of LiOH will be the trends of future research. Fluoride-based methods can achieve low energy consumption and high extraction efficiency but still need to be further investigated for a sustainable, economical and safe application. To compete with spodumene, the comprehensive utilization of all the valuable elements contained in lepidolite and zinnwaldite is crucial. In most of the recovery processes, more attention should be paid to the treatment of voluminous residue and waste for a safe disposal or further reuse. In addition, this study not only presents the methods for Li recovery, but also includes various downstream separation and purification steps to make the process integrated. It is expected that, as a review specialising in mineral resources of Li, the present study can provide insights for the development of this particular area.

The structure of AgBF₄ has been determined from single-crystal X-ray diffraction data collected at 200 K and refined to a conventional $R=0.0259$ ($wR=0.0276$) from 338 independent reflections (35 parameters). This compound crystallizes in the orthorhombic system, space group Pnma (No. 62) with $a=8.089(8)\text{Å}$, $b=5.312(6)\text{Å}$, $c=6.752(9)\text{Å}$, $V=290.1(10){\text{Å}}^3$, and $Z=4$. The Ag⁺ ion is coordinated by ten F atoms at 2.561(4)–2.950(2) Å distances. The metal polyhedron shares edges with three and apexes with four BF₄ tetrahedral units. [BF₄]⁻ anions deviate from ideal tetrahedral symmetry. The reduced symmetry can be seen in Raman spectrum, where splitting of the anion vibrational modes can be observed. Synthesis of the XeF₂ complex salt of AgBF₄ at ambient temperature failed.