Upcycling sulfide-laden wastewater offers way to shelter chemical industries from sulfur supply crisis

Most sulfur is applied to create sulfuric acid, which sector uses to synthesise phosphoric acid to fertilisers, extract metals from rocks or etch silicon wafers. Chemists also consumption elemental sulfur as a reactant, to instance to upcycle polystyrene discarded materials into valuable organic compounds.

Yet nearly 90% of sulfur is a byproduct of oil and gaseous refining, tying its price and availability to a evaporative 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 manufacturing effluent – which could contain spent sulfuric acid, to instance – means that companies can create 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 aquatic environments. Chemical-based approaches can be greater efficiently however are often costly and can generate additional discarded materials that is hard to manage 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 methodology 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 interaction 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 manufacturing 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 discarded materials from a petroleum processing plant that captures hydrogen sulfide exhaust fumes.

‘The process that the authors have developed is quite applicable to discarded materials streams with high hydrogen sulfide concentrations, where biological processes can be somewhat problematic due to the harmfulness 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 greater than $1000,’ says Cheng.

Using this method to convert sulfides into sulfur decreases the need to additional oxidants to neutralise wastewater before it enters the ecological stability, says Cheng. It also makes sulfur production greater circular by decreasing the amount of sulfur lost during chemical processes.

Cheng adds that his team is currently looking to scale up their methodology by collaborating with sector. Daigger cautions that ‘there are many questions to conclusion to scale this process up and convert it into practice’, including reliability, how easy the process is to manage and the physical characteristics of the sulfur particles formed. ‘I think the authors have done a good job of initial investigations, however much work remains.’