Degradation of phosphonate-based scale inhibitor additives in the presence of oxidizing biocides: "Collateral damages" in industrial water systems

Organic, phosphorus-based additives are commonly used in water treatment technologies such as mineral scale and corrosion inhibitors, and dispersing agents. Phosphonates find extensive use as anti-precipitation inhibitors for sparingly soluble salts such as calcium carbonates and phosphates, calcium/barium/strontium sulfates and others, commonly formed in supersaturated process waters in a wide spectrum of industrial applications. In open recirculating cooling water systems strong oxidizing biocides (eg. ClO-, BrO-, etc.) are also added to control microbiological growth but have detrimental effects on other water treatment chemicals that are sensitive to oxidative degradation. In this paper we report the effect of a hypobromite-based biocide towards the scale inhibitor AMP ( amino-tris-(methylene phosphonate)). AMP reacts rapidly with the biocide at room temperature. AMP degradation continues, but it slowly reaches a plateau after 1000 minutes. Even after 50 h the reaction time, only 20% of AMP has decomposed. AMP reacts with the biocide to give the orthophosphate much more rapidly at 43 degrees C than at 25 degrees C due to faster kinetics of decomposition. Results on various other oxidizing biocides on PBTC (2-Phosphonobutane-1,2,4- Tricarboxylic acid) are also presented. PBTC is a very "robust" scale inhibitor. This is confirmed by our degradation studies using biocides such as chlorine (ClO-), bromine (BrO-), their stabilized analogs, BCDMH, and ClO2. Degradation ( reversion to orthophosphate) of only up to 5% is observed in our experiments. These results are compared to others reported in the literature showing that PBTC degradation can be up to 25% under "harsher" conditions of higher biocide dosage and temperature. PBTC is virtually stable to the effects of a variety of oxidizing microbiocides, including chlorine, bromine and others.