Electrochemical calcareous deposition in seawater. A review

Abstract

Pollution and climate change issues are calling for advanced techniques of pollutant sequestration to decrease toxicity, of coral and costal remediation, and of carbon sequestration to decrease atmospheric CO2 levels. For that, calcareous deposition appears as an overlooked, but potentially efficient technique. The calcareous deposit is a well-known precipitation by-product of cathodic protection in seawater. The deposit is made of a mixture of CaCO3 and Mg(OH)2. A calcareous deposit is formed electrochemically when a metal connected to an electrical power source is immersed in seawater. So far, electrochemical calcareous deposition has seldomly found applications, except for speedup of coral growth, prevention of shore erosion, reinforcement of artificial marine structures and remediation of polluted seawater. Here, we review the principles and mechanisms of electrochemical calcareous deposition. The growth, composition and mechanical properties of calcareous deposits are controlled by several factors such as 1) the impact of electrochemical parameters on the Ca/Mg ratio. For instance, CaCO3 formation is favoured at low cathodic potentials and low currents, whereas Mg(OH)2 precipitates preferentially at high cathodic potentials and high applied current; 2) the nature of the metallic electrode: although lime could be deposited onto any metallic surface at a fixed potential, electrochemical reactions and deposit composition are controlled by the metal nature. Moreover, the state of the electrode surface, e.g. with the presence of oxides or biofilms, modifies the kinetics of deposit formation; and 3) electrolyte composition, pH, temperature and stirring. For instance, in seawater, Ca2+ and Mg2+ concentrations control the allotropic variety of CaCO3 formed, e.g. Mg2+ inhibits the formation of calcite and vaterite. Hydrodynamic conditions near the electrode also influence the deposit composition since conditions regulate the mass transport at the electrode–solution interface. Below 10 °C, aragonite is not formed and the deposit stays thin. In the first stage of the deposit formation, an Mg-based compound is formed, followed by the growth of a Ca-based compound with time.