The O’Hare group at The University of Oxford has developed a novel method of synthesising carbon-capture compounds from wastewater by-products. The waste water by-product is struvite (MgNH4PO4•H2O) is a naturally occurring mineral, which causes problems from being a major component of kidney stones to blockages in wastewater treatment plants (Figure 1).
Modern water treatment now incorporates a chemical sedimentation step to remove the struvite at an early stage. The addition of this chemical sedimentation step has increased the efficiency of wastewater treatment plants but resulted in the production of large amounts of waste struvite for which there is currently no commercial use. The O’Hare has developed a use for this struvite based on layered double hydroxides (LDHs). LDHs (Figure 2) are a class of ionic solid comprising sheets of metal hydroxides (indicated in yellow), interspersed with a layer of either water molecules or anions (represented by the green particles).
The interspersing anion layer is weakly bound to the hydroxide sheets, meaning these ions are easily exchangeable, resulting in LDHs having many useful and tuneable properties in fields as diverse as medicine, energy and the environment.
Usually LDHs are prepared industrially with a variety of expensive (and often toxic) metal salts. The O’Hare group have developed an alternative method, using the waste struvite prepare LDHs directly (Figure 3).
Adjustments to the ratios of reagents and the reaction conditions enable different double-layered structures with modified properties to be prepared using this method.
One such example of this is calcination (prolonged heating of the solid product in the absence of oxygen), and when applied to Mg3Al-CO3 SLDH, the resultant product is the mixed metal oxide Mg3Al-CO3 SLDO (struvite layered double oxide). This compound is of particular interest because of its ability to act as a carbon-capture material. The Mg3Al-CO3 SLDO product has a larger surface area and larger pore volumes than both struvite and the parent SLDH, making it particularly effective as a physical adsorber of carbon dioxide. Preliminary studies have already demonstrated that Mg3Al-CO3 SLDO is over four times more effective as a carbon capture material (Figure 4).
Such research clearly demonstrates the value of repurposing waste products of human activities. The full potential of struvite layered double hydroxides is still under investigation, but they could prove to be a revolutionary development in the fight against climate change.
Capturing Carbon Dioxide with Wastewater by Victoria A M Atkinson
Synthesis of dense porous layered double hydroxides from struvite, W. L. Joyce Kwok, Hongri Suo, Chunping Chen, D. W. Justin Leung, Jean-Charles Buffet and Dermot O'Hare, Green Chem., 2021, 23, 1616-1620
Figure 1: Serious pipe blockages caused by struvite deposits. ©apexengineering
Figure 2: Representation of a layered double hydroxide (LDH). Sheets of metal hydroxides (yellow) are interspersed by a layer of weakly-bound water or anion particles (green).
Figure 3: Synthesis of layered double hydroxides (LDHs) from struvite. Blue indicates treatments on the struvite. Red indicates side products removed during synthesis.
Figure 4: Comparison of carbon dioxide adsorption of struvite and Mg3Al-CO3 SLDO. Physisorption refers to physical adsorption processes. Chemisorption refers to chemical adsorption processes.