Enhanced Pretreatment of Natural Rubber Industrial Wastewater Using Magnetic Seed Coagulation with Ca(OH)2
To comprehend the hypothesized mechanism of the MSC process as described in this study, several analytical techniques were employed, including measurement of particle size distribution of the coagulum and FTIR and FD analysis. In the presence of MS, the sludge volume index (SVI) is generally lower (Figure 4), indicating more effective sedimentation. Specifically, Ca(OH)2 trials showed a tendency for a more compact sludge blanket over time compared to NaOH. This is likely due to the additional role of Ca(OH)2 as a coagulant, which could contribute to the formation of larger, denser flocs. The optimized doses of PAM and MS further enhance these effects, leading to improved removal mechanisms and higher sedimentation rates, as seen by the more pronounced reduction in the H+/H0 ratio. These observations are critical for understanding the dynamics of floc formation and the subsequent efficiency of the MSC process. According to Figure 5, the experiment demonstrated that treating the water with PAC alone, and then with additional PAM and MS, progressively increased the particle size. The most notable enlargement occurred when the pH was modified. However, the introduction of MS as a coagulant aid led to the formation of tangled, heavy flocs that were more resistant, which could potentially reduce the time required for the particles to settle. The pH and zeta potential can also be used to describe this phenomenon further, where changing the pH can alter the surface charge of particles, making them more amenable to coagulation and reducing the zeta potential through charge neutralization to enhance particle aggregation and floc formation. As per the literature evidence, zeta potential increased with rising pH value at pH values less than 6.5 and thereafter, zeta potential decreased in the basic range. PAC can reduce turbidity at low aluminum concentrations, and the lowest level was well before the compound’s isoelectric point when considering zeta potential. Due to stable Al species such as Al13, rapid adsorption occurs, leading to the aggregation and rearrangement of particles, eventually forming “electrostatic patches” on the particle surface [35]. During MS optimization, flocs optimized with NaOH had a fractal dimension of 1.7944, higher than flocs optimized with Ca(OH)2, which had a fractal dimension of 1.7392 (Table S4). The higher fractal dimension observed with NaOH treatment could be indicative of a more porous and less efficient floc structure, which correlates with its reduced contaminant removal efficiency compared to Ca(OH)2. Conversely, Ca(OH)2-optimized flocs showed a lower lacunarity value (L = 0.4771) compared to NaOH-optimized flocs (L = 0.5508), indicating a more symmetrical and compact structure (Table S4). Ca(OH)2 exhibits less lacunarity, indicating a more symmetric and compacted floc structure, which is more effective in contaminant entrapment and results in better removal efficiency. This aligns with the overall higher efficacy of Ca(OH)2 in the treatment trials for removing contaminants. Area under curve (AUC) values and D50 measurements (Table S5) of particle size distribution (PSD) curves indicate that Ca(OH)2 is more effective in terms of particle size reduction and sedimentation compared to NaOH. The reduced D50 during Ca(OH)2 trials suggests a faster and more efficient sedimentation process. The increasing trend in AUC values when going from raw waste to the MS optimization step in NaOH treatment suggests an increase in particle quantity. For Ca(OH)2, the AUC values initially increase until the PAC optimization step and then decrease during the PAM and MS optimization steps, indicating a change in both particle size and quantity. The decreasing trend in D50 during Ca(OH)2 trials supports the notion of faster sedimentation. This size variation of the particles as shown in Figure 5 can be explained by the solubility of NaOH and Ca(OH)2. NaOH is highly soluble and easily separates into the ions of Na+ and OH−. Due to strong disassociation, colloidal particles in the coagulum are smaller in size and more challenging to settle. The slower dissolution rate of Ca(OH)2 likely contributes to a more gradual release of Ca2+ ions, facilitating stronger floc formation and enhancing the MSC process’ mechanism for pollutant removal.
Findings of the FTIR results highlight NRIWW’s intricacy as well as a large range of chemical elements present in the raw wastewater. According to the FTIR results of the optimized dosage combination using NaOH and Ca(OH)2, as shown in Figure 6b,c, it is clear that Ca(OH)2 is more effective than NaOH in pollutant removal. However, neither NaOH nor Ca(OH)2 was effective in removing organic phosphate compounds contained in the NRIWW. Ca(OH)2 was efficient in removing dissolved contaminants such as aromatic compounds and C≡N triple-bond compounds from the wastewater. Some aromatic molecules can also directly adsorb onto the surfaces of PAC and PAM, which occurs as a result of chemical interactions such as hydrogen bonds or electrostatic interactions between the coagulants/flocculants and the aromatic molecules [44]. Based on the research outcomes of this study and previous studies [15,17,18,20], MS could act as cores to enhance the settling ability of coagulum with higher density, size, and strength. Due to the high suspended solid concentration of the NRIWW, short relative distances and high occurrence of impacts between particles can be observed and these result in high turbidity removal efficiency in the MSC process. According to FTIR data, aromatic/aliphatic carboxyl or hydroxyl organic matters present in NRIWW might interact with MS and form Fe–OH/Fe–OH+ bonds through surface complexing and hydrogen-bonding processes, which explain, in part, the relatively high removal of organic matter. Adsorbing–bridging is also another potential coagulation mechanism with MS-Al species, which elevated the removal of Al–hydroxyphosphate complexes. It was revealed that MS may function as adsorbents when combined with coagulants and suspended particles to form magnetic flocs with increased density, size, and strength that could be quickly and efficiently removed from suspension to coagulum.
