Removal Of Dissolved Organic Nitrogen In Aqueous Solutions Using Modified Wheat Straw

DON is present in low concentration, is highly hydrophilic and consists of a complicated mixture of components. Although there is a considerable amount of information in the literature concerning the removal of DON, high DON removal efficiency during water treatment has not been previously reported in the literature. Modified wheat straw (MWS) was employed to remove dissolved organic nitrogen (DON) from aqueous solutions. The main objective of this study was to investigate whether MWS could remove DON efficiently or not and to examine its adsorption mechanism. Sorption experiments were carried out as a function of MWS dosage, contact time, initial concentration, pH and temperature. The adsorption mechanism was explored through analysis of sorption isotherms, sorption kinetics and FTIR spectra of MWS. In addition, measures to further improve the DON adsorption capacity of MWS were investigated. This study mainly contain the following parts:(1) Several common sorbents such as activated carbon (AC), Permutit, and ion exchange resins were employed to compare the DON removal efficiency with MWS. Sorption experiments showed that the highest DON removal efficiency (approximately80%for DON1and94%for DON2) could be achieved. MWS has a good potential capacity for DON removal.(2) In this study, the sorption results were analyzed to see whether the isotherm obeyed the Langmuir, Freundlich and Dubinin-Radushkevich isotherm models. Results indicate that some heterogeneity on the surface or pores of the MWS sorbent will play a role in DON sorption. The sorption capacity of the MWS decreases with increasing temperature, indicating the possibility of exothermic sorption.(3) In this study, several kinetic models were used to analyze the sorption rate of DON onto MWS:the pseudo-first-order equation, pseudo-second-order equation and intra-particle diffusion equation. This result indicated that although intra-particle diffusion was involved in the sorption process, it was not the only rate-limiting step. The rate limiting step in the sorption of DON may be chemisorption involving valence forces through the sharing or exchange of electrons between sorbent and sorbate, complexation, coordination and/or chelation.(4) Further analysis of the sorption mechanism was carried out using FTIR spectrophotometry which illustrated the change of functional groups before and after the sorption process. And FTIR analysis revealed that no new functional groups were formed during the sorption process.As a summary of the analysis above, the adsorption mechanism of DON onto MWS was the formation of hydrogen bonds between the N atom of DON and the hydrogen nuclei of-OH in the grafted secondary amine and quaternary amine sections, but the quaternary amine section was redundant. In order to further improve the DON removal efficiency, the modified sorption material must possess functional groups which can interact with the lone pair of electrons on the N of DON.