Efficiency of five scale inhibitors on calcium carbonate precipitation from hard water: Effect of temperature and concentration

Abstract

The effects of temperature and concentration of five scale inhibitors (three polyphosphates, one polyphosphonate and one polycarboxylate) on calcium carbonate (CaCO₃) precipitation from hard water are reported in this paper. The CaCO₃ precipitation from calcocarbonically pure water was induced by imposed potential and studied on gold electrode by chronoamperometry technique. With this technique, the efficiency of preventing CaCO₃ precipitation was determined for all scale inhibitors. The composition of these scale inhibitors were analysed by ICP-AES technique and no or few toxic elements were detected. For each tested inhibitor, the optimal concentration was determined and their efficiency was studied in the range temperature 20–50 °C. SEM/EDS and Raman spectroscopy techniques were used to analyse the microstructure and the elemental composition of the formed scale. Only polyphosphonates remain efficient at 50 °C. All the tested inhibitors were shown to act by threshold effect.

Introduction

Hard water generates adherent deposits of scale on the internal walls of industrial or domestic equipments which cause severe technical problems with a great economical impact. The clogging of pipes is the consequence of the scale formation which reduces heat transfer efficiency, leading sometimes to the shutdown of an industrial plant in the worst cases [1]. Increased energy and maintenance costs, as well as plant shutdowns, are some of the economic penalties resulting from scale deposition [2]. Scale is mainly constituted of calcium carbonate which has three crystalline forms [3]: calcite (rhombohedric structure), aragonite (orthorhombic) and vaterite (hexagonal).

Scale deposition kinetics is generally long under natural conditions. Therefore, different techniques have been proposed to accelerate the phenomenon in the laboratory. One of them is the electrochemical method proposed by Lédion et al. [4]. It consists of increasing the pH in the vicinity of an electrode through an electrochemical reaction [4]. The principle is to cover with calcium carbonate a metal surface carried to a fixed negative potential compared to a reference electrode [5]. The application of this negative potential first involves, on the metal surface, oxygen reduction reaction according to the equation [4].

O₂ + 2H₂O + 4e⁻ → 4OH⁻

At more negative potential, water reduction reaction is also observed [4].

2H₂O + 2e⁻ → 2OH⁻ + H_2(gas)

The presence of the hydroxide ions involves a local pH increase. Then the following reaction below takes place [4].

HCO₃⁻ + OH⁻ → CO₃²⁻ + H₂O

Finally, CO₃²⁻ content increase induces the precipitation of CaCO₃ on the surface of the electrode.

Ca²⁺ + CO₃²⁻ → CaCO_3(S)

The formed deposit which is not conductive reduces the active area of the electrode and constitutes a barrier for oxygen diffusion. Then, the total cathodic current decreases and it can be followed by chronoamperometric measurements. Characteristic chronoamperometric curves are obtained providing the scaling time (t_s) reached when the electrode surface is completely covered by the insulating layer of CaCO₃. The scaling time gives information about the scaling power of a sampling solution [4]. This technique is widely used to study CaCO₃ precipitation and its prevention.

Scaling can be prevented by physical and chemical techniques. Antiscale magnetic treatment [6] and water treatment in electric fields or the use of sonic waves [7] are physical techniques currently used. However, the efficiency of these treatments is still controversial as the involved phenomenon is not clearly elucidated yet [8].

Chemical methods consist of adding chemical reagents like strong acids, to lower the pH, or chelating agents, to complex Ca²⁺. But these techniques are expensive and can have disastrous consequences on the environment. In addition, the presence of metal cations as Zn⁺ at low contents is sufficient to inhibit germination and growth of calcium carbonate [9], [10].

Chemicals for water treatment have been in use for more than a century. Subsequent studies on the use of chemical additives for scale inhibition have proven the efficiency of various polyelectrolytes on retardation of crystal growth [11], [12], [13], [14].

Threshold inhibition is the most appropriate method to control scale crystal growth [15]. A widely used technique for controlling such scaling problems is dosing trace amounts of chemical additives that are able to inhibit growth of scale layer and weaken its adherence to a flow surface. Significant scientific and technological efforts have been made to control scale phenomena by adding small amounts (i.e. some g/m³) of inhibitor agents [16], [17]. Such low dosages are far from the stoichiometric concentration of the scaling species.

Chemical additives, known as “threshold inhibitors” reduce or eliminate scale formation [18]. A large number of additive formulations are commercially available. Commonly used antiscalants are derived from three chemical families: polyphosphates, polyphosphonates and polycarboxylic acids.

More than half of the drinking water treatment utilities in the United States have adopted the use of phosphate-containing additives [19]. The phosphates prevent the scale deposition by sequestering calcium and inhibiting scale precipitation, even under conditions of calcite oversaturation [20]. Threshold inhibition of calcium carbonate formation by polyphosphates is particularly effective at a pH range of 8–10 in which carbonate scale in drinking water is a major problem. In solution, linear polyphosphates undergo slow hydrolysis. Under neutral pH and normal room temperature, this hydrolysis is relatively slow. Higher temperatures will increase the rate of hydrolysis because long chain polyphosphates will breakdown into shorter ones.

Phosphonates in various forms are also widely used to prevent scale formation [17]. Polymer compounds with small molecular weight groups as acryl and amino-acryl are frequently mentioned [20]. Characterized by one or more functional groups (–PO₃H₂), these compounds contain bonds P–C–P or P–C–N–C–P more stable than the P–O–P bonds of condensed phosphates. The shorter the chains are, the better are their adsorption and scaling inhibition. These molecules resist to hydrolysis and have a greater effectiveness as scaling inhibitors. In addition, these additives are used under stoichiometric quantities (threshold effect action), contrary to the sequestering agents, this is economically interesting.

Polycarboxylic acids which have a cyclic or linear structural [18] are other polyelectrolyte threshold inhibitors employed to reduce or eliminate mineral scale formation. Some of the more familiar polycarboxylates are: polyacrylic acid, polymethacrylic acid and polymaleic acid. Reddy and Hoch [18] showed that inhibition of calcite crystal growth inhibition by cyclic polycarboxyclic acids appears to involve blocking of crystal growth sites on the mineral surface by several carboxylate groups.

Inhibition phenomena do not entail chemical reactions and stem from complex physical processes, involving adsorption, nucleation and crystal growth processes. The fundamentals of inhibition mechanisms, particularly from their quantitative aspects, are poorly understood so that the effects on inhibition effectiveness are largely unpredictable.

Generally, the mechanism suggested for the scale inhibition is adsorption of antiscalant on the crystal surface, which blocks the active crystal growth sites. After the adsorption phase, several modes of actions are considered: delaying germination, slow down crystal growth rate, favoring homogeneous germination on the detriment of heterogeneous germination or deforming the crystals, giving them a friable structure that weakens its adherence to a flow surface.

There is large information available in the literature about the precipitation and the effects of inhibitors on scale kinetic [17], [18], [21], [22]. On the other hand, few studies compare the efficiency of polyphosphates, polyphosphonates and polycarboxylic acids used under the same conditions, and few investigations are devoted to the cross effect of temperature and inhibitors.

In this work, the inhibition of calcium carbonate precipitation by five commercial scale inhibitors (three polyphosphates, one polyphosphonate and one polycarboxylic acid) was studied. The efficiency of these inhibitors on CaCO₃ precipitation from calcocarbonically pure (CCP) water was determined with a gold electrode by a chronoamperometry technique. The combined effect of water temperature and inhibitor concentration was investigated. The precipitate morphology was examined by scanning electron microscopy (SEM). The main elements are identified by energy dispersive spectroscopy (EDS) technique and Raman spectroscopy.

In the semi arid areas, the main part of water requirements is ensured by groundwater and geothermal water. The lifespan of the water drainpipes in these areas is very limited in time because of the strong water hardness and the high temperature. The present study on the inhibitors scaling could find a good application for geothermal water where the water temperature can reach 50 °C.