Mechanistic insights into the interfacial adsorption behaviors of Cr(VI) on ferrihydrite: Effects of pH and naturally coexisting anions in the environment

Interfacial interaction of hexavalent chromium (Cr[VI]) with ferrihydrite (Fh) plays a key role in the behavior of Cr(VI) in the environment. In this study, H2PO4-, SO42-, NO3-, Cl-, and HCO3- were chosen as coexisting anions to explore their inhibition of the capacity of Fh to adsorb Cr(VI). We employed X-ray diffraction, scanning electron microscopy, attenuated total reflection Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy to thoroughly characterize Fh reaction products before and after adsorption of Cr(VI). The results clearly revealed that pH has a marked effect on the extent of Cr(VI) adsorption onto Fh, and this process is also highly dependent on the types of anions present. H2PO4- exhibited the most evident inhibition of Cr(VI) adsorption, even at low concentrations. Similarly, the inhibition of Cr(VI) adsorption by HCO3- increased markedly with increasing pH. In contrast, SO42- only slightly competed with Cr(VI) for reactive Fh surface sites. The anions Cl- and NO3- exhibited almost no inhibitory effect on Cr(VI) adsorption. The differential order of adsorptive affinity of all six anions for Fh was as follows: H2PO4- > HCO3- > SO42- ≈ HCrO4- > NO3- ≈ Cl-. Based on these results, we further provide mechanistic insights into the complexities of Cr(VI) adsorption/desorption behaviors on Fh surfaces. Using Fh as a geosorbent, these interfacial properties could be exploited to mediate the immobilization and release of chromate from and/or into contaminated environments such as aquifers.

Effect of molecular structure on the adsorption affinity of sulfonamides onto CNTs: Batch experiments and DFT calculations

In order to investigate the adsorption behaviors of sulfonamides onto hydroxylated multi - walled carbon nanotubes (CNTs) with a porous structure and large specific surface area, six typical sulfonamides including sulfanilamide (SAM), sulfamerazine (SMR), sulfadimethoxine (SMX), sulfadiazine (SDZ), sulfamethazine (SMT) and sulfametoxydiazine (SMD) were selected to be adsorbed respectively on CNTs, and in the same time the structural parameters of the six sulfonamides molecules were calculated according to the density functional theory (DFT). Based upon above mentioned experiments and the structural parameters, the quantitative correlation between the structural parameters of sulfonamides molecules and their adsorption affinity (e.g. adsorption capacity and adsorption rate constant) onto CNTs was established, respectively. The adsorption data of sulfonamides fitted well with the pseudo - second - order kinetic model and the Langmuir isotherm model. The order of both pseudo - second - order kinetic constant and maximum adsorption capacity of the six sulfonamides were SAM < SMR < SMX < SDZ < SMT < SMD. The frontier molecular orbital energy (E_HOMO) and dipole moment (μ) could be used as indicators for the adsorption affinity of sulfonamides onto CNTs. Accordingly, the possible adsorption mechanism was proposed.