| The widespread use of copper pesticides, and feed additives, as well as mining, electroplating,leather tanning, metal finishing, and chromate preparation industries release chromium effluentshave caused serious Cu-Cr compound pollution. Copper exists in the form of heavy metal cations,whereas chromium exists in the form of water soluble oxyanions (Cr6+) or hydroxide (Cr3+). Thecoexistence of the two elements can influence their own migration and transformation. Theadsorption of heavy metals on clay minerals, which exist extensively in surface rocks and soil,influences not only the bioavailability but also the migration, transformation, and fate of heavymetals in the environment. Nevertheless, limited research has been published on Cu-Cr combinedpollution. The difference and mechanisms of copper and chromium adsorpion on various clayminerals in single and binary solutions were less reported as well. Therefore, this study aims toexplore the adsorption behaviors and mechanisms of copper and chromium adsorpion on kaoliniteand montmorillonite, as well as to discuss the effect of co-existing ions on adsorption. The effects ofcontact time, concentrations, solution pH, ionic strength and humic acid content on adsorption wereinvestigated. The results of this study are expected to ascertain environmental chemical behavior ofheavy metals in soil and to serve as a theoretical basis for remedying heavy-metal contaminatedsoil.The main results are as follow:1. All results indicated that the adsorption capacity of copper or chromium on montmorillonite inboth single and binary solutions were stronger than that on kaolinite. The adsorption capacity ofcopper or chromium in single solution was qCr(III)> qCu(II)> qCr(VI).2. In single Cu and Cu-Cr binary solutions, our results showed that the adsorption behavior ofCu(II) on two clay minerals under different valence of chromium were qCu> qCu Cr. This indicatedthat Cr(III, VI) in the solution could reduced the adsorption of Cu(II) on both clay minerals.Moreover, the different valence of chromium iinhibitory effect on Cu(II) adsorption in two mineralswere difference. Our research found that Cr(VI) had stronger inhibitory effect on kaolinite, whilemontmorillonite inhibiting strongly by Cr(III). The adsorption of Cu(II) in both two Cu-Cr binary solutions were qCu Cr(VI)> qCu Cr(III)mostly.3. The results showed that the adsorption behavior of Cr(VI) on two clay minerals in single andbinary solutions mainly be qCr(VI)> qCr(VI) Cu. Therefore, Cu(II) could also inhibited the adsorptionof Cr(III) on both clay minerals. However, Cu(II) could promoted the adsorption of Cr(VI) onmontmorillonite only at the condition of pH≥6.0or0.1mol L1NaCl. In addition, both increasingionic strength and humic acid content would abated the inhibition of Cu(II) on Cr(VI) adsorption.As for Cr(III)-Cu(II) system, Cu(II) could promoted the adsorption of Cr(III) on kaolinite underdifferent pH and ionic strength, while decreased Cr(III) adsorption on montmorillonite. However,the promoting effect would be decreased with pH increasing or decreased ionic strength. In twohumic acid-clay mineral systems, Cu(II) could inhibited the adsorption of Cr(III) as well.4. Effects of Cr(III, VI) on adsorption behavior of Cu(II) in different factors were almost thesame. In both single and binary solutions, the adsorption of Cu(II) on the two clay minerals firstincreased and then decreased with the rising of pH. And at pH4.0~5.0had a significant influenceon Cu(II) adsorption (P <0.05). The adsorption capacity of Cu(II) mostly increased with rising ionicstrength, the maximum (qmax) was reached at0.001mol L1, and then the adsorption capacitydecreased. Humic acid could reinforced two clay minerals adsorbed on Cu(II), and qmaxappeared inhumic acid was20%. Only in the Cu(II)-Cr(VI) binary solution, a small amount of humic acidcould decreased Cu(II) adsorption.5. In all factors, there were some difference on adsorption behavior of Cr(VI) or Cr(III) in singleand binary solutions influenced by Cu(II). In single Cr(VI) solution, the adsorption capacity onkaolinite increased initially and then decreased with increasing pH. While a decreasing trend wasobserved for Cr(VI) adsorption on montmorillonite. In the binary solution, the adsorption of Cr(VI)on both minerals were on the contrary. In Cr(III) single and binary solutions, pH and Cr(III)adsorption on kaolinite was significantly positive correlation (P <0.05), whereas the trend onmontmorillonite was observed increased initially and then decreased. Nevertheless, the adsorptionof Cr(VI, III) mainly decreased with ionic strength rising, and increased with increasing humic acidcontent in both single and binary solutions. The influence of the adsorption rate of copper andchromium under different factors mostly were humic acid> pH> ionic strength.6. The dynamics results indicated that the adsorption process of copper or chromium on kaoliniteand montmorillonite in both single and binary solutions was fast at the beginning and then slowdown. Nearly90%of the adsorption were completed in the rapid adsorption and adsorptionequilibrium was observed within120min. The pseudo-second-order model showed the highestagreement with the adsorption of copper or chromium. And the film diffusion process was therate-limiting step, which mainly occurred at the edge and surface of clay minerals. Copper orchromium adsorption on two clay minerals were well fitted with the Freundlich and Langmuir isotherms.7. The adsorption mechanism of heavy metals on montmorillonite occurs not only ion exchangeand chemical adsorption but also entered the interlayer space. By contrast, hydrogen bondingbetween the layers of kaolinite reduces the inaccessibility of heavy metal into the layers, thereforethe adsorption process mainly occurs at the surface of kaolinite. Under different pH, ion exchangeadsorption was the primary mechanism for copper or chromium adsorption on montmorillonite andkaolinite, while coordination was the secondary mechanism for adsorption.In addition, theadsorption of Cu(II) on kaolinite in different ionic strength was specific adsorption, andmontmorillonite was non-specific adsorption on contrast. However, the adsorption of Cr(VI) andCr(III) on two clay minerals were specific or non-specific adsorption, respectively. Humic acid wasadsorbed by kaolinite mainly through ligand exchange mechanism.Whereas montmorillonite wasmainly by the weak forces, such as cationic key bridge, hydrophobic, hydrogen bonding and so on.Therefore, the humic acid had larger influence on kaolinite adsorption than that on montmorillonite. |
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