Foam fractionation for the separation of SDBS from its aqueous solution: Process optimization and property test
Introduction
Surfactants are a group of compounds with special amphiphilic molecular structure and they have been extensively used in the fields of washing, mining, petroleum exploiting, leather making and paper manufacturing, etc. The global market for surfactants is predicted to stand at USD 39.86 billion by 2021 [1]. According to diverse sources, surfactants can be divided into synthetic, natural-based and biological types. Due to the low cost (approximately $2 per kg) and good stability, synthetic surfactants hold over 75% of industrial consumption of all surfactants [2]. Wastewater analysis indicates that synthetic surfactants are the predominant organic contaminants in both concentration and frequency [3]. However, synthetic surfactants are difficult to be efficiently removed in sewage treatment plants because most of their constituents are non-biodegradable [4]. Once these surfactants are dispersed into environmental compartments, they can display detrimental effects on the survival of aquatic microorganisms (e.g., heterotrophic nanoflagellates and ciliates), degradation of other hazardous compounds (e.g., phenanthrene and pyrene) and breeding of amphibian organisms at a very low concentration [5], [6], [7], [8]. Moreover, the presence of synthetic surfactants in water may cause much damage to humans in the form of dermatitis, irritation and respiratory problems [9]. Many techniques had been proposed to eliminate synthetic surfactants from wastewater, involving flocculation, photocatalytic degradation, ion exchange and nano-filtration [10], [11]. Although these techniques have surfactant elimination capability, their popularity in application is limited by disadvantages, such as high operation cost, complex regeneration of substrate materials and easy secondary pollution.
Currently, foam fractionation has drawn considerable attention in the area of sewage treatment owing to its merits of simplistic device, mild operating conditions (inert gases and room temperature), free reagent consumption and large handing capacity [12]. This technique is a physical separation method employing bubbles as adsorbing media [13]. In a foam fractionation process, gas is injected in the solution for generating bubbles. Then, surfactant molecules with surface activity rapidly adsorb onto the gas-liquid interface of bubbles due to favorable thermodynamics [14]. This tends to lower the surface tension of the interface, thus producing a rising froth bed above the solution pool. As foam rises up in the column, entrained solution drains back along the Plateau borders and vertices due to gravity. Foam is collapsed to form a surfactant rich solution, called the foamate, when it is discharged out from the column. Obviously, foam fractionation process consists of three critical stages, including interfacial adsorption, foam drainage and foam collapse. Unlike other physical separation methods, foam fractionation allows for immediate reuse of both purified water and recovered surfactants [15]. At present, most researchers are mainly committed to improving surfactant elimination performance of foam fractionation through optimizing operating conditions and developing novel separation columns [16], [17]. However, on an industrial scale, entrepreneurs are more concerned about recovery and reuse of surfactant in terms of economic effectiveness and emission reduction. During foam fractionation, surfactant molecules would successively undergo interfacial adsorption and desorption processes, which easily induce structure transition. Bubble aging and coalescence could significantly affect adsorption capacity and arrangement state of surfactant molecules [18]. Moreover, the discharge of entrained solution from foam contributed to increasing the surfactant concentration in the foamate, even exceeding its critical micelle concentration (cmc). The self-association of surfactant molecules would prevent the decrease in surface tension of a solution [19]. These obstacles made the cyclic utilization and repetitive separation of surfactant even more challenging.
It had been well confirmed that protein molecules would suffer denaturation in foam fractionation because interfacial adsorption would induce the occurrence of molecular aggregation [20]. On the gas-liquid interface, the spatial structures of adsorbed protein molecules were partially unfolded in order to expose more hydrophobic groups. Once the interface disappeared, desorbed protein molecules were difficult to timely restore their original structures and they would be aggregated together via hydrophobic interaction. Aggregation phenomenon normally resulted in a large loss of protein functionality, especially catalytic activity of enzymes [21]. Compared to proteins, synthetic surfactants had lower hydrophile lipophilic balance and cmc values. Corti et al. (2000) had found that nonpolar hydrocarbon chains in synthetic surfactants had the trends to be stretched and gathered on the gas-liquid interface [22]. So far, there were not any references on reuse and separation properties of recovered synthetic surfactants by foam fractionation.
The objective of present work was to study the variations in recovery efficiency and physicochemical property of synthetic surfactant through multiple foam fractionation operation. As a model surfactant, sodium dodecyl benzene sulfonate (SDBS) was used due to its extensive industrial applications [23]. The cmc value of SDBS was 0.516 g/L [24]. Foam fractionation device was a normal vertical column. First, a suitable SDBS concentration in the feeding solution was determined to insure the smooth flow of entrained solution in foam. Subsequently, effects of volumetric air flow rate, height ratio of liquid phase and foam phase, and pore diameter of gas distributor on recovery efficiency of SDBS were investigated, respectively. Response surface methodology (RSM) was used to optimize the operating conditions of foam fractionation for separating SDBS. The repetitive separation of SDBS by foam fractionation was performed and property of recovered SDBS was tested.
Section snippets
Materials and reagents
SDBS (98% purity) was purchased from Shanghai Aladdin Biochemical Technology Co. Ltd., China. Pyrene (99%) was supplied from Tianjin Fengchuan chemical reagent Co. Ltd., China.
Determination of SDBS concentration
SDBS concentration of sample solution was determined by a spectrophotometer (752 N, Shanghai Precision Instruments, China) at a absorption wavelength of 223 nm. The linear-fitting equation was Y = 0.02883X + 0.0165, where Y and X are absorbance and SDBS concentration (0.005–0.025 g/L), respectively. Linear correlation
SDBS concentration
As seen from Fig. 2, the maximum liquid content of foam increased from 15.43% to 24.26%, and the liquid content half-life increased from 53 s to 224 s as SDBS concentration increased from 0.02 g/L to 0.10 g/L. A high SDBS concentration of feeding solution contributed to improving adsorption mass transfer of gas-liquid interface [26]. Before reaching saturation adsorption, the larger the surface excess of SDBS molecules was, the higher the liquid holding capacity of bubbles should get. The thick
Conclusion
In this work, the variations in recovery efficiency and physicochemical property of sodium dodecyl benzene sulfonate (SDBS) through multiple foam fractionation operation had been investigated. First, a suitable SDBS concentration of 0.05 g/L was determined for preparing feeding solution. The operating conditions of foam fractionation for separating SDBS were optimized by using RSM. Under optimal operating conditions of volumetric air flow rate 80 mL/min, height ratio of liquid phase and foam
Declaration of Competing Interest
None.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No.21908041), Science and Technology Project of Hebei Education Department, China (No. QN2018079), Natural Science Foundation of Hebei Province, China (B2020202070) and Graduate Student Innovation Foundation of Hebei Province, China (CXZZBS2020039).
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