Mechanisms of Calcite Dissolution Using Environmentally Benign Polyaspartic Acid:  A Rotating Disk Study

Abstract

The removal of calcium mineral deposits from metal surfaces is of practical interest for a variety of fields (i.e., food, petroleum, and chemical industries). This study investigated the mechanisms of calcite (CaCO3) dissolution using environmentally benign polyaspartic acid (PASP) under controlled hydrodynamic conditions by a rotating disk technique. The specific role of PASP conformation and surface interactions in the dissolution process was further studied using scanning electron microscopy and dynamic light scattering techniques. Using this combined approach, the dissolution mechanisms were investigated as a function of pH (3.5-10.0), rotating speed (150-1500 rpm), polymer concentration (0.001-0.1 M), and molecular weight (3000 and 10 000 M w). To quantify the effect of PASP on enhancing calcite dissolution, an enhancement factor, ηenh, was defined as a ratio of the rate of dissolution in PASP over the rate in water. Maximum enhancement was observed at pHs in the range 4-5, where an optimal combination of acid attack and chelant attack may appear. Dissolution is governed primarily by interfacial phenomena, including adsorption and surface reactions, at high pHs (>7), while it is controlled by mass transport at low pHs (<7). Increasing polymer concentration under acidic pH conditions, or increasing pH, raises the contribution of the interfacial controlled dissolution mechanism. Dissolution at high pHs is inhibited by small amounts of PASP (0.001-0.01 M) and enhanced by large quantities (0.1 M) of PASP, and proceeds via a surface complexation mechanism involving the chelation of calcium by PASP. A kinetic model incorporating surface adsorption and sequestration chemistry was developed to successfully calculate the dissolution kinetics of calcite at pHs above 7. In contrast, dissolution at low pHs occurs predominantly by acid attack from all acidic species (including H + and all partly dissociated PASP species), and PASP enhances dissolution over the entire concentration range (0.001-0.1 M). The lower molecular weight (3000) PASP is the more efficient dissolving agent at low pHs than the higher molecular weight (10 000) PASP. Dissolution at low pHs represents the most complicated case. An analysis of the dissolution mechanisms at low pHs was conducted on the basis of the consideration of mass transfer, polymer surface adsorption, conformation of polymers, and surface interactions.

Chelating Agents of a New Generation as an Alternative to Conventional Chelators for Heavy Metal Ions Removal from Different Waste Waters

“…In paper (Wu & Grant, 2002) it was found that aspartyl residues undergo protonation, deprotonation and complexation in solution. For sodium polyaspartate with a molecular weight of 10 000, 72 repeated units of aspartyl residues can be recognized as 18 independent hypothetical polyaspartic acid molecules (denoted as H 4 ds) for which the pKs of DS are equal to: pK 1 =2.27, pK 2 =3.60, pK 3 =4.09, pK 4 =5.17, At pH 1 about 100 % of DS occurs as H 4 ds, at pH 3 about 70 % occurs as H 3 ds -, at pH 4 about 48 % occurs as H 2 ds 2-, at pH 4,8 about 65 % occurs as Hds 3-and at pH above 7 100% occurs as ds 4- (Burns, et al 2003). The formation of metal complexes with DS can be summarized as:…”

Carbonation of Hydrous Materials at the Molecular Level: A Time of Flight-Secondary Ion Mass Spectrometry, Raman and Density Functional Theory Study

“…This is also shown by the growth of the Raman peak at 1083 cm -1 of 16 O-calcite over time ( Figure 3) and can be explained by the transformation of the initial metastable carbonates into more stable phases over time. [64][65][66][67][68][69][70][71][72][73][74][75][76][77] Figure 3 shows the disappearance over time of a peak at 495cm -1 that can be related to metastable hydrated phases 55 . Earlier, Rodriguez-Blanco et al 70 described the transformation mechanism of ACC to calcite as a two stage process: 1) ACC particles rapidly dehydrate and crystallize forming individual particles of vaterite; 2) the vaterite dissolves and re-precipitates as calcite (Ogino et al 71 also confirmed the latter stage also for aragonite O ratio within the first 24h.…”

Effects of L-Aspartic acid on the step retreat kinetics of calcite

“…The promotions of the dissolution rate and step velocity of calcite were observed in acidic organic compounds such as tartaric acid, alginic acid, and polyaspartic acid additive aqueous solution [31][32][33]. These acidic compounds might attack the calcite surface and promote calcite dissolution, although they can block reactive kink sites on calcite surface and inhibit the dissolution process [31].…”