Preparation of Calcium Carbonate Nanoparticles: Adsorption and Desorption Behavior of Organic Matter ^†

Abstract

In this research the adsorption of organic matter onto calcium carbonate (CaCO₃) nanoparticles was studied. CaCO₃ nanoparticles were prepared by purging CO₂ gas into lime slurry. Progress of the reaction was monitored by online measurement of pH and conductivity. Prepared particles were contacted with organic matter synthetic solutions. Organic matter removal rates during adsorption were higher in samples with a high initial organic matter concentration. This result would be a promising solution strategy for the problem of management of flows collected in the treatment concentrates of membrane filtration plants.

1. Introduction

Decomposition of organic substances results in the formation of high molecular weight humic substances that are known as natural organic matter (NOM). One of the major concerns that treatment plant operators have today is NOM. It is a major precursor to the formation of disinfection by products (DBPs), it facilitates the transport of organic and inorganic pollutants, and enhances biological growth in distribution systems [1,2]. Researchers have found that the formation of DBPs is directly proportional to the concentration of organic carbon in water. Removal of organics from water during treatment processes and before chlorination usually requires the addition of high amounts of chemicals. This process is known as enhanced coagulation. The other removal methods are adsorption and membrane filtration. High cost of chemicals used in enhanced coagulation and the formation of sludge, problems arising the use of powdered activated carbon during adsorption such as clogging in granular filters and performance loss encountered in membrane filtration because of organic fouling are among the concerns in selecting the appropriate technique for the removal of organic matter. Calcium-containing composite materials [3] and surface charge modified calcium carbonate particles [4] have been used in previous researches for the removal of organic matter from solutions. CaCO₃ particles were integrated in coagulation-flocculation process as a neutralizer and coagulant aid [5].

This research aims to investigate the use of calcium particles in adsorption of organic matter from waters. Effectiveness of the methods employed in preparation of particles and the repeated use of particles in removal of organic matter were also studied.

2. Materials and Methods

2.1. Preparation of CaCO₃ Nanoparticles

CaCO₃ particles were prepared by purging CO₂ gas into lime slurry. Progress of the reaction was monitored by online measurement of pH and conductivity [6]. Reaction was performed in a glass cell open to the atmosphere and equipped with a sintered glass disc with a nominal max pore size of 100–160 µm at the bottom for homogeneous transfer of CO₂ gas into the medium. Reaction was completed in approximately 20 min (Figure 1). CaCO₃ particles were washed and stored as a stock solution. Calcium content of this solution was determined by total solids measurement and measuring calcium concentration [7]. Particles were also characterized by conducting particle size distribution analysis (Malvern ZS90, Atomika Teknik, England) and scanning electron microscopy imaging (Jeol JSM-5900LV, SEM Tech., Massachusetts, USA). The fffect of ultrasonication on particle size, shape and dispersion was studied by applying high intensity ultrasonication to the particle samples for 5 min (Bandelin Sonopuls HD3220, Bandelin, Germany).

2.2. Preparation Synthetic Organic Matter Solution and Adsorption Studies

Organic matter containing synthetic solutions were prepared by dissolving humic acid powder (Sigma-Aldrich, Merck, Germany) in 0.1 mol/L NaOH solution. Organic carbon concentration was determined using a total organic carbon analyser (GE Sievers M5310C) and measuring UV absorption at 260 nm (Shimadzu 2450). Organic matter concentrations were determined in samples before and after contact (1–2 h) with CaCO₃ particles at 150 rpm and 20 ± 1 °C (GFL 3031). In order to observe the change in adsorption capacity during repeated use of particles, used particles were washed with deionized water and re-used in bottles containing the same initial concentration of organic matter.

Table 1. Experimental matrix.

		Organic Matter Loading (mg TOC/g CaCO3)
TOC (mg/L)	25	0.25	0.5	1
	50	0.5	1	2
	100	1	2	4

shows the experimental conditions studied in this work. TOC concentrations in solutions were 25, 50 and 100 mg/L. CaCO₃ particles were added in reaction bottles including these solutions. Organic matter loading which indicates mass of TOC (volume x concentration) put in reaction bottle containing a certain amount of CaCO₃ particles are given in Table 1.

3. Results and Discussion

3.1. CaCO₃ Particle Characterization

Prepared CaCO₃ particle suspension was first allowed to settle and excess water phase above the particle mass was decanted. Particle concentration, particle size measurements and scanning electron microscopy images are shown in Figure 2 and Figure 3.

Particle concentration was determined by gravimetric solids analysis. CaCO₃ stock suspension prepared and used in the adsorption experiments include 50.03 ± 0.44 g/L particles. Figure 2 shows particle size distribution of samples. Although ultrasonication applied following the preparation of particles produces particles with smaller mean sizes, particle size distribution of samples of both pristine and ultrasonicated particles were quite similar (20–80 nm). Ultrasonication improves homogeneity of the medium. These findings were consistent with observations obtained from scanning electron microscopy (Figure 3). It was observed that particles obtained after ultrasonication were more homogeneously distributed, with higher sphericity and sharp edges.

3.2. Organic Matter Removal

Figure 4 shows the overall results of organic matter removal after 2-h contact with CaCO₃ particles. Organic matter loading rates (mass of organics loaded per gram of CaCO₃ particles) studied were in the range of 0.25–4 mg TOC/g CaCO₃. In the experiments during which particles with ultrasonication treatment were used, organic matter removal rates were found to be around 30–45%. Removal rates were relatively stable compared to those obtained with particles used without any ultrasonication application (Figure 4b). Contact of organic matter with these particles resulted in removal rates ranging between 15–40%. In both sets, reusing particles after washing their surfaces with deionized water produced removal rates which are less than those obtained with freshly prepared particles.

Figure 5 shows TOC removal rates versus organic matter loadings ranging between 1–4 mgTOC/gCaCO₃. In this particular experimental set, a very high concentration of organic matter was used (approximately 100 mgTOC/L). Adsorption levels are around the same for both of the particles (ultrasonicated and no ultrasonication applied particles) without any significant improvement.

In all loading rates, adsorption capacities obtained with reusing particles for the second time are less than those achieved in organic matter contacts with fresh surfaces.

Figure 6a displays the effect of initial concentration of organic matter. All of the bars in this plot correspond to the same loading rate: 1 mgTOC/gCaCO₃. There was no significant change in removal rates for the reaction mixtures where ultrasonicated particles were utilized (removal rates are around 42–43% for 25, 50 and 100 mg/L initial concentrations of TOC). On the other hand, pristine particles without any dispersion or other effects of ultrasonication application showed improved adsorption rates when they are contacted with highly concentrated solutions (removal rates are 12%, 34% and 41% for 25, 50 and 100 mg/L initial TOC concentrations, respectively).

Figure 6a,b show organic matter removal efficiency of CaCO₃ particles for the two other loading rates: 0.5 and 2 mgTOC/gCaCO₃. Similar to the loading rate of 1 mgTOC/gCaCO₃, particles used after the application of ultrasound demonstrated approximately same performance in removing organic matter from water (light grey bars on the figures). On the contrary, the ones used as prepared (without any dispersal or other effects of ultrasound) were not as effective as ultrasonicated particles in adsorption of relatively lower concentrations of organics (25 and 50 mg/L TOC).

Ιt might be concluded that, sonication as a pretreatment before removal of low to moderate concentrations of organics (25 to 50 mg/L) would be effective since the results reveals an improved removal performance (20 to 30% increased removal) compared to those obtained in reaction bottles where particles without any pretreatment were employed.

TOC removal (mg) per gram of particles incorporated into the reaction medium was compared for particles used as prepared and particles sonicated after preparation (Figure 7). As explained above, ultrasonicated particles performed better in removing TOC from solutions after 2 h of contact. Their effectiveness were approximately the same when a highly concentrated solution was considered for treatment.

4. Conclusions

The conclusions and contributions of this study can be summarized as follows:

• Laboratory made calcium carbonate (CaCO₃) particles were considered as a medium for the removal of organic matter from waters.
• Two different particles were prepared and their physical properties were determined. These are: the particles used in organic matter removal as they were prepared, and the particles used in organic matter removal after ultrasonication following their preparation.
• Ultrasonicated particles demonstrated better performances than particles used without any dispersal or other effects of ultrasonication especially for the removal of relatively lower concentrations of organics.
• Experiments were conducted for the investigation of adsorption capacity of particles during repeated use.
• Results obtained in this study would be a promising solution strategy for the problem of management of concentrated flows collected in the treatment concentrates of membrane filtration plants.