Inhibition of Calcite Precipitation by Natural Organic Material:  Kinetics, Mechanism, and Thermodynamics

Abstract

The inhibition of calcite precipitation by natural organic material (NOM) in solutions seeded with calcite was investigated using a pH-stat system. Experiments were carried out using three NOMs with different physical/chemical properties. For each of the materials, inhibition was found to be more effective at lower carbonate/calcium ratios and lower pH values. The reduction in the precipitation rate could be explained by a Langmuir adsorption model using a conditional equilibrium constant. By identification of the type of site on the NOM molecules that is involved in the adsorption reaction, the "conditional" equilibrium constants obtained at different solution compositions converged to a single "nonconditional" value. The thermodynamic data determined at 25 degrees C and 1 atm suggest that the interaction between NOM molecules and the calcite surface is chemisorptive in nature and that adsorption is an endothermic reaction driven by the entropy change. The greatest degree of inhibition was observed for the NOM with the highest molecular weight and aromatic carbon content. For a given type of NOM, the degree of inhibition of calcite precipitation was dictated bythe balance between the enthalpy change and the entropy change of the adsorption reaction.

A simple reactive-transport model of calcite precipitation in soils and other porous media

“…Other authors have found similar degrees of inhibition of calcite precipitation by DOC in natural systems Suarez, 1996, 1998;Hoch et al, 2000;Lin et al, 2005), and also by dissolved phosphate (Mucci, 1986;Paquette et al, 1986;Dove and Hochella, 1993) and Mg (Nielsen et al, 2013). Dissolved phosphate and Mg concentrations in our soil were < 0.1 µM, which is below values causing inhibition.…”

“…Dissolved phosphate and Mg concentrations in our soil were < 0.1 µM, which is below values causing inhibition. The mechanisms of inhibition by DOC involve adsorption of DOC on nucleation surfaces, and the degree of adsorption and resulting inhibition depend on the nature of the DOC and the solution composition (Inskeep and Bloom, 1986a;Lin et al, 2005). Values of DOC of a few mM are typical of mineral soils under natural or seminatural vegetation, depending on the soil organic matter content, clay content, pH and other factors (Moore, 1997;Buckingham et al, 2008).…”

“…But there is little equivalent information for soil systems and other porous media. Soil solutions are often supersaturated with respect to pure CaCO 3 and precipitation is sensitive both to catalysis by existing CaCO 3 and other solid surfaces, and to inhibition by organic and inorganic ligands in the soil solution (Inskeep and Bloom, 1986a,b;Amrhein et al, 1993;Suarez, 1996, 1998;Hoch et al, 2000;Lin et al, 2005;Nielsen et al, 2013). Rates of transport in soil are generally far slower than in simple solution systems because most solutes are sorbed on soil surfaces and they are largely immobile in the sorbed state (Tinker and Nye, 2000;Sposito 2008).…”

Effect of coagulation pretreatment on membrane distillation process for desalination of recirculating cooling water

“…It is well known that operation conditions, NOM, many ions such as Mg 2+ , Ba 2+ , Sr 2+ , Mn 2+ and SO 4 2− affect CaCO 3 growth rate. Especially, NOM interacts strongly with calcium carbonate and acts as an inhibitor to calcium carbonate crystal growth in aquatic systems [23][24][25][26][27]. Hence, as a result of heating the feed and enhancement of CF, the solubility of CaCO 3 goes down with temperature rise, and the HCO 3 − ions present in the RCW undergo the decomposition and a considerable amount of CaCO 3 precipitates on membrane surface, which reduces the surface of water vapourization (pores).…”

“…The deposit of calcite crystals on the membrane surface was loosely packed and dispersive, and the color of inner membrane surface became brown. It is suggested that NOM acts as an inhibitor to calcium carbonate precipitation by blocking the active growth sites through adsorption reaction [24]. Otherwise, in the RCW, additional protection from scaling may be attained through the use of a number of proprietary antiscalant additives, such as polymaleic acid, phosphonates and non-ionic detergents.…”