Adsorption of semisoft pollutants onto Bi2S3/Ag2S-AC under the influence of ultrasonic waves as external filed
Graphical abstract
Introduction
There have been many synthetic colors extensively in the industry effluent such as textile, pharmaceutical industry and other chemical manufacturing [1], [2], [3], [4], [5], [6]. Between these dyes, the auramine-O as semisoft molecule applied in the textile mills, leather and carpet industry and the ponceau-4R as semisoft molecule many used in the food industry [7], [8], [9]. Unsuitable repulse of this semisoft molecule dyes infects groundwater and easily can be adsorbed on the solid surface of many inner and outer semisoft members of living organisms and causes injury to the living thing [10], [11], [12], [13], [14], [15], [16]. Therefore, removal of this pollutants from aquaes solution before entered into groundwater is vital and very important. Though a number of processes such as coagulation, precipitation, adsorption, electrochemical techniques, ozonation and biosorption are available for dyes removal from aqueous media [17], [18], [19], [20], [21], [22], [23], [24]. Amongst, adsorption techniques as one of the applicable purification and treatment of wastewater method due to the high selectivity, capacity and efficiency has been many used [14], [25], [26], [27], [28]. In adsorption method fabrication of such structures usually requires, which semisoft dyes easily physisorbed or chemically bonded to an atomically smooth semisoft surfaces [29], [30], [31], [32], [33], [34], [35]. In addition, the amount of dyes adsorbed, the adsorption rate, and the stability of the adsorbed layer depend on the physicochemical characteristics of the semisoft adsorbent (size, flexibility, and surface charge), the surface of the selected substrate (surface energy, charge, and morphology), and the environment for the interactions [36], [37]. Steps (occurring at different time scales) are involved in the attachment of dyes molecules onto solid surfaces of adsorbents [4], [5], [33], [38], [39], [40], [41], [42]. At the beginning of the process, the semisoft dyes molecules diffuse from the bulk of the solution to the interface of solid liquid. Then, the semisoft dyes are adsorbed to the solid surface of semisoft adsorbent by a combination of (mostly) electrostatic, H bond, semisoft–semisoft interaction and other hydrophobic and or hydrophilic forces [43]. Finally, and depending on the structure, semisoft dyes molecules can relax and spread to maximize the number of interaction points with the semisoft adsorbent surface [44]. In some rare cases, semisoft dyes can also desorb from the interface and subsequently leaching to the bulk in either a native or structurally modified surface of solid state [45]. Although these interactions can be controlled by the experimental conditions of the adsorption (pH, ionic strength, temperature, etc.) or by chemical modifications of the semisoft adsorbent substrate and the applied of an external filed for accelerating of dyes adsorbed on to adsorbent surface [46], [47]. In this regard, our group recently demonstrated ultrasonic waves for increases the interfacial potential and significant enhancements of adsorption kinetic of dyes molecule onto many of adsorbent [15], [48], [49], [50]. Utilization of ultrasound irradiation is mainly preferable due to its higher efficiency for formation of semi-stable and unique suspensions in aqueous media, causing excellent enhancement in the contact surface among the an adsorbent and adsorbates, ultimately leading to a facile and rapid microextraction of the analytes of interest [13], [16]. This positive effect is easily obtainable by the cavitation phenomenon. In other words, rapid pressure fluctuations at a liquid medium causes a significant push of the liquid quicker than it can react, resulting in many gas bubbles (or cavities) [49]. These bubbles have a larger surface area during the rarefaction (expansion) cycle, which increases the diffusion of gas, causing the bubbles to expand [51]. Overgrowth of these bubbles causes intense explosions with a very high temperature and pressure, reaching up to 5500 °C and 50 MPa, respectively, resulting in intense shock waves to the solid surface, and consequently, improvement in the adsorption process [52]. Soft dyes (with a medium band gap conformational entropy) tend to adsorb to surfaces under a wider variety of conditions by permitting rearrangements of the molecular structure upon the adsorption. As a consequence, it is generally accepted that while semisoft dyes can adsorb to a variety of surfaces (even under unfavorable electrostatic conditions), the interaction of hard dyes require a more careful selection of the experimental conditions (surface charge, solution pH, ionic strength, etc.) [44], [45]. Therefore, this article presents results related to simultaneous ultrasound assisted semisoft adsorption of a ponceau-4R (P-4R) and auramine-O (AO) semisoft dyes assisted by ultrasonic waves onto Bi2S3–Ag2S nanocomposite loaded on activated carbon (Bi2S3–Ag2S-AC) as semisoft adsorbent. At present, it is well documented that co-sensitization has greatly improved the physicochemical properties of nanomaterials, comparing to single sensitization. Ag2S and Bi2S3 as a low band gaps materials have high adsorption coefficients, good thermal stability, and no toxic composition. Furthermore, the quantities of the compositional elements can be easily adjusted. Therefore, co-sensitization can not only increase the adsorption capacity but also improve the physicochemical properties. Based on the above consideration, Ag2S and Bi2S3 have a potential application in the co-sensitized activated carbon based adsorbent [53]. Ultrasound assisted semisoft adsorption experiments were designed by central composite design followed by spectrophotometric determination of dyes concentration in effluent solutions.
Section snippets
Reagents and instruments
All chemicals including sodium sulfide nonahydrate (Na2S.9H2O), bismuth(III) nitrate pentahydrate (Bi(NO3)3.5H2O), silver nitrate (AgNO3), activated carbon, AO, P4R, HNO3, NaOH and HCl with the highest purity available were purchased from Merck company (Darmstadt, Germany). The pH was measured using a pH meter (Metrohm 691 pH meter, Switzerland). X-ray diffraction (XRD, Philips PW 1800) was recorded using Cu kα radiation (40 kV and 40 mA). The morphology of samples was analyzed using field
Characterization of samples
Molecules are classified as soft and hard: hard molecules have a large band gap, and soft molecules have a small band gap. When compared with hard and soft molecules, we can say the following: one of the characteristic features of soft acids and bases is high polarizability. So, soft molecules are more polarizable than hard molecules. Soft molecules have a small gap. So, their absorption bands are shifted toward the visible region. Soft molecules are convenient for easy reactions. So, soft
Conclusion
In the present study, activated carbon was modified with Bi2S3/Ag2S nanoparticles (Bi2S3/Ag2S-AC) laterally used for semisoft adsorption of AO and P4R from aqueous solution, while semisoft adsorption rate was accelerated via ultrasound irradiation. The Bi2S3/Ag2S-AC was characterized using FE-SEM, EDS, DRS and XRD analysis and the investigation of relative influence of different parameters on the ultrasound assisted semisoft adsorption of AO and P4R dyes percentage was conducted by central
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