Synthesis of polyaspartic acid-capped 2-aminoethylamino acid as a green water treatment agent and study of its inhibition performance and mechanism for calcium scales

Abstract

Polyaspartic acid (PASP), a well-known green scale inhibitor for industrial water treatment, might be decomposed with prolonged duration, and its anti-scaling performance against CaCO<small>₃</small> and CaSO<small>₄</small> is diminished at a low concentration (&lt;10 mg L<small>⁻¹</small>) and a high temperature. With semi-ethylenediaminetetraacetic acid (EDTA) tetrasodium salt as the mimicking model, novel phosphorus-free PASP-capped 2-aminoethylamino acid (PASP–ED<small>₂</small>A) containing side chains bearing multi-functional groups is rationally designed and successfully prepared <em>via</em> the ring-opening reaction of cheap poly(succinimide) under mild reaction conditions with the assistance of readily available 2-aminoethyl amino acid. The static scale inhibition method is used to evaluate the scale inhibition performance of the as-synthesized PASP derivative. Scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy are utilized to monitor the crystallization process of calcium carbonate and calcium sulfate scales, and density functional theory calculations are conducted to shed light on the relationship between the molecular structure and scale inhibition mechanism of PASP–ED<small>₂</small>A. Results show that the as-prepared PASP–ED<small>₂</small>A shows better scale inhibition performance for CaCO<small>₃</small> and CaSO<small>₄</small> than PASP with a low concentration, a high temperature, and an extended duration. Particularly, PASP–ED<small>₂</small>A with a concentration of 10 mg L<small>⁻¹</small> exhibits the best scale inhibition performance for CaCO<small>₃</small>; its scale inhibition capacity is about two times as much as that of PASP. The reason lies in that the coordination atoms in the molecular structure of PASP–ED<small>₂</small>A can chelate with Ca<small>²⁺</small> to inhibit the combination of Ca<small>²⁺</small> with anions and prevent the generation of CaCO<small>₃</small> and CaSO<small>₄</small> scales. The PASP–ED<small>₂</small>A derivative can more efficiently retard the formation and growth of CaCO<small>₃</small> and CaSO<small>₄</small> crystal nuclei and exerts better inhibition performance against CaCO<small>₃</small> and CaSO<small>₄</small> scales than PASP.