Well, in theory we could make the silicon semiconductor industry "clean."

[NNadir]

Here's a cutting edge paper in the current issue of Environmental Science & Technology from scientists in the country where most of our solar cells are made:

Recovery of Fluoride-Rich and Silica-Rich Wastewaters as Valuable Resources: A Resource Capture Ultrafiltration–Bipolar Membrane Electrodialysis-Based Closed-Loop Process, Yangbo Qiu, Long-Fei Ren, Lei Xia, Changmei Zhong, Jiahui Shao, Yan Zhao, and Bart Van der Bruggen Environmental Science & Technology 2022 56 (22), 16221-16229.

From the introductory text, which describes the current practice for these "clean energy" jobs in China:

1. Introduction

Wastewater is increasingly considered a renewable resource linked to the industry–resource–environment nexus. (1) Recovery of valuable resources from wastewater not only contributes to solving an ecological crisis but also gives additional economic benefits. (2) Traditionally, a variety of technologies such as precipitation, adsorption, and electrochemical systems have been developed for the removal of pollutants from wastewater. (3−5) However, the looming concerns of the generation and disposal of secondary pollutants such as waste solids cannot be ignored. (6) Commonly, traditional technologies focus on the removal of typical pollutants from one type of wastewater (e.g., fluoride removal from fluoride-rich wastewater). Few techniques focus on the recovery of valuable ionic resources from various wastewaters. If a variety of wastewaters can be recovered simultaneously, wastewater treatment capacity can be significantly improved.

For the semiconductor industry, large amounts of fluoride-rich and silica-rich wastewaters are discharged. The fluoride-rich wastewater with a high fluoride concentration (e.g., 1–50 g L–1) causes severe crises for human health, (7−9) while the fluoride emission discharge is limited to a median value of 15 mg L–1. (10,11) Silica is a typical waste in silica-rich wastewater. (12) The high concentration of nanosized silica particles (around 2.6 g L–1 SiO2) in wastewater causes environmental issues such as the production of hazardous oxyradicals. (13−15) Traditionally, the treatment methods such as precipitation and coagulation–flocculation are applied to treat the fluoride and silica in semiconductor wastewaters separately. (16,17) However, the waste solid (e.g., calcium fluoride and silica slurry) generation will cause secondary pollution to the environment. On the contrary, capturing fluoride and silica from different wastewaters (e.g., fluoride-rich wastewater and silica-rich wastewater) as well as recovery of the ionic products such as sodium silicofluoride (Na2SiF6) will eliminate pollutants from the environment and bring economic values simultaneously. Na2SiF6 is an odorless and white hexagonal structure crystalline salt, which has been widely used in water fluoridation, as well as the photovoltaic and frosted glass industry. (18) Previous studies have reported that the chemical reaction of hydrofluoric acid (HF) and silica generates silicofluoride (SiO2 + 6HF → SiF62– + 2H2O + 2H+). (19) Moreover, the precipitation reaction of silicofluoride with sodium ions can be used to produce Na2SiF6 (SiF62– + 2Na+ → Na2SiF6↓ . (20) Due to the fluoride ions being transformed into HF at lower pH (pH < 3.16), (21) it is possible to acidify the fluoride-rich wastewater to form HF using a strong acid. Then, the HF in fluoride-rich wastewater can react with SiO2 to generate Na2SiF6 using silica-rich wastewater. Based on these reactions, a corresponding efficient and sustainable separation system could be designed for resource recovery from the fluoride-rich and silica-rich wastewaters.

In view of sustainability and superior separation efficiency, membrane technology is preferential for consideration in a closed-loop process for zero liquid discharge of wastewater. In particular, ultrafiltration (UF) shows a unique advantage in removing macromolecules from wastewater with high water flux and low energy consumption. (22) For example, Liu et al. achieved over 99% rejection rate of calcium fluoride particles by UF during the treatment of fluoride-rich wastewater. (23) Ohanessian et al. carried out a crossflow UF for silica-rich wastewater treatment due to its efficient rejection of silica particles. (14) Therefore, it can be inferred that UF is feasible to recover Na2SiF6 macrocrystals by capturing the fluoride and silica from the mixed fluoride-rich and silica-rich wastewaters. However, the generation of Na2SiF6 requires the use of an acid and a base, thus limiting the large-scale production of Na2SiF6. Bipolar membrane electrodialysis (BMED) can be used to generate H+ and OH– through the dissociation of water molecules in a bipolar membrane (BM). Then, the cation/anion will migrate through cation/anion-exchange membranes (CEMs/AEMs) to form a base/acid, respectively, under an electric field. (24−26) Therefore, BMED could be designed to produce acid and base for the continuous generation of HF in fluoride-rich wastewater. (27)

In this study, a closed-loop process based on resource capture ultrafiltration–bipolar membrane electrodialysis (RCUF-BMED) was designed to recover fluoride and silica as Na2SiF6 from fluoride-rich and silica-rich wastewaters, and the feasibility of the RCUF-BMED system was demonstrated. Two key parts are essential for the RCUF-BMED system: (1) capture of the fluoride and silica to generate Na2SiF6 in an acid environment, and recovery of the Na2SiF6 using a base by the UF system and (2) acid/base generation and freshwater recovery by the BMED system...

The scariest day in my professional career by the way, was when some guys working for me allowed liquid HF to spill all over the hood. (I threw them out of the lab and cleaned it up myself, but it was scary, even fully suited up.)

They recovered 72% of the waste according to the conclusion, stated in "percent talk."

Don't worry. Be happy. It's all "green."

[brush]

1. Can any of these techniques be effective in recovering fracking water?

[NNadir]

2. Fracking water is a whole other game. Cleaning it up will fall to future generations, another...

...way we screwed them while patting ourselves on the back for being "green."

Nanofiltration may help in some ways (the chemistry though is very different) but my preferred option would be high temperature reforming, although this might release some of the carbon fracking didn't get.