Influence Of Aggregates Morphology And Algal Organic Mateirals On The Efficiency Of Modified Clays Deployed In Mitigation Of HAB Organisms

Modified clay was a promising method for mitigation of harmful algalblooms.The morphology of flocs and algal orianic materials (AOMs) were key factorsaffecting the removal efficiency (RE) of modified clays for HAB organisms. Thisstudy was conducted to investigate the effects of morphology of aggregates andAOMs on HAB cells removal using modified clays. Kaolinite modified withpolyaluminium chloride (PAC) was used as modified clay model substances. REs ofdifferent HAB species were determined and the fractal dimensions of HAB cell,clay-clay and algae-clay units, were assessed by non-intrusive optical sampling anddigital image analysis techniques, in different experiments, respectively. Experimentswere also carried out to study the mutual effect between AOMs and modified calys,for better understanding of the mechanisms and factors affecting the adsorption ofAOMs onto modified clays and the effect of AOMs on the coagulation rate and flocsformation of modified clays. Furthermore, the relation between the fractal dimensionsand REs, AOMs and REs, were analyzed, respectively. The main results were asfollows:The morphological characteristics of clay and algal cell were studied in this test.The results showed that the fractal dimensions of clay-clay flocs were affectedmarkedly by modifier dosage, pH and ionic strength. Compared with original clay, theflocs of modified clay was with a higher fractal dimension, which reflected the morecompact flocs. The optimum mass ratio between clay and modifier was10:1. Theflocs fractal dimension increased with increasing ionic strength, where the similartrend was found at pH<7, but the opposite was true above pH7. Thus, the optimumpH range for clay-clay aggregation was6-7. Furthermore, the fractal dimension was abetter indicater of the instant variation of flocs morphology. The fractal dimension ofalgal cell was determined by the algal species and its growth phase, e.g. the cells of P.donghaiense and S.costatum with a higher fractal dimension were in theirexponential phase, while H.akshiwo was in its stable phase, and the values were1.6785,1.7766and1.6757, respectively.In order to investigate the mutual effect between AOMs and modified calys, weused sodium alginate and amino acid as AOMs different component model substances.The adsorption of AOMs on modified clays was studied as a function of modifierdosage, contact time, solution pH and ionic strength through batch adsorptionexperiments. Kinetics revealed that the AOMs adsorption rate was described well by apseudo-second order model. Modifier effectively enhanced the adsorption capacity ofkaolinite and increased the adsorption rate, and the optimum additive amount ofmodifier was5%. The adsorption thermodynamics for AOMs onto modified clayssuggests that AOMs adsorption is a spontaneous process. The experimental data fittedboth the Freundlich and Langmuir adsorption equations well for alginate and was welldescribed by C isotherm curve for amino acid. The adsorption of AOMs ontomodified clays was highly dependent on pH and ionic strength. The effect of AOMson the coagulation rate and flocs formation of modified clay was also investigated.The flocculation kinetics of modified clay particles at various concentrations ofsodium alginate solutions were studied. And the flocs fractal dimension wascalculated by image analysis software. Our results indicate that the coagulation rate ofmodified clay and the fractal dimension of flocs were lower than that of control whenthe concentration of sodium alginate is below10mg/L. In the range of10-100mg/L,sodium alginate can improve the coagulation rate of modified clay; the flocs fractaldimension and flocs strength increased steadily to a peak and then decreased. Theoptimum concentration of sodium alginate was50mg/L, along with the maximumcoagulation rate of44.13%and the flocs dimension of1.6823respectively. However,the flocs fractal dimension and coagulation rate decreased when the concentration ofsodium alginate was higher than100mg/L, resulting in subsequent increase of theflocs voidage. For amino acid, in a certain concentration rang, the fractal dimensionsof flocs increased with the concentration of amino acid increasing, which reflectedmore compact and lower voidage flocs. In addition, the fractal dimension of modifiedclay was higer than original clay’s.Finally, the morphology of algae-clay and its effect on the RE of HAB organisms,the variation characteristics of AOMs in algal growth phase and its effect on REs,were studied. The fractal dimensions of algae-clay and REs were both increase with the increase of modifier dosage. The relation between the fractal dimensions and REssuggested that the fractal dimension can effectively indicate the variation of REs. Theimpacts of different component of algal suspensions were different. The REs ofA.tamarense cell in suspensions by modified clays in different growth pahse were: lagphase> decline phase>stable phase>exponential phase. For cell resuspensions, thelowest RE was in exponential phase, however, there was no significant differencebetween the phases. The different component of AOMs, such as TON, TOP, TOC,showed different trend with A.tamarense growth. The relation between AOMs and dRE,which was the difference between REs of algal cell resuspension and algal suspension,was analysed and the result showed that there was no significant relation betweenTON and dRE, but a stronger positive correlation between the lower concertration ofTOC (≤1.0mmolC/L) and dRE. It indicated that a lower concertration of TOC wouldreduce the flocculation effect of modified clay on HAB organisms.