Abstract
Among natural materials, clays occupy a prominent position being low cost, available in abundance, environmentally friendly, and having good adsorption properties. Bentonite, which has strong affinity towards cationic dyes such as methylene blue (MB) is able to remove dye effectively. It is very economical if the spent adsorbent can be recovered and reused for many cycles. Hence, this study investigated the ability of sodium persulfate (Na2S2O8) (SPS) to regenerate bentonite loaded with MB. The modification of raw bentonite with SPS was also investigated. The results showed that after added with SPS, the bentonite and MB adsorbed and flocculated together to form large flocs that were firm and compact. The process adsorption and flocculation was very efficient and equilibrium was reached within one hour. However, the modification of raw bentonite may not be a good option as the flocs formed were loose and less compact. High water retention of the flocs would cause disposal issue during dewatering process. The flocculation mechanism of MB-loaded bentonite can be explained in macroscopic and microscopic level. In macroscopic level, charge neutralization and bridge formation are the main mechanism while in microscopic level, increase in interlayer spacing and extensive cation exchange with MB are the one that contribute to flocs formation. Besides that, dimerization and trimerization also contribute to total MB adsorption which also allow bigger flocs formed. However, the ability of SPS to degrade MB adsorbed to bentonite through oxidation process does remain uncertain. Instead of regeneration, flocculation of MB-loaded bentonite upon addition of SPS occurred to remove the dye effectively. For reusability study, the MB-loaded bentonite can be used up to 3 cycles with percentage removal of 95%. Hence, SPS, as a flocculation promoting agent can be further studied and investigated, to be used in large scale in colour wastewater treatment.
Highlights
At current times, the regeneration method of low-cost adsorbents is still not well-developed
The process adsorption and flocculation was very efficient and equilibrium was reached within one hour
Charge neutralization and bridge formation are the main mechanism while in microscopic level, increase in interlayer spacing and extensive cation exchange with methylene blue (MB) are the one that contribute to flocs formation
Summary
The regeneration method of low-cost adsorbents is still not well-developed. Persulfate is a non-selectively reactive and relatively stable at room temperature It has relatively long lifetime in water, as compared to hydrogen peroxide. Activated persulfate (TAP) is currently a new novel method that emerges as an excellent choice of advanced oxidation processes (AOPs) for the treatment of organic solvents. TAP system emerges as the alternative method as it has longer lifetime and it is able to produce sulfate radicals SO4- directly as well as hydroxyl radicals OH- indirectly Both radicals play important roles in oxidizing organic contaminants (OCs). This current work is attempted to regenerate and recycle bentonite clay after use for adsorption by thermal activation of sodium persulfate. The success of bentonite regeneration will become a great breakthrough in coloured wastewater remedy
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