The researchers from the Kazan State Power Engineering University, together with their colleagues from the University of Ruse in Bulgaria have proposed a new method for processing one of the most hazardous effluents from the petrochemical industry: sulfur-alkaline wastewater. These extremely toxic liquids form during purification of gases and petroleum products: their pH exceeds 13 – practically a concentrated alkali. Moreover, the sulfur compound content reaches several grams per liter which makes such effluents hazardous even in small quantities. Their release into the environment leads to rapid destruction of aquatic ecosystems. The proposed technology not only neutralizes this waste but also converts it into three useful products: purified alkali, hydrogen, and elemental sulfur.
Classical methods used to treat such effluent have long been known, but each has significant limitations. Neutralization with acids leads to formation of large volumes of salts, thermal methods require significant energy consumption, and membrane systems quickly fail due to the aggressive environment. Therefore, the researchers propose a different approach: electrochemical treatment. They developed a four-chamber electrodialyzer with bipolar nickel electrodes and cation-exchange membranes, designed specifically for these conditions.
Anion-exchange membranes had to be abandoned: in an aggressive environment, they deteriorate within days, losing strength and dramatically increasing resistance. Cation-exchange membranes, on the other hand, demonstrated stability and preserved their properties for a long time.
The unit’s operating principle is based on a simultaneous occurrence of several processes. Contaminated water enters the anode chambers, where under the influence of an electric current, sulfide compounds are oxidized and converted into elemental sulfur which precipitates and is easily separated. Simultaneously, sodium ions pass through the membranes into the adjacent chambers where water decomposes producing hydrogen and hydroxide ions. These combine to form a sodium hydroxide solution – the same alkali used in production.
The result is a highly pure alkali solution suitable for reuse. Moreover, the impurity content is minimal, and most of the electrical energy is used to form a useful product.
The purification effect also turned out to be significant. The level of organic contaminants in the source water reduces by tens of times – up to 95-99% of organic matter gets effectively destroyed. Furthermore, virtually all the alkaline component is recovered from the effluent and returns as a finished solution.
Nickel electrodes play a key role in the process efficiency. In an alkaline environment, active compounds form on their surface, accelerating oxidation of contaminants. This eliminates the need for expensive metals and reduces the technology cost. Energy consumption remains very low, while productivity remains high, with potential for further growth through scaling.
Based on these results, the researchers proposed a closed-loop process flow. The sulfur-alkaline effluent enters the unit where sulfur is separated, hydrogen is produced, and the alkali is recovered and returned to the production cycle. The purified water is partially reused.
Thus, virtually no waste accumulates: all the main components return to the economy. This reduces environmental burden and at the same time, increases the efficiency of the production itself.
