Synthesis and evaluation of phosphate-free antiscalants to control CaSO4·2H2O scale formation in reverse osmosis desalination plants

Abstract Polysuccinimide (PSI) of various molecular weights (MW) was synthesized from aspartic acid. The PSIs were hydrolyzed with NaOH to obtain sodium polyaspartates. Partial hydrolysis of a PSI gave the copolymer poly(SI- co -sodium aspartate). PSIs were treated with one equivalent of cyclic amines: pyrrolidine, piperidine, and azepane to afford the corresponding polyaspartamides in excellent yields. Treatment of PSI with 0.5 equivalents of the cyclic amines led to the formation of 1:1 copolymer poly(SI- co -aspartamide) which on basic hydrolysis gave the corresponding 1:1 copolymer poly(sodium aspartate- co -aspartamide). The antiscalant behavior of the newly synthesized compounds was evaluated at various concentrations (2.5 ppm–10.0 ppm) for the inhibition of gypsum scale formation. The antiscalant evaluation experiments were performed by measuring conductivity and using concentrated brines. The study revealed the performance of low MW antiscalants to be superior to their high MW counterparts. Thus, a polyaspartate is found to be a better antiscalant than its corresponding copolymer of SI/aspartate, i.e. a partially hydrolyzed PSI. Derivatives of 7-membered cyclic amine azepane performed better than their 5- and 6-membered counterparts. At the same degree of polymerization, performance with decreasing order of effectiveness was found to be: polyaspartate > poly(aspartamide- co -aspartate) > (SI- co -aspartate) > poly(SI- co -aspartamide) > polyaspartamide.