Federal Institute of Rio Grande do Norte1, Federal University of Rio Grande do Norte2
The scenario of water scarcity, present in various parts of the world, is accompanied by the search for alternatives for obtaining drinking water. In northeastern Brazil, for example, the wells become indispensable to the survival of the inhabitants of the areas affected by droughts. It turns out that, generally, this water is accompanied by components that, when in very high concentrations, are harmful (WHO, 2011). Two of them are calcium ions (Ca2+) and magnesium (Mg2+), added in quantities which result in a parameter of water quality called hardness. Given this reality, there is need for creating methods that meet the purposes of the treatment of this natural resource. For this, the application of Green Chemistry becomes essential for defending the sustainable, environmental and social guidelines essential to living context.
Highlighting the need to develop low cost processes, and taking advantage of local raw materials to facilitate logistics, this paper aims to study the efficiency of using clays in water softening and their application in column filtration, with parallel analysis of methodologies for the activation of clays (bentonite and vermiculite) and tracking of fundamental physicochemical parameters such as conductivity, total dissolved solids and pH.
The methodology basically consisted of planning and constructing the filtration device, activating the selected adsorbent materials for the tests, studying the parameters initially investigated for controlling water quality, testing the treatment using a laboratory-produced calcium standard solution.
Through extensive bibliography - represented by scientific articles focused on the chemical and structural properties of the clays as well as their activation processes - chemical methods were selected in order to obtain a variety of materials to be tested for the ability to adsorb undesirable components present in waters.
Activations of bentonite clays and vermiculite used acidic, basic and salt solutions, using the methods described in Morales-Carrera (2009), and Franca Arruda (2006), Quartatone et al. (2012) and Bertella et al. (2010).
For determining the crystalline phases of the clays and subsequent verification of the desired chemical modifications, X-ray diffraction tests were performed in a Bruker-D2PHASER equipment. The graphs were compared to those obtained from the literature, in order to confirm the nature of the used clay.
In order to prove the efficiency of the activated clay in water treatment, two procedures have been established to measure their adsorptive capacity: one using a magnetic stirrer, as seen in the literature (FranÃ§a; Arruda, 2006); and the other using a column filtration with standard calcium solution being introduced, at constant flow, fed by peristaltic pump, adopting the clay with best results in the first adsorption procedure. In this, 1 g of each clay was put in contact with 100 mL of a standard calcium solution at 1000 ppm in a 250 mL beaker. After being stirred on magnetic stirrer for 1 h, the mixtures were filtered and then collected for further analysis.
For the column procedure, the same process was applied, but instead of stirring, the calcium standard solution passed through the prepared clay into a filtration column. 1-micron filtration meshs were used to contain the clays.
At the end of this procedure, the clay with the best results was used in successive column filtration tests, until no further substantial reductions in hardness could be observed. This was done in order to obtain the maximum amount of calcium ions adsorbed per 1 g of clay.
The solutions obtained from the filtration processes described went through measurements of hardness (complexometric titration), and salinity, total dissolved solids, conductivity and pH (HANNA equipments), making it possible to compare the parameters with those of the calcium standard solution, which would indicate the effectiveness of each clay.
RESULTS AND DISCUSSION
According to the literature, the interplanar distance for natural vermiculite revolves around 14 ÃÖ (ALBERS et al., 2002). Santos et al. (2002) found the value of 13.78 ÃÖ (2θ 6.42Âº). Regardless of that, the diffraction patterns obtained indicate that activation with sodium chloride generated a small increase of that distance, reaching 14.004 ÃÖ (2θ 6.31). Although very expressive, this finding is in agreement with gifts Avelino et al. (2009) for sodium vermiculite. The modification with sodium hydroxide generated a slightly larger increase in the interplanar spacing, moving to 14.515 ÃÖ (2θ 6.09Âº), which can identify early deep changes in the sipes. The organization of vermiculite, for these two processes, remained.
Treatment with hydrochloric acid (Figure 3), on the other hand, led to profound changes in the clay lamellar structure, with a drastic reduction in the characteristic peak, although it is still present at 2θ 6.64Âº. This, however, did not extinguish the adsorption capacity of the material, which was determined by agitation and column testing. An explanation for that is that the delamination in the clay mineral's structure caused an increase in the superficial area of contact of the material.
For bentonite, the XRD analyzes confirmed the nature of the clay, with the existence of the peak at around 2θ of 21.50Â°, coinciding with gifts in the literature (Zhang, 2011). The treatments demonstrated no significant differences between them because the peak shifts were very similar. However, they existed and show that there was change in the interatomic distance. The tests confirmed the efficiency of adsorption processes, with considerable improvement in the results of the agitations.
The results obtained by the filtration tests are shown in the tables below. Tables 1 and 2 refer to agitation tests, while 3 and 4 refer to column filtration tests.
It is evident that all materials tested have considerable efficiency in the water softening, taking into account the amount of each material (1 g) and also the using prospect of the water. It was noticed, by the data obtained, (Table 2 and Table 4) an increased efficiency of vermiculite in water softening. Bentonites, however, have potential for water softening, but they need more treatments for enhance their adsorptive properties.
The results show a reduction of pH of the solution after contact of the tested adsorbents with the standard solution, though it remains high. It is important to consider that this is a fact of minor importance, since the reuse water usually does not have such high pH, and it is a parameter that is easy to fix when needed. Significant reductions were also noted in other parameters.
The test involving successive filtrations showed that for every 1 g of vermiculite activated with HCl, it is possible to remove about 60 mg of calcium ions.
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