Ion chambers and PIPS detector at beamline 1811, Max-lab. Credit: Charlotta Tiberg
Phosphate outcompetes arsenate, a form of arsenic, for sorption in aluminium-rich oxides according to scientists at the Swedish University of Agricultural Sciences (SLU). Their study, published in Chemosphere, holds important implications for the management of arsenic contamination in soils and groundwater.
The research, which analysed the extent of phosphate competition, evolved from earlier work with arsenate mobilisation from contaminated soils by principal investigator Charlotta Tiberg. “I realized that most mechanistically oriented research had been performed on pure sorbents, for example, ferrihydrite, goethite or gibbsite, and little on more complex matrices. Pure sorbents are not so common in natural systems,” said Tiberg, now Environmental Chemistry Researcher at the Swedish Geotechnical Institute (SGI) in Stockholm. “We wanted to understand the competition between arsenate and phosphate on mixed aluminium-iron sorbents.”
Arsenate and phosphate are known to adsorb strongly to iron and aluminium hydroxides and also compete for sorption sites on both these sorbents. Arsenic is a highly toxic, naturally occurring element which contaminates soil and groundwater in industrialised and developing nations. High concentrations of arsenic are typically found in areas of hazardous anthropogenic activity or in minerogenic rock.
Arsenic on the move
The research team investigated how phosphate and arsenate self-partition when applied to the sorbents, poorly crystalline aluminium hydroxide (Alhox) and ferrihydrite (Fh). X-ray absorption (XAS) spectroscopy revealed the distribution of absorbed minerals in mixed Alhox-Fh solution. Extended X-Ray Absorption Fine Structure (EXAFS) spectra at the arsenic K-edge was taken at I811 beamline at Max-lab in Lund, Sweden.
“Addition of phosphate to contaminated soil mobilises arsenate that is adsorbed to soil hydroxides. Our study shows that mobilisation would probably be larger in a soil with more aluminium oxides compared to a soil with more iron oxides,” explained Tiberg.
With equivalent parts arsenate, phosphate had higher adsorption in Alhox-containing samples. In consequence, fewer sorption sites were available for arsenate to take hold, causing the release of arsenate, and therefore its movement, in mixed iron-aluminium substrates.
According to the study, greater insight of arsenic transport in the environment could be gained with the development of new surface complexation models which reliably describe the sorption mechanism at play. By combining models for the iron hydroxide (ferrihydrite) and the aluminium hydroxide the competition between arsenate and phosphate could be analysed in more detail. This may lead to the development of remediation methods for contaminated soils and water, such as adsorption filters. Currently, Tiberg and colleagues are looking at remediation solutions with iron-based reagents.
Charlotta Tiberg, Carin Sjöstedt, Ann Kristin Eriksson, Wantana Klysubun, Jon Petter Gustafsson. Phosphate competition with arsenate on poorly crystalline iron and aluminum (hydr)oxide mixtures. Chemosphere, Volume 255, 2020, 126937, ISSN 0045-6535. https://doi.org/10.1016/j.chemosphere.2020.126937.