Enhancement of silicate solubility by use of “green” additives: linking green chemistry and chemical water treatment

The inhibition performance of three synthetic amine/ammonium-containing cationic polymers in colloidal silica particle growth is reported. The three additives are compared to control (no additives present during silicate polymerization). The three polymers, polyethyleneimine (PEI), polyallylamine hydrochloride (PALAM) and poly(acrylamide-co-diallyl-dimethylammonium chloride) (PAMALAM), were screened in two sets of experiments: long term (24–72 h) and short term (0–8 h). Silica inhibition performance is profoundly dependent on the polymeric additive dosage. The most effective dosages were found to be 10 ppm for PEI, 20 ppm for PALAM and 80–100 ppm for PAMALAM. The inhibitory efficiency of PEI (at 10 ppm dosage) reaches 55% at 24 h (inhibitory efficiency is defined as reactive silica in ppm at the time of measurement divided by 500 ppm, times 100). PALAM reches 65% inhibitory activity at 20 ppm, after 24 h. PAMALAM at 80 ppm dosage shows 60% inhibition. Inhibitory activity drops on longer silicate polymerization times (48 and 72 h). All three inhibitors show activity higher than the control during the first 8 h, exhibiting small variations in performance.

In this work novel approaches based on polymer chemical technologies are studied as effective silica scale growth inhibitors in process waters. The most important conclusions are summarized below: C Cationic polymers enhance silicate solubility at the dosages used. C PEI (10 ppm), PALAM (20 ppm) and PAMALAM (80–100 ppm) cationic polymers are effective inhibitors of SiO2 scale growth at different dosage levels each. C Their structure and cationic charge density affect inhibitory activity. High positive charge density results in inhibitor co-precipitation with anionic colloidal silica, whereas low charge density renders the inhibitor ineffective at lower dosages. C These amine/ammonium-containing polymers also act as silica aggregators forming SiO2polymer composites with subsequent loss of inhibitor efficiency over time due to inhibitor entrapment within the amorphous colloidal silica matrix. The ultimate decision on which additive is the most appropriate to use is dependent on several factors. Inhibitory activity, potential hazards, ease of application, cost-effectiveness are some of the criteria that dictate such a decision [25]. Based on the data presented herein, it appears that PEI is the additive of choice when the lowest dosage is the criterion. However, if inhibitor performance is the criterion, then PALAM is the additive of choice. Lastly, it should be emphasized that control of colloidal silica by inhibition is a challenging issue. Colloidal silica is profoundly different than other, crystalline sparingly soluble salts [26], because it is amorphous. An added challenge, is