Using Zeolite

Zeolite is a naturally occurring mineral containing the elements silicon, aluminum, and oxygen. Researchers Dr. Marcia Silva and Dr. David Garman at the University of Wisconsin - Milwaukee, working in the freshwater sciences department, patented a way of using it to remove PFAS containing phosphate from lakes and rivers. The mineral is modified with metal oxides (iron, calcium, lanthanum, zirconium, or magnesium) in order to attract the phosphate ions to the mineral. Thus, their filters containing this engineered zeolite can be placed in water systems, efficiently collect phosphate, and then be removed, cleaned, and replaced.

Research on this phenomenon reveals exactly how it works. For example, a 2017 study published via ScienceDirect discusses how scientists used zeolite modified with calcium hydroxide in order to remove phosphate from water. 

They used adsorption, or particle adhesion to a surface, to achieve this. Adsorption refers to the phenomenon of substances collecting on a surface (as opposed to absorption, where substances collect inside of another substance). Adsorption can be modeled through many different equations, most notably the Langmuir Adsorption Model:

XA = V/Vm = (KAeq*M)/(1+KAeq*M)

The occupancy of the adsorption sites (X) equals the ratio of the volumes of the substance adsorbed v. the volume that could be adsorbed given the surface area of the solid (V/Vm) which equals the equilibrum constant (K, the point where the substance has fully adsorbed onto the solid leaving no surface uncovered) times the molecules of the substance over itself + 1. It's complicated, but you only need the first two sections of the equation to see the idea.

The zeolite-phosphate connection is an example of physical adsorption, where the phosphate is held to the zeolite via electrostatic attraction between the negatively-charged phosphate ions and the positively-charged metallic ions the zeolite has been treated with.

The equivalence capacity (how much phosphate can be removed) of this adsorption can be expressed by the following equation:

qe = [V(Co−Ce)]/m

Adsorption capacity (mg/g) is equal to the volume (L) times the difference in phosphate concentration at the equivalence (in mg/L) divided by the mass of the zeolite (g). Thus, the bigger the difference in molarity/concentration in relation to the mass, the more efficient the removal method is.

The results of modified zeolite on phosphate pollution are favorable. A solution of phosphate ions with an initial molarity of 1.1×10-4 M (moles/L) reduced to a molarity of between 8.5×10-7 and 2.51×10-6 M after being exposed to a small sample of the calcium-modified zeolite. Other studies of zeolite-phosphate adsorption have shown 97.29-98% effectiveness in removing phosphates from different sources of water.

Zeolite has great capacity to help reduce phosphate pollution in water. Further introduction of zeolite into water filtration systems around the Great Lakes would allow for significant declines in algae blooms, increasing the wildlife population and making the lakes safer for both humans and animals.