Researchers in China have found a profitable way to create elemental sulfur from sulfide-containing wastewater, which could help protect the chemical industry from price fluctuations and shortages of the material. This method could also help reduce the impact of industries that use sulfur-containing chemicals by incentivising the clean-up of wastewater before its released into the environment.
Most sulfur is used to produce sulfuric acid, which industry uses to synthesise phosphoric acid for fertilisers, extract metals from rocks or etch silicon wafers. Chemists also use elemental sulfur as a reactant, for example to upcycle polystyrene waste into valuable organic compounds.
Yet nearly 90% of sulfur is a byproduct of oil and gas refining, tying its price and availability to a volatile market controlled by a few countries, such as those in the Middle East, the US and China. Estimates suggest that an irregular supply of sulfur could lead to a shortfall of sulfuric acid of between 40 and 130% of current production levels by 2040. This situation is likely to worsen as society moves away from fossil-fuel production and towards renewables.
Recovering sulfur from industrial wastewater – which could contain spent sulfuric acid, for example – means that companies can produce sulfur on-site, rather than outsourcing it. However, existing biological methods – converting sulfide to sulfur using bacteria – are often slow, unselective and lead to sulfur that is hydrophilic, making it difficult to separate from water. Chemical-based approaches can be more effective but are often costly and can generate additional waste that is hard to dispose of.
‘We have [instead] developed a new process with ferrihydrite as a catalyst to recover elemental sulfur from wastewater,’ says Hao-Yi Cheng at the Harbin Institute of Technology Shenzhen in China. ‘It’s just three steps.’
Cheng explains that mixing ferrihydrite, a hydrated iron oxide, in de-oxygenated sulfide-laden wastewater produces elemental sulfur and iron sulfides. Aerating the reaction mixture then converts the iron sulfides back into ferrihydrite and additional sulfur.
Once this process has gone through several cycles, Cheng’s team separates the compounds by letting the heavier sulfur particles sediment while the lighter iron compounds float. Currently, the team can generate around 100kg of sulfur per tonne of wastewater per day.
This approach also works with industrial wastewaters that often contain a mixture of ions, organic matter and suspended solids. The team tested their method with wastewater from an electronics manufacturer, as well as caustic waste from a petroleum processing plant that captures hydrogen sulfide exhaust fumes.
‘The process that the authors have developed is quite applicable to waste streams with high hydrogen sulfide concentrations, where biological processes can be somewhat problematic due to the toxicity of hydrogen sulfide,’ says environmental engineer Glen Daigger at the University of Michigan in the US.
Assuming a market price of elemental sulfur of $250 (£185) per tonne – the average price before they rocketed to $1300 following the conflict in the Middle East – Cheng’s team estimates that using this recovery method can return a profit of around $200 per tonne of sulfur. ‘If we consider the sulfur market price this year, [the profit] goes to more than $1000,’ says Cheng.
Using this method to transform sulfides into sulfur decreases the need for additional oxidants to neutralise wastewater before it enters the environment, says Cheng. It also makes sulfur production more circular by decreasing the amount of sulfur lost during chemical processes.
Cheng adds that his team is currently looking to scale up their technology by collaborating with industry. Daigger cautions that ‘there are many questions to answer to scale this process up and convert it into practice’, including reliability, how easy the process is to control and the physical characteristics of the sulfur particles formed. ‘I think the authors have done a good job of initial investigations, but much work remains.’
