*3.7. Analytical Figures of Merit and Method Validation*

Analytical method validation has been accounted for irrespective of the applicability of the developed procedure for gaining useful data. Following the optimum experimental parameters, the calibration plot for Hg(II) analysis was obtained in the range of 0.2 to 100 µg L−<sup>1</sup> of Hg(II), with a good correlation coefficient, R<sup>2</sup> = 0.9998. The limit of detection (LOD) and limit of quantification (LOQ), obtained as the concentrations equivalent to three times and ten times of the standard deviation of eleven blank runs, were found as 0.06 µg L−<sup>1</sup> and 0.2 µg L−<sup>1</sup> , respectively [37]. Thus, it allows for the ultra-trace determination of Hg(II) in water samples. The relative standard deviation (RSD) that characterizes the method's precision, evaluated for eleven replicate samples containing 5 µg L−1 of Hg(II), was found in the range of 3.0–4.5%. The validity of the proposed method was observed by analyzing the standard reference material (SRM 1641d). The results are shown in Table 3. The closeness of measured value with the certified values is in good agreement, indicates the accuracy of the developed method. In addition, the spiking analysis with two levels of Hg(II) concentration was carried out using different environmental water samples such as household water (tap), industrial wastewater and river water samples (Table S1). The recoveries of the added amount of Hg(II) were satisfactorily recovered with a 95% confidence limit, and the mean percentage recoveries range between 99.0% to 100.2%, with an RSD value in the range 0.35–2.26%. This suggests the accuracy of the method to preconcentrate the trace analytes in real water samples for accurate determination.

**Table 3.** Analytical method validation by analyzing standard reference material (SRM) after column preconcentration (column conditions: sample volume 100 mL, flow rate 8 mL min−<sup>1</sup> , eluent 5 mL HCl, sorbent amount 0.5 g).


#### **4. Conclusions**

A novel organic–inorganic hybrid adsorbent was synthesized by surface modification of bare MoS<sup>2</sup> using PANI. The prepared PANI–MoS<sup>2</sup> hybrid material shows selective extraction of Hg(II) in presences of co-existing ions. The fast and selective Hg(II) adsorption may be attributed to the soft acid-soft base interaction between the Hg(II) and–S ions of the PANI–MoS<sup>2</sup> adsorbent. A comparative data on the Hg(II) adsorption capacity of prepared material with previous literature was compared and is shown in Table 4. The PANI–MoS<sup>2</sup> adsorbent shows comparable adsorption capacity over the previously reported nanoadsorbents. The proposed method's accuracy was validated by analyzing reference material and the standard addition method (RSD < 5%). The proposed methodology is simple and successfully used in the quantitative analyses of trace Hg(II) to monitor the Hg(II) level in real environmental water samples.

**Table 4.** Hg(II) adsorption capacities of different nanomaterials based on previous literature.


**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4360/12/11/2731/s1, Figure S1: Zeta potential envelope of bare PANI and bare MoS<sup>2</sup> , Figure S2; ATR-IR spectra of PANI-MoS<sup>2</sup> before and after Hg(II) adsorption, Table S1: Solid phase extraction and preconcentration of trace Hg(II) in real samples analyses after to determine Hg(II) concentration by ICP-OES (column conditions: sample volume 250 mL, flow rate 8 mL min−<sup>1</sup> , eluent 5 mL HCl, sorbent amount 0.25 g).

**Author Contributions:** Conceptualization, H.A.; methodology, H.A.; software, R.A.K.; validation, H.A.; formal analysis, H.A.; investigation, H.A.; resources, A.A.; data curation, H.A., I.I.B., and R.A.K.; writing—original draft preparation, H.A. and R.A.K.; writing—review and editing, H.A., R.A.K., I.I.B., and A.A.; visualization, H.A.; supervision, A.A.; project administration, R.A.K. and A.A.; funding acquisition, A.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors extend their appreciation to the Deputyship for Research and Innovation, "Ministry of Education" in Saudi Arabia, for funding this research work through project no. IFKSURG-1438-006.

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