*3.8. Regeneration Studies*

The reusability and regeneration of the prepared akaganeite nanorods were evaluated by performing a series of adsorption–desorption cycles (Figure S4). Desorption of the analyte was achieved using 100% acetonitrile. Thereafter, the adsorbent was washed with an ethanol–water solution and dried in an oven at 50 ◦C. The removal efficiency of the nanoadsorbent toward β-estradiol slightly decreased after the 6th usage/regeneration cycle; however, it remained at >90%. These findings suggested that the akaganeite nanoadsorbent could be regenerated and subjected to multiple usages with suitable robustness and reusability properties.

#### **4. Conclusions**

Akaganeite nanoparticles generated via a facile and simple synthesis route using a precipitation technique are highly useful for the adsorptive removal of β-estradiol. A variety of characterization techniques, including XRD, SEM, EDX, and TEM, confirmed the formation of akaganeite nanoparticles that are predominantly rod-shaped. The method was demonstrated to remove β-estradiol at concentrations up to 8 mg L−<sup>1</sup> from a variety of spiked aqueous solutions with removal efficiencies of 93–100%. The adsorption process was characterized using both linear and adjusted non-linear models. The adsorption process followed pseudo-second order kinetics and a Langmuir isotherm model with a maximum adsorption capacity of 97.0 mg g−<sup>1</sup> for the linearized isotherm model. The adjusted non-linear Langmuir isotherm resulted in a maximum adsorption capacity of 102 mg g−<sup>1</sup> , which was comparable to the linear model. The akaganeite nanoparticles also could be reused during up to six adsorption/desorption cycles maintaining an approx. 90% removal efficiency. Hence, a low-cost and environmentally friendly adsorbent was demonstrated, which is a suitable candidate for water treatment via adsorption for hormone-like substances such as β-estradiol.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2227-9717/8/9/1197/s1, Figure S1: FTIR of akaganeite after adsorption; Figure S2: Pareto chart of the standardized relevance of the individual variables affecting the adsorbing performance of the synthesized materials. The red line indicates the 95% confidence interval. The linear interactions of these factors are: 1Lby2L (pH–MA interaction), 2Lby3L (MA–CT interaction) and 1Lby3L (pH–CT interaction); Figure S3: Response surface plots showing the interaction effects of the main parameters pH, adsorbent material mass (MA), and contact time (CT); Figure S4: Desirability function for the main parameters pH, adsorbent material mass (MA), and contact time (CT); Figure S5: Regeneration studies. **Author Contributions:** A.M., B.M. and P.N.N., conceptualization; A.M. and P.N.N., method development; P.N.N., chemometrics; A.M. and A.N., sampling; A.M. and A.N., investigation; A.M., synthesis and characterization, A.M. and A.N., data collection; A.M., writing—original draft; A.M., A.N. and P.N.N., writing—review and editing; P.N.N., resources; B.M. and P.N.N., supervision. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Research Foundation (NRF, South Africa, grant nos. 113010 and 91230).

**Acknowledgments:** The authors would like to acknowledge the Department of Chemical Sciences, University of Johannesburg (Doornfontein Campus) for providing laboratory space and other resources required in this project

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


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