Synthesis Of Carboxymethyl Chitosan-conjugated Magnetite Nanoparticles And Their Sorption Behavior For Heavy Metal Ions Pb(?), Zn(?), Cr(?) And As(?)

The pollution of heavy metal ions to water (surface water and groundwater) is increasingly serious and has become a global environmental problem. The development of method and technology about treatment of wastewater contaminated by heavy metals is one of the important subjects in the field of environmental science and engineering. Adsorption is widely used to treat wastewater for the advantages such as convenient operation, high efficiency, low cost and so on. The technology of adsorption has been extensively studied currently and the key is the development of highly efficient sorbents. A magnetic composite formed through carboxymethyl chitosan (CMCS) conjugated magnetite (Fe3O4) nanoparticles (MNPs), which are denoted as CMCS-MNPs, is a new type of magnetic adsorbent developed recently. CMCS-MNPs have good adsorption capacity for heavy metal ions and can be separated from aqueous solution quickly under external magnetic field. So it is convenient for recycle use and has promising application prospects. However, the loading amount of CMCS in CMCS-MNPs is low (-5 wt%) in the current research. Increasing its loading capacity can enhance its adsorption ability and this is important for practical application.The phenomenon of "sorbent concentration effect" (Cs-effect), in other words, the adsorption equilibrium constant (or saturated adsorption amount) is related to the concentration of adsorbent (Cs), is found during the study of adsorption thermodynamics at soild-liquid interfaces and it seems to be contrary to the theory of chemical thermodynamics. Recently, our group developed a surface component activity (SCA) model to describe and explain Cs-effect. Currently, the general applicability of the SCA model need to be verified and the physical significance of model parameters need to be investigated, so did the effects of the medium.We first explored the method to improve the loading amount of CMCS in CMCS-MNPs in this paper. Afterwards, we investigated the adsorption behaviors of CMCS-MNPs towards heavy metals including the cations of Pb(II) and Zn(II) and the complex anions of Cr(VI) and As(V). In particular, the applicability of the SCA model and the influence of various factors on Cs-effect are investigated to increase the understanding of adsorption phenomena and provide the basis for the development of magnetic nano adsorbents and the application of adsorption technique in wastewater treatment.The main research contents and conclusions are listed as follows:(1) We explored the way to increase the loading amounts of CMCS in CMCS-MNPs. Two kinds of CMCS-MNPs with CMCS loading amount 24.7 wt% and 58.8 wt% respectively, which are denoted as CMCS-MNPs-A and CMCS-MNPs-B, were synthesized by covalently binding CMCS onto the surface of the MNPs via carbodiimide activation in a paraffin-acetic acid medium. The loading amounts are separately about 5 and 10 times that of reported value (-5 wt%) in literature. CMCS-MNPs-A and CMCS-MNPs-B were all monodisperse and spherical with a mean diameter of 15 nm. They show good water dispersity and a strong magnetic response. Their magnetic saturation intensity are 62 and 56 emu/g respectively. They are easy to be separated from aqueous solution quickly under external magnetic field.(2) Adsorption of Pb(?) and Zn(?) on the CMCS-MNPs-B in aqueous solutions was investigated and the effects of sorbent dosages (Cs:1-10 g/L), pH (3.0-6.0), electrolyte (NaNO3) concentration (CNaNO3:0.001-0.500 M), and temperature (T:25-55?) as well as competitive adsorption on the sorption were determined. CMCS-MNPs-B shows good adsorption capacity for cations of heavy metals. The adsorption equilibrium is in accordance with the classical Langmuir and Freundlich isotherms and the adsorption process is in accordance with the quasi two stage kinetic equation under a given Cs. The sorption amount of Pb(?) and Zn(?) on the CMCS-MNPs-B increases with increasing pH, but decreases with increasing CNaNO3 or T and both adsorption system are spontaneous and endothermic reaction under the studied conditions. Competitive adsorption can lead to the reduction of their respective adsorption capacity, but the removal rate of Pb(?) is higher than that of Zn(?). CMCS-MNPs has good recycling ability and is a promising magnetic adsorbent in practical application. The apparent Cs-effect phenomenon existed in the adsorption experiment of Pb(?) and Zn(?) on CMCS-MNPs-B. Cs-effect observed in the adsorption system can be described by SCA model and the value of fitted parameter ? (characteristic for the strength of Cs-effect) is about 0.61 L0.5/g0.5. The variation of environmental factors such as pH, CNaNO3 and T and competitive adsorption have no influence on the strength of Cs-effect (y) and this indicated the strength of Cs-effect is mainly determined by the nature of adsorbent and adsorbate.(3) The adsorption of complex anions Cr2O72- and HAsO42-, abbreviated as Cr(VI) and As(V), on CMCS-MNPs-B were investigated. The effects of sorbent dosages (Cs: 1-10 g/L), pH (3.0-6.0), electrolyte concentration (CNaNO,:0.001-0.500 M), and temperature (T:25-55?) as well as competitive adsorption on the sorption were determined. The result is similar to the adsorption for the cations and suggested that CMCS-MNPs-B shows good adsorption capacity for complex anions of heavy metals. The adsorption equilibrium is in accordance with the classical Langmuir and Freundlich isotherms and the adsorption process is in accordance with the quasi two stage kinetic equation under a given Cs. The sorption amount of Cr(VI) and As(V) on the CMCS-MNPs-B increases with increasing T, but decreases with increasing CNaNO3 or pH under the studied conditions. Both adsorption process is endothermic process and driven by entropy. Competitive adsorption can lead to the reduction of their respective adsorption capacity, but the removal rate of As(V) is higher than that of Cr(VI). The apparent Cs-effect phenomenon were also existed in the adsorption experiment of Cr(VI) and As(V) on CMCS-MNPs-B. Cs-effect observed in the adsorption system can be described by SCA model and the value of fitted parameter y is about 0.40 L0.5/g0.5. This value is obviously lower than that of adsorption cations on CMCS-MNPs-B. The variation of environmental factors such as pH, CNaNO3 and T and competitive adsorption have no influence on the strength of Cs-effect (the value of y) and this indicated the strength of Cs-effect is mainly determined by the nature of adsorbent and adsorbate.(4) We investigated the adsorption of Pb(II) behavior on the CMCS-MNPs-A and compared with the results of adsorption Pb(II) on CMCS-MNPs-B to study the effect of CMCS loading amount on the adsorption behaviors. The results show that the loading amount of CMCS has no influence on such adsorption behaviors as the type of isotherm and the dependence of environmental factors (pH, CNaNO3 and T). However, the greater the load capacity of CMCS, the stronger the adsorption capacity (adsorption capacity is larger). Moreover, the adsorption capacity of CMCS is essentially unchanged at per unit mass, indicating that the increase of loading amount of CMCS does not significantly reduce its specific adsorption capacity (the adsorption capacity of unit CMCS). This can increase the utilization efficiency of magnetic particles and has important significance for its practical application. Furthermore, the loading amount of CMCS has no influence on the y value in the SCA model and also indicated that the strength of Cs-effect is mainly determined by the nature of adsorbent and adsorbate.