3.1. Physical and Morphological Observation
Figure 2 demonstrates a comparison of features. Activated dolomite, which has a more greyish-white colour from carbonisation, is significantly greyer than raw dolomite. An increase in pore density following heat treatment, essential to adsorption as the active site for the dye binding site, makes activated dolomite lighter in weight [
29].
After carbonisation, the activated dolomite contains more significant concentrations of trace elements such as O, Ca, C and Mg that exhibit heat treatment that changes their physical and chemical properties as stated in
Table 1. After the heat treatment, observing other elements, including Fe, Ti, Mg, Si and Al, is feasible. The elements’ weight indicates the elements’ concentration on the dolomite surface. The weight of oxygen and carbon elements increased after the heat treatment of the raw dolomite, while magnesium and calcium decreased.
The presence of elements on the dye-treated dolomite exhibits a modest weight increase compared to the control dolomite, indicating that adsorption has occurred.
Dolomite is a physical adsorbent similar to any other activated carbon and gravel filtration compound. It can absorb the contaminants onto its surface, which is rich with pores. In this study, the observation of dolomite was conducted using a dissecting microscope (DM) and Field Emission Scanning Electron Microscopy (FESEM) to investigate and characterise the surface morphology and fundamental physical characteristics of the adsorbent surface, which was advantageous for mapping out the particle shape, porosity and size distribution of the activated dolomite. FESEM is a non-destructive form of electron microscopy in which the sample’s surface is contacted by an electron beam to produce signals at extremely high magnifications in the nm range. The DM and FESEM images of dye-treated and untreated activated dolomite are displayed in
Figure 3.
SEM analysis evaluates the dolomite adsorption on the dye compounds (
Figure 3).
Figure 3 shows FESEM images of inactivated and activated dolomite. Raw dolomite is depicted in
Figure 3A using SEM images, and the surface is very disordered with various-sized pores.
Figure 3B, showing activated dolomite, exhibits smaller, more prominent pores that are evenly distributed; these display the reduction in dolomite due to carbonate decomposition and the thermal cracking of dolomite fragments.
When compared to raw dolomite and untreated activated dolomite as a control, activated dolomite exhibited dye adsorption (pores: green arrows in
Figure 3A,B), which was followed by increasing densities of clumps on the pores, indicating that the dye molecules had coated the dye-adsorbed dolomite. The control dolomite in
Figure 3A,B (left pictures) is grey, indicating that it is unsaturated, which is consistent with the findings of [
17,
27]. The blue arrows are in
Figure 3C.
3.2. Optimisation of RTDW Decolourisation
Table 2 displays the ANOVA for RTDW dye decolourisation. The difference between the predicted R
2 (0.77) and adjusted R
2 (0.69) is less than 0.2, indicating reasonable agreement. The model
p-value in
Table 2 is <0.005 (
p < 0.0015), indicating high-significance for the model research. Given that this model’s precision is greater than 4.0, it is functional in the design space. The significance of the model was derived from the regressed adjusted coefficient of determination (adj. R
2 = 0.77), as stated in Equation (9). The F-value of 8.14 for the model illustrates its applicability. The 0.42 lack of fit F-value shows that noise is likely to cause a significant lack of fit F-value in 81.8% of occurrences (not significant).
Figure 4 includes 3-D graphs that depict the effects of temperature, size and weight. Each panel displays the two components at optimal levels while the other component’s experimental range remains constant.
Figure 4A,C demonstrates that weight and adsorbent size were insignificant factors in dye decolourisation, whereas activation temperature was highly significant (
p = 0.0001).
Figure 4B shows how the weight of the adsorbents affected decolourisation typically, whereas the size of the adsorbents was planar since it did not significantly affect the process.
3.3. Optimisation of STDW Decolourisation
Table 3 displays the ANOVA for STDW dye decolourisation. The difference between the predicted R
2 (0.76) and adjusted R
2 (0.56) is less than 0.2, indicating reasonable agreement. The model
p-value in
Table 2 is <0.005 (
p < 0.0018), indicating high-significance for the model research. Given that this model’s precision is greater than 4.0, it is functional in the design space. The significance of the model was derived from the regressed adjusted coefficient of determination (adj. R
2 = 0.76), as stated in Equation (10). The F-value of 7.75 for the model illustrates its applicability. The 0.54 lack of fit F-value shows that noise is likely to cause a significant lack of fit F-value in 73.7% of occurrences (not significant).
Figure 5 shows 3-D graphs exhibiting the effects of temperature, size and weight. Each panel depicts the two factors at their optimal levels while keeping the experimental range of the third factor constant.
Figure 5A,C demonstrates that, in the dye decolourisation process, the activation temperature was very significant (
p = 0.0001), while the other parameters were only marginally significant.
Figure 5B demonstrates how the adsorbents’ weight minimally impacted decolourisation, whereas the adsorbents’ size was planar because it had no appreciable influence on the process.
3.10. Limitations, Challenges and Future Strategy of Dolomite Decolourisation
The utilising of raw dolomite as an adsorbent is possible. However, consideration of the standard reaction and natural weathering process that occurs over thousands or even millions of years to achieve the objective is a huge consideration. To enhance the reaction process, pretreatment must be performed on the dolomite, such as thermal activation. Thermal activation removes chemically bound water, which may increase the porosity, resulting in surface area effects and disordering of the mineral lattice [
34]. Accurate control of the thermal activation process is necessary to prevent overheating, which induces the recrystallisation of minerals and dramatically lowers the carbonation rate [
35]. Due to this concern, a vast amount of utilised energy might be necessary for the pretreatment to continuously produce activated dolomite.
Although we have been using activated carbon for a long time, activating dolomite in water treatment is a novel technological technique. The construction cost for each unit process is presented as a function that was found to be the most practical and adaptable under many circumstances, such as those that can alter due to the designer’s preference or regulatory agency regulations [
36]. The most significant components of material and labour determining the cost of water treatment are the upkeep for applying activated dolomite to the system and its associated cost components [
37]. Higher carbonisation temperatures produce activated dolomite with superior properties than activated dolomite generated at lower temperatures. However, these higher temperatures require significant energy, which has a high indirect cost [
38].
In the limestone caverns of Perlis and Perak, Malaysia, dolomite is prevalent. Nevertheless, being a renewable resource, dolomite must initially be formed by replacing the calcite ions with magnesium ions, since naturally occurring dolomite requires a long time to develop [
13]. Numerous natural limestone caverns must be explored and mined to maintain the dolomite supply, disrupting the surrounding environment. When using activated dolomite in water treatment, a thorough analysis of the toxicity level and potential harm to human health from ingesting treated water must be considered.
Carbonation enables the valorisation of underused raw dolomite, reducing manufacturing and transportation costs and pollution issues whenever readily available materials are considered [
38]. Activating dolomite provides calcium (Ca
2+) and magnesium (Mg
2+), resulting in higher base saturation. At the same time, aluminium (A1
3+) ions are replaced by Ca and Mg, and are neutralised by OH− ions. Consequently, the dolomite application increases the pH of the water [
10]. The increment of pH is relatively high, exceeds the acceptable range and requires further treatment to reduce the pH. The adsorption of dolomite is surface-dependent, as the contaminants will bind on the available sites. At some point, the adsorption reaches equilibrium as the active sites are saturated by contaminants [
39]. A sustainable method will help to sustain the treatment in the future.
Once the adsorbent has become saturated with the contaminant, it can be considered a waste, generating a new environmental problem if it is not adequately disposed of [
18]. Toxins may be released into the environment by the waste-saturated activated dolomite leachate. Additionally, considering dolomite is an inorganic mineral, it cannot further disintegrate unlike other biological components utilised in the filtration system [
39]. Since environmental regulations for handling waste from dolomite materials are new, it may be difficult for some industries and companies to comply.
Dolomite that has been activated has the potential to acquire various beneficial properties for use in varying sorts of wastewater treatment. These characteristics enhance the water treatment process and water quality while supplying our water with essential minerals. They benefit our bodies since they contain the necessary minerals for good health. Mineral deficiencies increase an individual’s likelihood of developing conditions such as osteoporosis, goitre, and migraines. Therefore, high-quality treated water is required for environmental sustainability and human health mortality.
Dolomite is an example of an inorganic substance that is difficult to break down and cannot be used by other organisms. However, it can be employed for other things, such as improving concrete or amending soil. Dolomite’s crystalline structure enables us to reuse spent dolomite in water treatment at a reasonable cost to consumers. Today’s industries seek more reliable, stable and cost-effectively produced technologies. Due to environmental concerns, renewable resources are one of the trendsetters in technological advancement.