Scale deposition mainly includes insoluble salts of calcium and magnesium, such as CaCO3, Ca3(PO4)2, CaSO4, MgCO3, and Mg(OH)2. Scaling phenomenon commonly occurs in desalination processes, cooling water systems, and oil fields, which may bring some deleterious effects, such as membrane fouling in desalination processes, aggravation of corrosion, and reduction of heat transfer efficiency. Addition of antiscalant is widely applied in scale formation control due to its effectiveness, convenience, and relatively low cost.
Multiple scale inhibition mechanisms working together contribute to the effectiveness of the antiscalants, including chelation, dispersion, lattice distortion, and threshold effects. The use of antiscalants allows reduction of membrane scaling, stable operation of cooling water systems, and high recovery in desalination processes.
Scale inhibitors could be classified as phosphorus-containing and phosphorus-free inhibitors, with the former as the most popular inhibitors worldwide for their good cost-effective performance. However, those phosphorus-containing materials increase the phosphorus content in water, resulting in heavy eutrophication and increased biofouling of membrane systems. Thus, low phosphorus or phosphorus-free scale inhibitors are more favorable. Many studies have been done on scale inhibition using synthetic polymers without phosphorus, such as poly(acrylic acid), poly(maleic acid), and their various copolymers. However, these polymeric antiscalants are not biodegradable, and some of these compounds are ter- or tetra- copolymers, which are complex and costly. Thus, the development of green scale inhibitors, which are phosphorus-free, biodegradable, and inexpensive, has attracted significant attention.
“Green” antiscalants, such as polyaspartic acid (PASP) and polyepoxysuccinic acid (PESA), have been receiving increasing attention owing to their non-phosphorus, low toxic, and biodegradable features. These compounds contain abundant active functional groups, such as carboxyl and amino groups, and exhibit good scale-inhibition performance due to their efficient chelation, dispersion, and lattice distortion. Moreover, they are especially suitable for high-hardness systems, such as membrane treatment technology, boiler water system, circulating cooling water treatment, and other fields.
Polysaccharide-based “green” antiscalants
Polysaccharide, such as starch (St), chitosan, inulin, and alginate, is a class of natural polymers and also contains abundant functional groups including hydroxyl, carboxyl, and amino groups, resulting in good chelation and dispersion effects. Moreover, these natural polymers are inexpensive, widely available, and easily biodegradable compared to PASP and PESA. Thus, polysaccharides have significant potential as green scale inhibitors.
However, although the use of polysaccharides and their derivatives as antiscalants have been reported, polysaccharide-based scale inhibitors could not be used widely because of the difficulty in controlling their complex molecular structures to meet the application requirements, such as molecular weight, degree of functional groups substitution and its distribution on the polymer backbone, and chain architectures (linear, branching and star-like forms).
St is one of the more popular polysaccharides because of its abundant source and low cost. However, St could not be used directly as an antiscalant because of its poor water solubility. Given that anionic groups, such as carboxylic and sulfonic groups, usually play important roles in scale inhibition, several anionic St-based scale inhibitors have been reported using various chemical modifications. These scale inhibitors were mainly obtained using oxidation or etherification, and the anionic groups were distributed on the St backbone.
Grafting copolymerization is also a useful modification process and could introduce functional groups onto St at branch chains. This approach generates distinct structural morphologies and potential application performances of grafted St derivatives. Therefore, the relationship between the molecular structural morphology of a grafted St, namely, grafting ratio and grafted-chain distribution, and its scale inhibition properties is an interesting topic.
St-graft-poly (acrylic acid) as antiscalants
St-graft-poly (acrylic acid) (St-g-PAA) is one of the simplest grafted St samples, which contains abundant hydroxyl and carboxyl groups. St-g-PAA may thus own good scale-inhibition performance and have significant application potentials as a commercial scale inhibitor. In this work, various St-g-PAA samples with different grafting ratios and grafted-chain distributions were easily obtained. The effects of dosage, grafting ratios and grafted-chain distributions on scale inhibition performance were studied. Furthermore, the scale-inhibition mechanisms were studied systematically from the molecular level based on changes in the apparent morphology and crystal structure of CaCO3 scale by scanning electron microscopy (SEM) and X-ray diffraction (XRD).
Results showed that the scale inhibition efficiency increases as dosage increases, while efficiency remains unchanged or slightly decreased after the optimal dosage. An excessive addition may cause flocculation effects and intermolecular hydrogen bonding, which can lead to the reduction of scale inhibition effect. The molecular structure of St-g-PAA has a significant effect on its scale inhibition performance. St-g-PAA with relatively low grafting ratio but a higher number of grafted chains showed better inhibition performance than the other samples. This phenomenon was due to the synergistic effect of two adjacent PAA branched chains for efficient chelation effect and increased activity of St-g-PAA resulting from its higher number of terminal groups of branched chains. And the scale inhibition efficiency of St-g-PAA significantly weakened with an increase in grafting ratio because of the increased molecular weight. The reduction of scale inhibition was owing to enhanced bridging flocculation effect and formation of intermolecular hydrogen bond.
In short, multiple short PAA branches generally increased the effectiveness of St-g-PAA in inhibiting precipitation of CaCO3 from solution. This work provides a reference for the development of other green scale inhibitors and investigation of water treatment agents’ structure-activity relationship.
These findings are described in the article entitled Evaluation of the structural morphology of starch-graft-poly(acrylic acid) on its scale-inhibition efficiency, recently published in the journal Water Research. This work was conducted by Wei Yu, Yawen Wang, Aimin Li, and Hu Yang from Nanjing University.