Phosphorus is widely used in many industrial processes and products, such as cleaning products and agricultural fertilisers, and is a principal source of environmental pollution. It is typically produced from phosphates which are mined. Many natural stocks of suitable phosphate ores are becoming depleted.
Phosphates are naturally present in small quantities in water. However, as a pollutant, large amounts can trigger potentially toxic algal blooms, which cause oxygen depletion when they decay. Several chemical methods are available for removal of phosphorus in wastewater, though many may be too expensive to use as standard treatments.
The new research used a gel prepared from the waste products of orange juice production. This contains around 10 per cent pectin, which can be converted to pectic acid with the addition of calcium hydroxide. When ions of zirconium are added to the gel, it shows strong adsorption of phosphates. After use, treating the gel with sodium hydroxide then removes around 95 per cent of the adsorbed phosphorus. This means that both the gel and the phosphorus can be re-used. Tests showed that the gel remained effective, even when it had been used many times.
The gel was also effective on artificial and real phosphate solutions and was not affected by the presence of other particles in the waste solution, such as sulphate, carbonate and chloride, which may have competed with the phosphate for adsorption to the gel. The best performance occurred in acid solutions (pH3), but was still very effective at neutral pH7.
The experiments tested the gel against two other existing phosphate removal products. It had around one third higher adsorption capacity than a commercially-available resin product, and four times higher adsorption capacity than zirconium ferrite, also sold commercially. Due to the high capacity of the gel and the low cost and easy availability of orange juice waste, the authors expect widespread use of the orange gel preparation in commercial processes in the near future.
They also report that recovery and recycling of phosphates may be needed to maintain commercial phosphorus supplies. This technique, employing other industrial waste products, may be a significant step towards industrial sustainability.