| There are a lot of heavy metals, natural organic matters and clay particles in natural water. It has been paid more attention to the contaminants for their polluting hazard and poor biodegrability, especially, combined pollution caused by the interactions between pollutants. Adsorption is a common action on the solid liquid interface, which is important to form transformation and migration of heavy metals in water. Therefore, it is essentially to study on the adsorption properties and relationships between aqueous pollutants in theory and actual in practice.Synthetizing references, relationships of humic acid (HA), kaolin particles and heavy metals including Cr, Cu and Zn were studied. The adsorption properties and complexing constants of metal ions and HA were investigated to define the species transformation of metal elements. And the difference of two forms of HA combined with kaolin was discussed. Furthermore, the influence of dissolved HA on the metal adsorption onto kaolin was explained mainly.The main contents and results are following:1. Adsorption/desorption properties of particulate HA for Cr2O72-, Cu2+ and Zn2+ were studied by static adsorption method and mechanism under different conditions was analyzed. The results showed that the reducibility of HA in process of Cr2O72-adsorption was the control step, while the mechanism of Cu2+/Zn2+ adsorption onto granular HA was related to pH. The adsorption force was ion exchange in acidic solution and complexation in neutral condition. Three kinetic models were used to fit the experimental data, and it was found that the pseudo-second-order equation was most appropriate to describe the adsorption kinetics with different initial concentrations and s. The saturated adsorption uptake of Cr(VI), Cu2+ and Zn2+ were 0.77 mg/g,8.59 mg/g and 1.52 mg/g, respectively. With increasing concentration, the saturated amount adsorbed increased but the adsorption rate declined, which meant that it needed more time to reach the same percentage in high concentration. In addition, the equilibrium adsorption amount and rate constant increased with increasing temperature, which indicated the adsorption mechanism. Increasing temprature favored for the adsorption proceeding.2. Combined with the influence of different experimental condition on the adsorption of kaolin and the two basic forms of dissolved HA, the reaction mechanism was studied. The results showed that the adsorption took place mainly by the complexation between the charged groups on kaolin surface and carboxyls of HA in pH range 4-7. Above pH 7, the complexing ability of carboxyl was weaker than that of hydroxy because of electrostatic repulsion, so the form of salicylic acid (SA) became dominant rather than the form of phthalic acid (PA). The results of adsorption thermodynamics indicated that the adsorption could be described well by Freundlich model and the adsorption process was favourable. The positive AH indicated the endothermic process and its value was about 40 kJ/mol accounted for the surface complexation. Additional salt ions, e.g. Na+, Ca2+ and Mg2+ were also influential to HA removal, which accured by compression of double charged, flocculation and bridging effect.3. The adsorption mechanism was concluded based on the study of effect of HA on Cr(VI) adsorption onto kaolin. The results showed that in the kaolin/Cr2O72-/HA ternary system, the adsorption mechanism of kaolin for Cr(VI) actually included complexation between Cr2O72- and SOH2+, as well as reduction reaction, by which Cr2O72- was reduced to Cr3+ by HA molecule. The whole adsorption process could be simulated by the Langmuir adorption model. Both the Van der Waals force, hydrogen bond force and hydrophobic bond force and the increase of entropy of unadsorbed Cr in solution were the driving force for the adsorption process. The inhibited effect of ionic strength on adsorption was remarkable because HA molecules could be flocculated and thereby reducibility was weakened. So the Cr removal decreased. The order of salt ions affected the adsorption percentage of Cr(VI) was Mg2+>Ca2+>Na+ which fact confirmed the non-specific adsorption system and the main adsorption force was surface electrostatic attraction.4. Based on the static adsorption method, the complexing stable constants of Cu-HA and Zn-HA were determined and the precipitation interaction was investigated. By studying the influence factors on the adsorption, the mechanism of HA affecting the Cu2+/Zn2+ adsorption on kaolin was revealed. The results showed that the increasing amount of HA in solution declined the metal cations adsorption obviously due to the formation of dissolved complex. On the same condition, the complexing stable constants of Cu-HA and Zn-HA were 2.54 and 1.85, respectively. Through the study of precipitation interaction between HA and metals, the precipitation mechanism was concluded. It is found that the stability of HA-metal complex was related to the holding capacity of HA for metals. The precipitation mechanism was comprehensive, including ion exchange, complexing reaction and coprecipitation, etc. In low concentration, dissolved HA-metal complex was the primary reason to decrease metal adsorption. The pseudo-second-order model best described the adsorption behavior of Cu2+/Zn2+ onto kaolin particle in planar and ternary system. It is indicated that the existing of HA in solution didn't change the adsorption mechanism. |
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