Application of CaCO 3 –Rich Solids

Limestone materials have been used in different sectors of the industry, which mainly includes construction, cement and agriculture. As already discussed throughout this section and based on the different analysis performed in the recovered solids, the results indicated that the composition of the solids recovered from the LLP at the evaluated pH conditions such as 12 were rich in CaCO3. Also, important features such as relatively low concentration of heavy metals and impurities such as organic matter and a large fraction of the solids with particle size less than 100 µm made the recovered solids a good material for potential applications, which might include those related to limestone material.

The explored application for the CaCO3-rich solids recovered from the landfill leachate permeate at pH condition of 12 with the mixed Na2CO3-NaOH reagents as precipitant and washing ratio of 1:50 included the neutralization of acid soil from lignite mines as illustrated in figure 83.

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Figure 83. Acid soil neutralization with recovered CaCO3-rich solids from the LLP; Left: Acid soil from lignite mine;

in Between: Acid soil to solids ratios; Right: CaCO3-rich solids

The results obtained in the pH analysis at the different acid soil to recovered solids mass fractions were compared with pure CaCO3 at the same mass fraction conditions as illustrated in figures 84 and 85.

Figure 84. pH analysis: Neutralization of acid soil with recovered CaCO3-rich solids 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0

2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Recov.-CaCO₃ mass frac.

Cond. (mS/cm)

pH

Acid soil mass frac.

pH Cond.

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Figure 85. pH analysis: Neutralization of acid soil with pure CaCO3

As illustrated in figures 84 and 85, for the mass fraction of 0,1 the neutralization effect of the recovered CaCO3-rich solids with respect to the pure CaCO3 was almost the same reaching pH values of 6.5 and 6.8 respectively. However, as the solids mass fraction increased a deviation with respect to the pH value was observed where for the solids mass fraction of 0,3 and 0,5 the recovered solids reached pH values of 7,5 and 8,8 respectively, which were larger than the pH values of 7,0 and 7,2 obtained with the pure precipitated CaCO3 at the same mass fraction conditions respectively.

Furthermore, the slightly increased in conductivity with respect to the acid soil value was observed with both neutralizers at mass fractions of 0,1 and 0,3 and then with solids mass fraction of 0,5 the conductivity values of the solids and acid soil mixture were below the value of 2,3±0,1 mS/cm corresponding to the acid soil.

The optimum pH for a particular soil depends on the kind of plant that is planned to be grown. For example, conifers can tolerate acid soils with values around 4 but for grass the recommended pH value in soil is greater than 6 with optimum values found around the neutral pH value (Oates, 1998). Thus, based on the results, the mass fractions of CaCO3-rich solids that seemed to be suitable for the increase of pH in the acid soil could range in between 0,1 to 0,3.

Additionally, leachability tests were performed in the acid soil, recovered CaCO3 rich solids and the mixtures of 70% and 50% acid solid with the recovered solids. The results obtained for the pH and conductivity at a temperature of 23,0±0,2°C are given in figure 86.

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Figure 86. Leachability analysis: Neutralization of acid soil with recovered CaCO3-rich solids

As seen in figure 86 and 84, the pH in the acid soil and recovered CaCO3-rich solids were almost the same in both the pH and leachability analysis with pH values about 2,5 and 10 for the acid soil and recovered solids respectively. However, for the recovered solids mass fractions of 0,3 and 0,5 the pH were higher than the values obtained in the pH analysis and with respect to conductivity in average the values obtained in the leachability test were about twice the values obtained in the pH analysis for all of the solid conditions.

Furthermore, Ca, Mg, Fe, TOC and SO4 were analyzed in the collected eluates collected from the leachability test. The results for Ca, Mg, TOC and Fe are given in figure 87 and for SO4 in figure 88.

Figure 87. Leachability analysis: Ca, Mg, Fe and TOC conc. in Eluates generated from different mass percentages of acid soil and recovered CaCO3-rich solids

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Recov.-CaCO₃ mass frac.

Cond. (mS/cm)

104

As illustrated in figure 87, in the eluate from the acid soil the concentration of important nutrients in soil such as Ca and Mg were less than 5 and 3 mg/L respectively. However, with the addition of the recovered solids the concentration of Mg was considerable in the eluates corresponding to the acid soil mass percentage of 70 and 50% with value of about 800 and 650 mg/L respectively. The same was observed for Ca but at lower scale with a value of about 65 mg/L in the eluate from the acid soil mass percentage of 70%. Furthermore, the TOC leached from the acid soil was almost twice the TOC leached out by the solid mixtures and about 1,5 times higher than the TOC concentration in the eluate from the recovered solids.

Furthermore, the concentration of Fe in the eluate generated by the acid soil was very high with a value of about 850 mg/L but this value decreased greatly compared to the eluate generated by the mixture with the recovered solids to values below 1 mg/L.

Figure 88. Leachability analysis: SO4 conc. in Eluates generated from different mass percentages of acid soil and recovered CaCO3-rich solids

Additionally, as seen in figure 88 for the case of SO4 based on the high concentration of about 4000 mg/L measured in the eluate from the acid soil, the concentration of the eluates generated in the solids mixtures almost corresponded to the fraction of the acid soil in the mixture. Thus, SO4 was not affected by the addition of the recovered CaO3-rich solids. Other nutrients such as N and P were measured in the generated elutes but their concentrations were relatively low with values less than 10 mg/L as illustrate in figure 89.

0 1000 2000 3000 4000 5000

A-Soil

pH:2,5 70% A-Soil

pH 9,0 50% A-Soil

pH 9,4 R-Solids pH 10,0 SO4 Conc. (mg/L)

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Figure 89. Leachability analysis: N and P conc. in Eluates generated from different mass percentages of acid soil and recovered CaCO3-rich solids

PO4-P NO3-N

TN 0.0

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

A-Soil

pH:2,5 70% A-Soil

pH 9,0 50% A-Soil

pH 9,4 R-Solids pH 10,0

Conc. (mg/L)

106

3.4. MEMBRANE CONTACTOR EVALUATION AND APPLICATION FOR THE