Mercury is a source of toxic pollution in water, air and soil, arising from many industrial processes such as mining, cement production, coal combustion and more. Methods for removing mercury pollution from the environment can be expensive and inaccessible in poorer regions of the world, creating the need for less costly solutions that are both effective and easy to use. Researchers at Flinders University, led by Professor Justin Chalker, have now expanded testing for a novel sorbent material made from reused waste materials, creating possibilities for a new inexpensive and sustainable method to remove mercury from water.
The sorbent material is a free-flowing powder consisting of silica coated with a combination of sulfur and limonene. This coating, poly(S-r-limonene), can be sustainably sourced from industrial waste, with sulfur recovered as a byproduct of petroleum production and limonene extracted from orange peels discarded in the citrus industry, explained study co-author Max Worthington. This novel chemical combination has been shown to effectively absorb waste mercury, and is able to selectively bind to mercury but not other metals like iron, copper, cadmium, lead, zinc and aluminum, said Worthington.
The researchers tested the ability of the sorbent to remove mercury from water under a range of pH and salt conditions. The studies showed mercury uptake occurred rapidly in the pH range from 3 to 11, although spent sorbent degraded at higher pH levels which could lead to mercury leaching. At neutral pH, the sorbent could remove up to 90% of mercury within one minute from a 100 mL solution containing 5 ppm of mercury(II) chloride (HgCl2), and after five minutes, as much as 99% of the mercury was removed from the solution. Sodium chloride was found to dramatically reduce mercury uptake rates and capacity when present at concentrations comparable to seawater, although the inhibiting effects of sodium chloride could be overcome by using more sorbent.
Production of the silica-based sorbent material – coated with a nanometer-thin layer of poly(S-r-limonene) – can be completed in a fast process that prepares hundreds of grams of powder in an hour. The silica used in the material could further be sourced from agricultural waste, meaning there is a potential to make the removal process even more sustainable, said PhD candidate and study coauthor Alfrets Tikoalu. This study was published in the Royal Society of Chemistry journal Physical Chemistry Chemical Physics (PCCP).
“The particles contained in just 27g of this free-flowing orange powder has an approximate surface area of a soccer field, and it can be quickly produced in large enough volumes to suit contamination levels,” said co-author Max Mann.
The researchers’ experiments were further supported through mathematical modeling to qualitatively understand the rate of mercury uptake, which can further help the team measure and optimize the sorbent in real-world remediation efforts. The ability to produce the sorbent quickly using reused waste materials provides an easier, less expensive and environmentally friendly method that could be used in low-resource regions.
Photo: Poly(S-r-limonene)-coated silica is a free flowing orange powder and fast acting mercury sorbent. 300 grams produced in a single batch pictured. Credit: Flinders University