A facile, versatile approach to hydroxyl-anchored metal oxides with high Cr(VI) adsorption performance in water treatment

In this study, a facile and versatile urea-assisted approach was proposed to synthesize Chinese rose-like NiO, pinecone-like ZnO and sponge-like CoO adsorbents. The presence of urea during syntheses endowed these adsorbents with high concentration of surface hydroxyl groups, which was estimated as 1.83, 1.32 and 4.19 mmol [OH−] g−1 for NiO, ZnO and CoO adsorbents, respectively. These surface hydroxyl groups would facilitate the adsorption of Cr(vi) species (e.g. HCrO4−, Cr2O72− and CrO42−) from wastewater by exchanging with hydroxyl protons or hydroxide ions, and hence result in extremely high maximum adsorbed amounts of Cr(vi), being 2974, 14 256 and 408 mg g−1 for NiO, ZnO and CoO adsorbents in the pH range of 5.02–5.66 at 298 K, respectively. More strikingly, the maximum adsorbed amounts of Cr(vi) would be greatly enhanced as the adsorbing temperature is increased, and even amount to 23 411 mg g−1 for ZnO adsorbents at 323 K. Based on the kinetics and equilibrium studies of adsorptive removal of Cr(vi) from wastewater, our synthetic route will greatly improve the adsorptivity of the as-synthesized metal-oxide adsorbents, and hence it will shed new light on the development of high-performance adsorbents.


Introduction
With the accelerated process of modernization, overuse of industrial chemicals, organic compounds and fossil fuels have attracted growing attention [1,2]. Abundant existence of heavy metal ions (e.g. Ba(II), Pb(II), Cd(III), Cr(III), Cr(VI), As(III), As(V), Co(II), Cu(II), Ni(II), Zn(II) and Hg(II)) in water has become a topic of worldwide concern as it brings many severe challenges to the public health and environmental ecosystems [3,4]. Especially, the exorbitant usage and indiscriminate disposal of these metal ions in the metallurgical industry and chemical manufacturing have resulted in serious consequences. Up to now, many methods such as coagulation and flocculation [5], membrane separation [6], chemical precipitation [7], ion exchange and adsorption [8][9][10][11][12] have been used for removing these heavy metal ions from water bodies. Among these methods, adsorption techniques have been proved to be very effective and attractive for the purification of wastewater [13][14][15].
One of the most important factors in designing an adsorbent is the understanding of its adsorption mechanism. With regard to metal-oxide adsorbents, their most accepted adsorption mechanism should be the ion-exchange process reported many times in the literature [11][12][13]16,17]. The surface of metal oxides in aqueous solutions is hydroxylated due to dissociative chemisorption of water molecules, and the surface hydroxyl groups adsorb heavy metal ions from solution by the exchange with hydroxyl protons or hydroxide ions [18]. In this scenario, increasing the amount of surface hydroxyl groups on metal oxides will enhance ion-exchange capacity of these oxides, and hence will improve their adsorption performance in water treatment.
Along this line, a facile and versatile solvothermal approach was proposed in this work to fabricate metal oxides (including NiO, ZnO and CoO) with a high density of surface hydroxyl groups. In water treatment experiments, Cr(VI) was chosen as the adsorbate because of its high toxicity, carcinogenicity, mutagenicity to living organisms and extreme mobility [19,20]. The effluents from certain industries were reported to contain 50-100 mg l −1 of Cr(VI), which is over 1000 times higher than the maximum allowed concentration of standard for wastewater discharge [21]. Based on our kinetics and equilibrium studies of adsorptive removal of Cr(VI) from wastewater, this approach greatly enhances the adsorptivity of metal-oxide adsorbents, and hence will shed new light on the development of high-performance adsorbents.

Characterization
The XRD patterns were recorded on a powder X-ray diffractometer (Rigaku D/max-rA) equipped with a rotating anode and a Cu-K α1 radiation source (λ = 1.5406 Å) at a step width of 0.02°. Scanning electron microscope (SEM) images were collected on a field-emission scanning electron microscope (JEOL JSM-6700F). The iso-electric point (IEP) of the sample was measured by testing the zeta-potential in aqueous solution on a Zetasizer Nano ZS (Malvern Instruments). Surface charge density was determined by conductometric titration method.

Batch adsorption experiments
Solutions with different Cr(VI) concentrations were prepared using K 2 Cr 2 O 7 as the source of heavy metal ions. For the adsorption kinetic study of Cr(VI), 40 mg of adsorbent was added into 60 ml Cr(VI) solution with initial Cr(VI) concentration of 25 mg l −1 for different adsorbing time. To obtain the adsorption equilibrium isotherms, 20 mg of adsorbent was added into a set of 30      NiO

Results and discussion
ZnO CoO The facile syntheses of these NiO, ZnO and CoO adsorbents with well-defined morphologies share a common feature of the usage of urea during their solvothermal reactions. Generally, urea acts as a chelating agent of metal ions and in this way prevents their aggregation [22]. This peculiarity of urea can thus provide fine control of the final morphologies of these metal-oxide adsorbents. Most notably, the urea-involved hydrothermal procedure has been found to exhibit lower Lewis acidity, but higher concentration of surface hydroxyl groups [23]. To verify this point, the surface charge densities of these NiO, ZnO and CoO adsorbents were measured by conductometric titration method. In a typical measurement, 10 mg of the adsorbent was firstly dispersed into 100 ml of de-ionized water and stirred with a magnetic bar. After adding a certain amount of hydrochloric acid, the solution was titrated with sodium hydroxide (5 mmol l −1 ) and meanwhile the conductivity was recorded (figure 2a-c). Figure 2d shows that the calculated surface hydroxyl densities are 1.83, 1.32 and 4.19 mmol [OH − ] g −1 with regard to NiO, ZnO and CoO adsorbents, respectively. These values are extremely large and probably due to the usage of urea in our experiments. In this scenario, Cr 2 O 7 2− /HCrO 4 − anions in wastewater will exchange with these numerous hydroxyl groups to promote the chemisorption process [11,16,17], and hence greatly improve the adsorptive capacity of these adsorbents. It should be mentioned that these surface hydroxyl densities are seemingly irrelevant to the crystallite sizes of NiO, ZnO and CoO, revealing that the assembling manner of primary particles has a critical impact on the number of their surface hydroxyl groups. Figure 3a shows the kinetics of Cr(VI) adsorption onto NiO, ZnO and CoO adsorbents with an initial concentration of 25 mg l −1 at 298 K. Generally, the adsorption equilibrium of Cr(VI) on these metal-oxide adsorbents was achieved within 30 min. The pseudo-second-order kinetic model was used to fit these experimental data (figure 3b), and the obtained parameters are summarized and listed in table 1. The high correlation coefficients (R 2 ) of ca 0.99 indicate a good description of these kinetic data by this pseudosecond-order model. More strikingly, the initial adsorption rates h, which can be determined by h = kq e 2 (where k is the rate constant (g mg −1 min −1 ), q e is the equilibrium adsorption capacity (mg g −1 )), are    figure 4. Most strikingly, the adsorbed amounts of Cr(VI) increase fast at any given Cr(VI) concentration. The Langmuir [24] and Freundlich [25] models were then used to fit these isotherm data, and the fitted parameters are summarized and listed in table 2. The high R 2 -values, being very close to or even equal to unity, indicate that the Langmuir and Freundlich models can both describe the adsorption isotherms in this study. The Langmuir isotherm model provides a unitless constant separation factor R L , defined by R L = (1 + K L C 0 ) −1 [26], wherein K L is the Langmuir adsorption constant related to the affinity of binding sites (l mg −1 ) and C 0 is initial concentration of Cr(VI) (mg l −1 ). The value of R L indicates the types of Langmuir isotherm of irreversible (R L = 0), favourable (0 < R L < 1), linear (R L = 1) or unfavourable (R L > 1). By calculation, R L is between 0 and 1 for all adsorbents at any initial Cr(VI) concentration, indicative of the favourable Langmuir isotherm in this study. According     values rival or even surpass the reported values in the literature [27][28][29], strongly evidencing that our versatile urea-assisted approach is remarkably advantageous in synthesizing hydroxyl-anchored metaloxide adsorbents with high Cr(VI) adsorption performance. On the other hand, the Freundlich model describes the adsorption on an energetically heterogeneous surface on which the adsorbed molecules are interactive. The Freundlich constants K F ((mg g −1 )(l mg −1 ) 1/n ) and n (unitless) are related to the adsorbed amount and adsorption affinity, respectively. The values of 1/n are less than 1 for all Cr(VI) adsorbents, indicating the degree of nonlinearity between the solution concentration and amount of Cr(VI) ions adsorbed [30].
To seek deep insight into the extremely high adsorptive capacity of these hydroxyl-anchored metaloxide adsorbents, IEP values were determined by measuring pH-dependent zeta-potential values in aqueous solution. It is clearly observed from figure 5a that the IEP values can be estimated as 11.2, 9.4, 9.4 for NiO, ZnO, CoO adsorbents, respectively. The pH dependence of their equilibrium adsorption capacity is shown in figure 5b. As is known, Cr(VI) exists mainly in the soluble forms of both HCrO 4 − and Cr 2 O 7 2− at pH from 2.0 to 6.5 [20,26], and HCrO 4 − and Cr 2 O 7 2− anions account for about 80% and 20%, respectively, in the pH range of 2-5 [31]. As the pH value increases above 6.  two aspects. In the first place, desorption and reusability of these adsorbents should be investigated. In our desorption procedure, the NiO, ZnO and CoO adsorbents after Cr(VI) adsorption were firstly centrifuged from solution. After that, 1 mol l −1 NaOH aqueous solution was used to convert the adsorbed Cr(VI) species to soluble Na 2 CrO 4 . Finally, these adsorbents were separated by centrifugation, washed successively with distilled water and absolute ethanol, and dried in the air at 60°C for 4 h. Figure 6 shows the morphologies of these adsorbents after Cr(VI) adsorption and desorption. Obviously, the particle size and morphology of these adsorbents are virtually unchanged, and their chemical compositions (figure 7a-c) are also quite similar to their counterparts before Cr(VI) adsorption. This excellent retention of morphology, size and composition is of great benefit to their reusability. Figure 7d shows the equilibrium adsorptive experiments at 298 K for six cycles of desorption-adsorption process. It is obvious that the equilibrium adsorption capacity renders virtually the same values, manifesting the good reusability of our adsorbents in water treatment. Additionally, the detection limit (or sensitivity) of these adsorbents is of fundamental importance in their applications. Given that the maximum contaminant level of chromium in domestic water supplies has been set by WHO as 50 µg l −1 [12], thus a set of solutions with initial Cr(VI) concentration of 50 µg l −1 were used to perform the kinetics and equilibrium adsorption study once again. It is clear from figure 8 that all of these adsorbents can still exhibit obvious adsorptive performance in 50 µg l −1 of Cr(VI) solution, revealing that their detection limit is much lower than 50 µg l −1 . This is conducive to their practical water purification. Notably, the inset of figure 8 shows CoO adsorbent, being of the highest surface hydroxyl densities (figure 2d), exhibits the least adsorption capacity for low concentration Cr(VI) solutions. This adsorption behaviour differs significantly from the case of 100 mg l −1 , and can   be explained by considering the adsorption mechanism. At first, the negatively charged Cr 2 O 7 2− or HCrO 4 − ions transfer to the periphery of adsorbents by electronic attraction, and then the enriched Cr 2 O 7 2− /HCrO 4 − ions will exchange with the hydroxyl groups [12] on the adsorbent surfaces to promote the adsorption process. However, if the surface hydroxyl density is extremely high in the CoO case, the initial electrostatic adsorption process is greatly inhibited, and therefore the subsequent ion exchange process is restrained. This means the proper amount of surface hydroxyl groups will be of great benefit to high adsorption capacity.

Conclusion
In summary, Chinese rose-like NiO, pinecone-like ZnO and sponge-like CoO adsorbents were synthesized by a facile and versatile urea-assisted solvothermal procedure. Their surface hydroxyl densities were determined to be 1.83, 1.32 and 4.19 mmol [OH − ] g −1 for NiO, ZnO and CoO adsorbents, respectively. The presence of urea endowed the hydrothermal procedure with lower Lewis acidity, but higher concentration of surface hydroxyl groups, which would be conducive to the ion-exchange process between the adsorbents and Cr(VI) species in wastewater. The maximum adsorbed amounts of Cr(VI) were as high as 2974, 14 256 and 408 mg g −1 for NiO, ZnO and CoO adsorbents at 298 K, respectively. Most notably, these adsorbed amounts would be greatly enhanced as increasing the adsorbing temperature. It is anticipated that this work will shed new light on the development of high-performance adsorbents.