Acridine orange (AO) is a cationic fluorescent dye commonly used in DNA analyses. Extensive studies were conducted for its metachromasy under different solution concentrations and different amounts of AO sorbed on a solid surface. Meanwhile, for the safe disposal of wastewater, AO removal from water using different materials was also evaluated extensively. Clay minerals, due to their large specific surface area, high cation exchange capacity, and vast reserves, have been evaluated as potential sorbents for the removal of a variety of different types of contaminants, including color dyes. In this study, the sorption of AO on different types of clay minerals was contrasted. The sorption of co-presenting Zn2+ was much less than the sorption of AO, suggesting that clay minerals have higher affinities for AO in comparison to inorganic Zn2+. The desorption of exchangeable cations was linearly related to AO sorption, and the amounts of AO sorbed were close to the CEC values of the minerals, confirming that cation exchange is the dominating mechanism for AO sorption. Molecular dynamics simulation results showed that, under low and high AO loading levels, the sorbed AO formed monolayers and bilayers on the mineral surfaces of non-swelling clay minerals, except halloysite, as well as in the interlayer of swelling clay minerals, due to its relatively large dimer constant in solution. Overall, clay minerals are good candidates for the removal of cationic dyes from solution even in the presence of competing inorganic cations.
The increased use of color dyes in industry imposes a great threat to the environment. As such, developing cost-effective techniques for dye removal from wastewater attracted great attention. Earth materials, particularly those with large specific surface area (SSA) and high cation exchange capacity (CEC), were evaluated for their potential use for wastewater treatment. In this study, palygorskite, sepiolite, and clinoptilolite were evaluated for their removal of cationic dyes using safranin O (SO+) as a model compound. The CEC values of the materials played a key role in SO+ removal while other physicochemical conditions, such as temperature, equilibrium solution pH, and ionic strength, had less influence on SO+ removal. Sorbed SO+ cations were limited to the external surfaces of the minerals, as their channel sizes are less than the size of SO+ cation. Molecular dynamic simulations showed dense monolayer SO+ uptake on palygorskite due to its relatively large CEC value. In contrast, loosely packed monomer SO+ uptake was adopted on sepiolite for its large SSA and low CEC. Dense multilayers or admicelles of SO+ formed on zeolite surfaces. As such, for the best SO removal, palygorskite is better than sepiolite, though both are fibrous clay minerals.
The extensive use of color dyes in modern society has resulted in serious concerns of water contamination. Many organic dyes bear charges; thus, materials of opposite charges have been tested for sorptive removal. However, the results from several studies also showed that anionic dyes methyl orange (MO) and alizarin red S (ARS) could be removed from water using minerals of negative charges, but the mechanisms were not addressed. In this study, negatively charged clinoptilolite was tested for its removal of anionic dyes MO and ARS from water under different physico-chemical conditions and to investigate the mechanism of Mo and ARS removal. The sorption capacities were 166 and 92 mmol/kg for MO and ARS, respectively, confirming the uptake of anionic dyes on negatively charged framework silicates. The influence of solution pH and ionic strength on MO removal was minimal, indicating the strong affinity of anionic dyes for clinoptilolite in comparison to other inorganic species. It was speculated that the N in the dimethyl group may bear a partial positive charge, which may have a net electrostatic attraction to the negatively charged mineral surfaces for MO sorption. For ARS, sorption may involve hydrogen bonding formation between the dye and the clinoptilolite. Moreover, under the experimental conditions, the MO molecules form dimers in solution via dimeric π-π interactions. Thus, the sorption of the dimers or aggregation of the MO monomers and dimers on clinoptilolite surface was attributed to additional MO removal, as suggested by molecular dynamic simulations. The speculation was supported by FTIR analyses and molecular dynamic simulations. As such, negatively charged Earth materials may be used as sorbents for the removal of certain anionic dyes via sorption, a new perspective for the innovative use of Earth materials.
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