•Np(V) sorption on Na-bentonite is studied under various physicochemical conditions.•We model the sorption of Np(V) on Na-bentonite with CCM model.•Increasing temperature will not influence the sorption of Np(V) on Na-bentonite.•HA promotes Np(V) sorption at low pH values while restricting it at high pH values.•The sorption consists of ion exchange and inner-sphere complexation.Effects of ionic strength, pH, temperature, humic acid (HA) and adsorbate concentration on Np(V) sorption to Na-bentonite were investigated in detail. The sorption of Np(V) to Na-bentonite was significantly dependent on pH and independent of temperature. In the presence of HA, Np(V) sorption was enhanced significantly at low pH; while an obvious negative effect was observed in higher pH range. Np(V) sorption followed the Freundlich isotherm indicating a heterogeneous sorption of Np(V) on the surface of Na-bentonite. The surface complexation model (SCM) suggests that sorption of Np(V) on Na-bentonite was mainly dominated by ion exchange at low pH values, and two dominant monodentate inner-sphere complexes of ≡ SiONpO20 (logK = − 4.55) and ≡ AlO(NpO2OH)− (logK = − 13.80) contributed to Np(V) sorption on Na-bentonite over high pH range.The sorption species of Np(V) on Na-bentonite as a function of pH.

Oxidation debris (OD) and graphene oxide (GO) before and after OD removal were characterized by Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, mass spectroscopy, X-ray diffraction, transmission electron microscopy and potentiometric titration, respectively. OD removal decreased GO absorption intensity in UV/Vis spectra, caused changes in peak position and absorption intensity in FTIR spectra, and resulted in the decrease of ID/IG in Raman spectra. OD was amorphous and had higher content of acidic groups than purified GO. OD contributed 10–25% of overall surface charge density to unpurified GO in spite of small amount (ca. 1% mass). OD removal decreased significantly GO dispersibility in aqueous solution, but increased obviously the electrical conductivity of reduced graphene oxide (rGO) and the apparent density of compacted rGO. The removal of OD was necessary because of its striking effects on both GO spectroscopic and macroscopic properties. Batch desorption in NaOH solution was recommended for OD removal from as-prepared graphite oxide because of slow OD desorption kinetics.

Humic substances (HS) substantially affect heavy metal (M) adsorption on mineral surfaces. However, quantitative descriptions of ternary systems involving M, HS and mineral surfaces remain unclear. This study examines adsorption in a model ternary system including Eu(III), fulvic acid (FA) and silica, and describes the adsorption of Eu(III) and FA by combining a double-layer model (DLM) and the Stockholm humic model (SHM). SHM explains the binding of H+ and Eu3+ to FA and the DLM for FA and Eu(III) adsorption on silica. Experimental results showed that the presence of FA promotes Eu(III) adsorption at acidic pH values, but decreases it at basic pH values, which indicates the formation of ternary surface complexes. Modeling calculations have shown that two ternary surface complexes are required to describe the experimental results in which Eu3+ acts as a bridge between the surface site and FA. The present study suggests that the discrete-site approach to HS is a promising method for interpreting the adsorption data for M, HS and mineral ternary systems.

•Experiment data for the interactions between SA with Cu(II) at the surface of MWCNTs.•π–π Bonds sites perform quantitative modeling of adsorption by DLM model.•Cu(II) and SA sorption to MWCNTs can be well described with the DLM model.•The interactions with SA/Cu(II)/MWCNTs can be well predicted with the DLM model.Batch experiments were conducted to elucidate the adsorption of a widely used salicylic acid (SA) and copper(II) on oxidized multi-walled carbon nanotubes (MWCNTs) as a function of pH and concentration. X-ray photoelectron spectroscopy and potentiometric titration were performed to study the concentration at the surface oxygen-containing functional groups of them. π–π Bond's sites were evaluated by the Dubinin–Ashtakhov model. Data analysis indicated that the adsorption mechanism was mainly due to chemical interaction among copper(II) and SA and the surface functional groups of them. The adsorbed SA on them leads to the modification of their surfaces and partial complexation of copper with the SA adsorbed on them and thus had a positive effect on the adsorption of copper. Copper was also shown to increase SA adsorption from pH 3 to 7. The surface complex model was used to describe the surface charge density, as well as adsorption equilibrium. The copper removal was due to adsorption of surface–metal complexes, while the SA uptake resulted from the formation of surface–organic complexes. Furthermore, when they coexisted in the system, it appeared that the surface/metal/organic chemical complexes were the main forms.

U(VI) sorption on kaolinite was studied as functions of contact time, pH, U(VI) concentration, solid-to-liquid ratio (m/V) by using a batch experimental method. The effects of sulfate and phosphate on U(VI) sorption were also investigated. It was found that the sorption kinetics of U(VI) can be described by a pseudo-second-order model. Potentiometric titrations at variable ionic strengths indicated that the titration curves of kaolinite were not sensitive to ionic strength, and that the pH of the zero net proton charge (pHPZNPC) was at 6.9. The sorption of U(VI) on kaolinite increased with pH up to 6.5 and reached a plateau at pH >6.5. The presence of phosphate strongly increased U(VI) sorption especially at pH <5.5, which may be due to formation of ternary surface complexes involving phosphate. In contrast, the presence of sulfate did not cause any apparent effect on U(VI) sorption. A double layer model was used to interpret both results of potentiometric titrations and U(VI) sorption on kaolinite.

The sorption of U(VI) and phosphate on γ-alumina was investigated in binary (phosphate/γ-alumina; U(VI)/γ-alumina) and ternary (U(VI)/phosphate/γ-alumina) systems as functions of contact time, pH, ionic strength, solid-to-liquid ratio and U(VI) and/or phosphate concentrations by using a batch experimental method. It was found that the sorption of phosphate on γ-alumina increases with pH from 2.5 to 5.2 and then decreases with pH from 5.2 to 9.4. The sorption of phosphate on γ-alumina is insensitive to ionic strength. On the other hand, the sorption of U(VI) on γ-alumina increases with increasing pH over the range of 4–6. The sorption of U(VI) on γ-alumina increases slightly with decreasing ionic strength. In the ternary sorption system, it was found that the presence of phosphate increases the sorption of U(VI), whereas the presence of U(VI) has little effect on the sorption of phosphate. The sorption of U(VI) and phosphate in binary and ternary systems were interpreted in terms of surface complexation models. The effects of γ-alumina dissolution and CO2 in the sorption systems were considered in modeling calculations. Four surface complexes of phosphate, XOHAlHPO4+, XH2PO4, XHPO4− and XPO42−, and two surface complexes of U(VI), XOUO2+ and XOUO2(OH)2−, were respectively used to reproduce the sorption of phosphate in phosphate/γ-alumina system and U(VI) sorption in U(VI)/γ-alumina system. The co-sorption of U(VI) and phosphate in the ternary sorption system was interpreted by a model which combines the surface complexation models for the binary sorption systems together in addition to considering the formation of a ternary surface complex, XOUO2HPO4−.

The sorption of Se(IV) on TiO2 (anatase) was investigated as functions of contact time, pH, ionic strength, solid-to-liquid ratio (m/V) and Se(IV) concentration by using a batch sorption method. It was found that the sorption kinetics of Se(IV) can be described by a pseudo-second-order model. Both the amount of Se(IV) sorbed at equilibrium and the pH of aqueous solution affect the rate constant of Se(IV) sorption on anatase. The results at sorption equilibrium indicate that Se(IV) sorption is pH-dependent and insensitive to ionic strength. A constant capacitance model was used to quantitatively interpret Se(IV) sorption on anatase. Inner-sphere surface complexes, SSeO3− and S2SeO3, were considered.

In this paper, the physico-chemical, titration and Eu(III) adsorption/desorption characteristics of a purified Na-bentonite have been measured. Potentiometric titration at three ionic strengths (0.01, 0.1 and 0.5 mol/L NaCl) at 25 °C indicates that the point of zero charge (PZC) of the Na-bentonite increases with decreasing ionic strength. The parallel titration curves at three ionic strengths were interpreted by considering a layer site and two edge hydroxyl sites in the framework of surface complexation model. The adsorption/desorption of Eu(III) on the Na-bentonite was investigated by a batch experimental method. pH adsorption edges of Eu(III) in the absence and presence of CO2 at variable Eu(III) concentrations (6.74 × 10−8 and 3.33 × 10−6 mol/L) were collected and reasonably interpreted by a surface complexation model. Adsorption and desorption isotherms at pHs 4.04 ± 0.05, 6.01 ± 0.05, 6.53 ± 0.05 and 7.46 ± 0.05 indicate that the adsorption/desorption of Eu(III) on the Na-bentonite is reversible. The adsorption isotherms of Eu(III) were successfully reproduced by the proposed model.

•A GC model for Eu(III) adsorption on granite was verified and extended to Am(III).•Temperature effect on Eu(III) adsorption was negligible at 25–80 °C.•No aging effect of freshly crushed granite on Eu(III) adsorption was observed.•Eu(III) was incorporated in calcite when pH value of groundwater was increased.•Biotite was an important mineral for Eu(III) adsorption on granite.Granite has been chosen as a promising host rock for geological repository of high-level radioactive waste in many countries. However, the description of radionuclide adsorption on granite is subjected to its complicated composition and still a challenge. We studied the adsorption of Eu(III) and Am(III) on Beishan granite, a preliminary selection of host rock in China, as a function of pH, adsorbate concentration, ionic strength and the composition of background electrolytes. A surface complexation model was set up using Generalized Composite approach based on Eu(III) adsorption in NaCl electrolyte, supported by X-ray photoelectron spectroscopy and verified by Eu(III) adsorption in the presence of CO2 at PCO2=10-3.58atm and in CaCl2 electrolyte. The model was successfully extended to describe Am(III) adsorption in both NaCl and CaCl2 electrolytes. It was also found that temperature effect on Eu(III) adsorption was negligible at 25–80 °C, that the adsorption of Eu(III) on freshly crushed granite with “new” surfaces and aged granite with “old” surfaces were identical to each other, and that the presence of fulvic acid of 2–20 mg/L significantly declined the distribution coefficients of Eu(III) and Am(III) in high pH range. Electron probe micro analyses indicated that biotite had higher affinity for Eu(III) than other major minerals, such as oligoclase, quartz and orthoclase.Graphical abstractDownload full-size image