alginate was 6.7 nm/s, which was more than five times higher than with 10 ppm LowG (G = 1.2 nm/s) or HighG (G = 0.94 nm/s) alginate present. The initial growth rates for 100 ppm LowG and HighG alginate were 0.58 and 0.48 nm/s, respectively. The growth rate in absence of alginate was higher than with alginate present for all values of relative supersaturation. The growth model described in eq 5 in the Experimental Section (G = kgσg) was fitted to the growth rate data and shown as solid lines in Figure 6. The model fitted well to the experimental data both with alginate present and absent. It is important to remember that the model requires the decreased calcium concentration to be caused by growth of the seeds, not by formation of new crystals (see below). The overall growth rate constant, kg, was found to decrease from 1.71 ( 0.12 nm/s to
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Olderøy et al. Table 1. Change in Calcium Concentration in Reactor between Two Chosen Time-Points Measured Both by Titration and by Coulter Counter (CC) Measurementsa
alginate no alginate 10 ppmLowG 100 ppm HighG
ΔCCa2þ (mmol/kg) 1-30 min
ΔCCa2þ (mmol/kg) 1-60 min
titration
titration
CC
3.9
3.9
2.6 1.4
CC 3.3 1.4
a The values obtained with the two different techniques are in good aggreement.
Table 2. Growth Rate Constants Determined from Growth Rate Model Fitted to Experimental Data in Figure 6 as Described in the Experimental Section
Figure 5. Coulter counter Multisizer measurements at selected time-points during growth experiments in absence of alginate (a), with 10 ppm LowG alginate (b) and with 100 ppm HighG alginate (c). Volume contribution was plotted as function of volume per particle. Both agglomeration (peaks moving distingctly to the right) and growth (increase in the area under the volume distribution curve) of the seeds can be seen from the plots. In some experiments small peak around 100 μm3 could be observed (as in panel a and c). These peaks are most likely a result of nucleation, but it represents not more thaen 10% of the total mineral growth and therefore nucleation is not significantly influencing growth rate calculations.
0.28 ( 0.02 nm/s when 10 ppm LowG alginate was present and to 0.20 ( 0.04 nm/s for 10 ppm HighG alginate (see Table 2). Reduced growth of calcium carbonate in the presence of additives is usually accounted for by adsorption of the additive onto active growth sites on the crystal surface.36,37 Because of alginate’s high affinity to calcium, it is probable that alginate was adsorbed onto active growth sites on the vaterite surface.
alginate
Growth rate constant kg (nm/s)
g
absent 10 ppm LowG 10 ppm HighG 100 ppm LowG 100 ppm HighG
1.71 ( 0.12 0.28 ( 0.02 0.20 ( 0.04 0.11 ( 0.004 0.091
2.02 ( 0.11 2.33 ( 0.06 2.37 ( 0.23 2.52 ( 0.15 2.54
It could be speculated that the growth rates decreased because alginate binds up calcium ions from the solution, and thus reduces the activity-based calcium carbonate supersaturation. However, the molar concentration of G-blocks used in the experiments was not high enough to significantly change the calcium concentration in solution. This supports that the growth rates were lowered because alginate interacts with the active growth sites. It has previously been shown that peptides rich in aspartic and glutamic acid (contain carboxylic side-group) interact strongly with calcium carbonate crystal surfaces.38 In alginate, each monomer contains a carboxylic group, which increase the polymer/mineral interactions. The different growth rates seen for LowG and HighG alginate maybe accounted for by the difference in G-block length, which is an important parameter related to calcium affinity. The HighG alginate with longer G-blocks may thus bind more strongly to the crystal surface. The growth order parameter g was found to be approximately 2 in the absence of alginate (see Table 2), characteristic of surface integration control.18 The parameter g increased with increasing alginate concentration, which may indicate a change in growth mechanism t
