Is Phosphonate a Chelating or Sequestering agent? Why Phsphonates are threshold scale inhibitor?

Chelating agent, sequestering agent and phosphonates are all antiscalants and corrosion inhibitors. But their mode of action is different.

Chelating agent

It is a chemical substance with molecular structure which can envelop certain types of metal ions into soluble complexes. A chelating agent converts the positive charge on metal ions into negative charge. Those who want to know more, chelating agents have atoms with lone pairs of electrons , oxygen is an example. These lone pairs are donated to a metal ion (metal ions are always positively charged) through coordinate covalent bond that gives a negative charge on metal ion. A negatively charged metal ions cannot join a negatively charged anion and their ability to form any compound is lost. This is the fundamental point about how chelating agent deactivates metal ions. When enveloped or clawed and grabbed by chelating agent, the metal ions lose their ability to react with other ions. The trapped ions have no mobility of their own. Neither they can agglomerate nor they can deposit – practically the ions become dead particles. In water treatment, EDTA finds its application as chelating agent particularly to deactivate hardness ions in water.

This is how a molecule of EDTA looks like. A chelating agent can bind with a single metal ion at a time.

Sequestering agent

A sequestering agent does the same job what a chelating agent does to cover a metal ion and form a stable soluble complex and thus prevents metal ions from taking part in chemical reactions. A sequestering agent can envelop multiple ions at a time because it has multiple active sites which can bind with more than one metal ion. Thus the only difference between a chelating agent and a sequestering agent is that chelating agent can bind with a single metal ion at a time while sequestering agent can envelop multiple ions at a time. Sodium tripoly phosphate is a typical sequestering agent.

This is how a sodium tripolyphosphate molecule looks

Both chelating agent and sequestering agent act on metal ions by mole to mole chemical reactions. Both chelating and sequestering are chemical reactions while phosphonates physically adsorb on molecules and work internally within crystal network to prevent growth of crystals and eventual deposit of solids.

Phosphonates

Phosphonates work by adsorbing physically on crystal surface by surface forces. It is a physical process. Most important application of phosphonate is that they inhibit formation of scale forming substances and corrosion. Phosphonates also work as sequestering agent for metal ions like iron, copper, zinc and manganese. In addition, phosphonates also exhibit some dispersancy properties towards clay, silt and calcium carbonate.

Phosphonate as scale inhibitor

Phosphonates prevent the growth of crystals in water by restricting the molecules or atoms within crystal to grow bigger. Phosphonates would work when the crystals are still soluble and in their growth stage. Phosphonates would work more efficiently on crystals because of large surface area than on an individual particle. The intermolecular forces like Van der waals forces become bigger as the molecules agglomerate adding more surface area. As intermolecular forces become bigger, the particles have more chances to agglomerate and deposit on surface. The surface binds big deposited agglomerated particles by Van der Waals intermolecular attraction and form hard scale. This is the most basic fundamental of deposition of scale on a surface.

Let us recap what is Van der Walls attraction.

What are Van der Waals forces?

A molecule as whole is neutral. However, every molecule has some surface charges. The electrons are mobile. At any one instant they might find themselves towards one end of the molecule, making that end – ve. The other end will be temporarily short of electrons and so becomes +ve. This constant "moving around" of the electrons in the molecule causes rapidly changing, – ve and + ve temporary dipoles. The +ve end of one molecule is attracted to the –ve end of the other. This brings the molecules closer and this is what is called Van der Waals intermolecular attraction. Larger the size of aggregate of molecules larger would be the intermolecular attraction because there are larger sites of temporary dipoles. The graphite is an example where layered carbon sheets are held by Van der Waals forces.

Mode of action by phosphonates

Crystal growth inhibitor

Let us take a case of calcium carbonate. Calcium carbonate in water exists as soluble crystals until they do not exceed the solubility. The crystals consist of stack of layers of calcium carbonate molecules held by Van der Waals forces arranged in a three dimensional network called crystal lattice. When phosphonate is added to water, it adsorbs on the molecules of calcium carbonate in the crystal and blocks the growth sites of the molecules and makes the Van der Waals forces ineffective to draw the molecules closer to agglomerate into eventual insoluble solids. When this happens the nucleation of calcium carbonate crystals is delayed and as a result of this supersaturation point of calcium carbonate in water increases. Water can thus dissolve more calcium carbonate.

Crystal distortion

Adsobed phosphonates on calcium carbonate molecules in the crystal, change the shape of molecules. The distorted molecules increase the stress in the crystal lattice and crystals collapse back into soluble particles. The phosphonate molecules which had adsorbed on calcium carbonate molecules are released when crystals collapse and they are available to repeat same jobs again and again. These inhibitors are often referred to as threshold inhibitors because they are effective at very low concentrations, typically less than a few ppm. 

Why phosphonates are called threshold scale inhibitors?

As opposed to a chelating agent / sequestering agent which require stoichiometric quantity to react chemically with scaling molecules, the physical adsorption of phosphonates require a fraction of its stoichiometric quantity that is needed for a chemical reaction to inhibit scale deposition. Phosphonates regenerate itself in scale inhibition process as explained above and repeat scale inhibition again and again. The regeneration of phosphonate in the scale inhibition process makes it effective at very low concentrations, typically less than a few ppm.