The effect of pH on the formation of a gypsum scale in the presence of a phosphonate antiscalant

A massive gypsum scale in a discharge pipe of a desalination plant occurred despite the presence of an antiscalant (PermaTreat, PC-191). The scaling occurred following a sharp drop in the reject-brine pH when an acidic effluent (pH<2) was introduced. Batch experiments revealed that the antiscalant is effective in postponing nucleation and hindering the crystal growth kinetics of gypsum in the supersaturated brine (Ω _gypsum>2) at neutral pH. However, as pH decreases both nucleation and crystal growth accelerate.A single general rate law that consistently described the crystal growth of gypsum as a function of pH in the batch experiments was found to be:RateSBET=k1{dot operator}(Ω1/2-1)10+k2{dot operator}(Ω1/2-1)2,where k _i is the pH-dependant rate coefficient (molm ^-2s ^-1) and S _BET is the BET surface area of gypsum (m ²). The dependence of the rate coefficients on pH is described quantitatively by an adsorption model which considers the protonation of the antiscalant functional groups.Finally, the precipitation rate of gypsum in the pipe was investigated with the laboratory rate law and adsorption model. It is shown that gypsum precipitation in the pipe occurs even in the absence of the acidic effluent and despite the fact that it could not be analytically detected.