Characterisation and tuning of DOLLOPs in potable waters

It is worth softening the tap water by removing the divalent cations for many hardness-sensitive applications. Softening is applied to increase the lifetime, reduce energy consumption of water cookers, boilers, showers, etc., and reduce the use of surfactants in dishwashers and washing machines. Softening technologies are associated with relatively high chemical use (cation exchange, chemical precipitation), brine streams (ion exchange, membrane filtration) and/or energy consumption (membrane filtration). A better understanding may limit these issues or may even provide the industry with new softening technologies (magnetic fields).

Technology

Recent reports suggest that thermodynamically stable prenucleation clusters exist in undersaturated CaCO3 solutions and can account for as much as half the calcium in the solution. These “sloppy” objects have earned the felicitous name “DOLLOPs” (dynamically-ordered liquid-like oxyanion polymers). The hydrated multinuclear carbonate complexes of calcium and other divalent cations are found to aggregate into much larger (4–100 nm) particles, forming a liquid emulsion at neutral pH. Independent from that, nm-sized objects in natural waters were discovered consisting mostly, but not entirely, of CaCO3. This is of imminent importance for the drinking water industry since the ratio of Ca2+ in DOLLOPs compared to the overall Ca2+ concentration may provide a completely new physical water quality parameter. It may also provide plausible mechanisms for not well-understood and unconventional magnetic treatment devices that are widely applied for scale prevention.

Challenge

Little is known about the concentration, size distribution, structure and dynamics of DOLLOPs in potable waters. Investigating these parameters offers the chance to extend the parameters of water quality. The chemistry of aqueous systems can be understood and dealt with more accurately if DOLLOPs are taken into account and characterised properly. Moreover, it has been suggested recently that DOLLOPs provide the basis for a plausible mechanism for magnetic treatment based on the gradient of the applied field rather than its absolute strength. That is of imminent importance for the drinking water industry since the ratio of Ca2+ in DOLLOPs compared to the overall Ca2+ concentration provides a completely new physical water quality parameter.

Solution

DOLLOP and amorphous carbonate nanostructures are low in number (some thousands per mL) and spread over a large size distribution (a few to some hundreds of nanometers). Their detection and identification is not trivial. Supported by the water technology laser-optical lab at Wetsus, the proper laser-optical detection techniques will be developed, namely a dynamic light scattering (DLS) system tailor-made for DOLLOPs and other nanostructures detection.

This project aims to establish a new, more precise definition of the hardness of drinking water. This will be achieved by characterizing how Calcium and Magnesium are dissolved in water in addition to their total amount, namely how much is dissolved in ionic form and how much is in colloidal (“DOLLOPs”) form. Moreover, shifting their population between ionic and colloidal phases will be investigated.