| With the increasing of environmental pollution by waste plastics seriously, waste plasticsrecycling had become a priority for governments and enterprises. As separation techniquebased on the surfactant, plastic flotation can greatly improve purity of the sorting of wasteplastics to provide favorable conditions for subsequent recycling. In the plastic flotationprocess, the attachment between plastic particles and air bubbles was the key to the success ofthe process. The adsorption of the surfactant on the plastic surface and the changes ofinterfacial tension between bubbles and plastic had an important influence on the interaction.Therefore it was particularly important to choose suitable surfactant concentration and typefor a wide variety of waste plastics. This paper studied the impact of surfactants on bubbleattachment behavior, and also discussed whether the biosurfactants can replace chemicalsurfactants via the compare of the effect between biosurfactants and chemical syntheticsurfactant. To this end, we used high-speed photography technique and set two kinds ofimpact plates （glass and polypropylene） to compare the speed variation, adhesive effect aswell as the attachment time in process of bubbles impacting plates in Sodium dodecyl sulfate（SDS） solution, Rhamnolipid solution and Tea soaponin solution respectively.The experimental results showed that the change in concentration of the surfactants had asignificant effect on bubble attachment behavior. The time at which bubble attachment occurswould decrease or increase with the increase of the concentration, but the time of thethree-phase contact formation （tTPC） was remarkable extended when the concentration reachedthe CMC of the surfactant. The time of the three-phase contact formation was155ms whenthe larger bubble began to attach to the surface of the polypropylene plate in1×10-5mol/LSDS solution, but that time reduced to140ms when the concentration was3×10-5mol/L. Theconcentration of SDS solution increased only2×10-5mol/L, whereas the tTPCwas reduces byapproximately10%. Under the same conditions, the time of the formation of bubbleattachment changed form190ms to835ms with the increase of concentration of the Teasaponin solution from1×10-5mol/L to3×10-5mol/L, and then the tTPCincreased by320%. Asthe concentration of surfactant increased to its CMC, the three-phase contact time wassignificantly prolong. During the experiment, the tTPCreached more than300ms under CMC of SDS solution. Furthermore there was no attachment occurred within the time of normalobservation when the concentration of Tea saponin solution reached its CMC.The study also showed that biosurfactant affected in such significant degree of thebubble attachment and the effect was stronger than the chemical surfactant SDS. Unlikehydrophilic plate, there would form a three-phase contact interface and the attachmentoccurred during collisions of the flotation bubbles with hydrophobic surface due to thepresence of microbubbles hydrophobic attraction. As we know, the more biosurfactant weadded, the more stable liquid film which was intermediate between the solid surface and airbubbles would be. The time of the TPC formation was805ms in the3×10-5mol/L Tea saponinsolution, much longer than in the case of pure water（157ms）, Rhamnolipid solution（125ms）and SDS solution（140ms） at the same concentration. The biosurfactant significantly hinderedthe formation of TPC. In the three kinds of surfactants, at the concentration of CMC, therestill would appear TPC phenomenon only in SDS solution and Rhamnolipid solution, butthere was nothing happened in Tea saponin solution within the observation time. By contrast,the biosurfactant Tea saponin is the most significant impediment to the formation of TPC.Finally, this study verified that the bubble size was also an important factor affecting theformation of attachment. It was documented that bubbles with the smaller size facilitatedattachment at the surface of polypropylene. The tTPCof the bubbles with the larger size at thesurface of polypropylene in pure water is158ms, while the attachment of small-sized bubblesneed only97ms. After the addition of biosurfactant the Tea saponin with concentration of3×10-5mol/L, the bubbles with larger size took805ms to form the three-phase contact.However the tTPCdecreased significantly to140ms as the bubble diameter decreased. At thesame time, the small-sized bubbles needed120ms and111ms to form the attachment inRhamnolipid solution and SDS solution when the concentration is also3×10-5mol/L. |
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