Reverse osmosis (RO) technologies are widely used throughout the world for drinking, industrial and waste water treatment purposes. Proper and economically attractive operation of RO installations should be provided by the correct maintenance of all technical process parameters. One of the significant operational problems is precipitation and deposition of sparingly-soluble salts on the membrane surface, which can be prevented by dosing special chemical additives into the feed solution. The aim of the present work is to reveal the structure/function relationship of phosphonate scale inhibitors, which possess systematic structural similarities and differences. Specifically, two classes of tetraphosphonate (5 additives) and diphosphonate (6 additives) have been tested as calcium carbonate scale inhibitors on the lab membrane unit. It was found that among the family of tetraphosphonates and diphosphonates the inhibitor efficiency increases with elongation –(CH2)– chain, but the longest additives have a sharp drop in inhibition efficiency.

Oilfield scaling is one of the most prevalent water-based oilfield production problems. Chemical inhibitors are often used to combat and prevent scale deposition. The phosphonate functional group is commonly used in many scale inhibitors. A series of bolaamphiphilic alkyldiamine tetrakis­(methylenephosphonic acid)­s have been synthesized from 1,X-diamines, where X in the polymethylene spacer groups has an even number of 2–12 carbon atoms. These inhibitors were coined CX-T (X is the number of C atoms between the N atoms on the inhibitor backbone, and T stands for tetraphosphonates). All tetraphosphonates showed excellent compatibility with calcium ions at 100 ppm or less, i.e., normal scale inhibitor dosage levels. At higher dosages, the calcium concentration and length of the alkylene spacer chain between the nitrogen atoms also determine the compatibility. Crystal structures were determined for the calcium “complexes” with the tetraphosphonates C2-T, C4-T, C6-T, and C8-T. All tetraphosphonates were investigated for calcite and barite scale inhibition under dynamic conditions using oilfield produced waters based on the Heidrun field water composition, offshore Norway. For calcite scaling, tetraphosphonates with ethylene (2 carbons) and hexamethylene (6 carbons) spacer groups gave the best inhibition performance. For barite scaling, the best inhibition was observed for the tetraphosphonates with ethylene and tetramethylene spacer groups, which relates to the ability of both disphosphonate moieties to interact with a single barium ion.