Being “green” in chemical water treatment technologies: issues, challenges and developments

Mineral scale deposits in industrial waters supporting a number of process industries are a major problem that causes unexpected shutdowns and costly chemical or mechanical cleaning operations. Some deposits, although less common, are exceptionally troublesome. These include silica and magnesium silicate. Accumulation of these deposits greatly diminishes system performance. Scale prevention can be achieved by use of scale inhibitors that are added to any given treatment in minute (parts per million) quantities. Use of environmentally friendly water additives for colloidal silica scale control is a topic of great interest and intense research efforts in our laboratory. Inhibition and dispersion are two generic approaches for preventing silica scale and fouling. Inhibition stops scale-forming precipitates from forming, whereas dispersion keeps scale particles from being attached onto critical surfaces. We have pursued silicate polymerization inhibition by use of “green”, environmentally-friendly cationic macromolecules. These utilize inulin as the backbone, which has been chemically modified to introduce quaternary ammonium moieties. Three CATIN polymers (CATIN = cationic inulin) were tested: CATIN-1 (DS = 0.22), CATIN-2 (DS = 0.86), CATIN-3 (DS = 1.28), DS = degree of substitution at dosages 40, 80 and 100 ppm in supersaturated silicate solutions (500 ppm) adjusted to pH 7 for 8 h. Measurements for soluble silicate were performed every hour. After 8 h polymerization time, CATIN-1 achieves stabilization of 275 ppm silicate (∼90 ppm above the control), whereas CATIN-2 and CATIN-3 exhibit virtually identical performance, stabilizing ∼340 ppm silicate (∼150 ppm above the control). Inhibitor dosage increase to 80 ppm does not induce any additional inhibitory performance. Further inhibitor dosage increase to 100 ppm appears to have additional detrimental effects on inhibitory activity. CATIN-1 maintains 247 ppm soluble silicate (only ∼50 ppm above the control), CATIN-2 keeps ∼300 ppm soluble silicate (∼100 ppm above the control), and finally CATIN-3 maintains ∼270 ppm soluble silicate (∼80 ppm above the control). These CATIN additives present promising alternatives to treat silicate-supersaturated process waters.