Thiol Modification For Natural Graphite Particles: Its Effect On Deposition Of Silver And Absorption Properties Of Particles

Since the thiol group has strong affinity to some heavy metal ions such as Ag and Pb, the thiol-functioned materials could be widely used to prepare silver-coated composite particles and remove the heavy metals from aqueous solution. This thesis aiming at the problems of existing silver-coated natural graphite particles(NG@Ag), such as incompact shell, separate Ag particles, high silver content and density and of the existing absorbent for Pb(?), such as the low absorption rate and hard separation, developed a method to achieve dense thiol groups on NG, which enabled us to obtain thin and compact Ag shell on NG and synthesize a kind of absorbent with high adsorbing capacity and effectiveness and easy recovery. We discussed the possible formation mechanism of thin and compact silver shells of the composite particles and the electrical conductivity of epoxy resin conductive adhesives(ECAs) which contain them. We had a discussion on the adsorption behavior of Pb( ?) on MPTMS-functioned natural graphite particles(MNG) with different density of thiol groups and the effect of pH values on the adsorption capacity and the adsorption efficiency at different cycles for MNG. The major results are as follows:Firstly, we obtained high density of thiol group(NSH) on the surface of NG which contains low hydroxyl group. The influence of reaction parameters(including concentration of MPTMS, H2 O and ammonia) on NSH were systematically investigated. It is found that water in the presence of ammonia could not only promote the condensation of free hydrolyzed 3-mercaptopropyltrimethoxysilane(MPTMS) molecules with the hydroxyl groups on the NG surface, but also enhance the self-polymerization of them with the MPTMS anchored on the NG surface. This can benefit to form high density of thiol group(NSH) on the surface of NG. Besides, the NSH can be controlled by changing the volume ratio of H2 O to MPTMS.Secondly, we obtained NG@Ag composite particles with compact shells and controllable thickness by simply tuning the density of thiol group(NSH). We derived the relationship between NAg and the minimum thickness(h) corresponding to a compact shell of the NG@Ag core-shell composite particle. Then the effect of shell thickness of NG@Ag composite particles on the electrical conductivity of ECAs containing them was also illustrated. Besides, a high value of NSH can subsequently create dense Ag nuclei and produce thin and compact silver shells on the NG surfaces. Compared with other prepared nonmetal@Ag core-shell composite particles, the resultant NG@Ag core-shell composite particles obtained here have a low Ag content of 32 wt%, low density of 3.06 g/cm3 and high conductivity of 1.7×106 S/m. Moreover, the as-obtained NG@Ag particles as conductive fillers have advantages of high conductivity, low filling volume percentage and low costs, which can be widely used in the fields of conductive materials and electromagnetic shielding materials.Lastly, the NG particles with higher specific surface area were chosen to be functioned by MPTMS molecules to obtain MNG with different density of thiol groups. It is found that the Langmuir adsorption model agrees well with the experiment data and the maximum adsorption capacity for Pb(II) removal can be up to 55.67 mg/g and 5.89 mg/m2 at pH ~7. The MNG can effectively reduce the Pb(II) concentration from 1 mg/g to 0.012 mg/g well below the acceptable limits in drinking water standards(0.015 mg/g) in a few minutes. The value of the adsorption rate constant k2 was determined to be as high as 3.07 g mg-1 min-1, which further indicates fast removal ability of MNG. The MNG can be easily separated from aqueous solution because they have a micron-sized diameter. In addition, the absorption ability of the absorbent performs best in neutral solution. After three cycles, its absorbtion efficiency remains at more than 90%, which suggests it has a long service life. This kind of absorbent not only shows the advantage of low cost, easy synthesis and easy recovery but also has high adsorbing capacity and fast removal ability.