*3.3. Preparation of Ag2S*

For control experiments, Ag2S nanoparticles were prepared using the procedures used for CNTs-Ag2S without CNTs.

#### *3.4. Adsorption Experiments*

The stock solution of Cd(II), with a concentration of 1 g/L, was prepared in DI water using cadmium chloride and was diluted to the desired concentrations. The kinetic Cd(II) adsorption experiments were conducted to find the contact time needed to attain equilibrium. In the typical experiment, 100 mg of CNTs-Ag2S was dispersed into 500 mL of a Cd(II) solution with a concentration of 0.5 mg/L and stirred at room temperature. After the required contact time, an adequate sample was collected, and the dispersed CNTs-Ag2S was separated by centrifugation. The concentration of residual Cd(II) in the collected samples was estimated by atomic absorption spectrometer. The amount of Cd(II), adsorbed by CNTs-Ag2S was monitored as a function of time for 120 min. The quantity of Cd(II) adsorbed was calculated using Equation (8).

$$q\_t = \frac{\left(\mathbb{C}\_0 - \mathbb{C}\_t\right)V}{M} \tag{8}$$

where *q<sup>t</sup>* is the amount of adsorbed Cd(II) (mg/g) at time t; *C<sup>0</sup>* is the initial concentration of the Cd(II) (mg/L), and *C<sup>t</sup>* is the concentration of the Cd(II) (mg/L) at time t; V is the volume of the solution (L), and M is the amount of adsorbent (g).

Further, the efficiency of CNTs-Ag2S in Cd(II) adsorption was estimated using Equation (9).

$$\text{Removal efficiency (\%)} = \frac{\left(\text{C}\_0 - \text{C}\_t\right)V}{\text{C}\_0} \times 100\tag{9}$$

For adsorption isotherm experiments, 10 mg of CNTs-Ag2S was mixed with 50 mL of the Cd(II) solution and stirred at room temperature for 24 h to reach equilibrium in the concentration between 50 and 140 mg/L. After separating the dispersed CNTs-Ag2S, the concentration of Cd(II) in the solution was measured using an atomic absorption spectrometer. The amount of Cd(II) adsorbed at equilibrium, q<sup>e</sup> (mg/g), was determined by Equation (10):

$$q\_{\ell} = \frac{\left(\mathbb{C}\_{0} - \mathbb{C}\_{\ell}\right)V}{M} \tag{10}$$

where *q<sup>e</sup>* is the amount of Cd(II) adsorbed (mg/g) at equilibrium.

#### *3.5. Photocatalytic Activity*

To evaluate the photocatalytic activity, 10 mg of CNTs-Ag2S was added to the 100 mL aqueous solution of AYR with a concentration of 10 mg/L. It was allowed to stir in the dark for 30 min to reach adsorption/desorption equilibrium of the AYR molecules over the surface of CNTs-Ag2S. This suspension was transferred to a photocatalytic reactor having a water jacket with a water circulation system to maintain a constant temperature, and the suspension was exposed to sunlight. At the required time, 5 mL of the reaction mixture was withdrawn, and the suspended CNTs-Ag2S was separated using centrifugation. The concentration of AYR after photocatalysis was assessed with UV-vis spectrophotometer by recording the absorbance at 373 nm. The normalized concentration of AYR after photocatalysis was calculated as C/C0, where C<sup>0</sup> is the initial concentration of AYR and C is its concentration after photocatalysis. All photocatalytic experiments were conducted in the month of June, between 1 pm and 4 pm. The intensity of sunlight measured during photocatalysis was 800–900 W/m<sup>2</sup> .

#### *3.6. Characterization*

The UV-vis absorption spectra were recorded using a Jasco V-770 UV-vis-NIR spectrophotometer (Easton, MD, USA), and ATR-FTIR spectra were collected with a Smiths ChemID diamond attenuated total reflection (DATR) spectrometer (Smiths Detection, Inc., London, United Kingdom). The XRD was obtained by a Scintag X-ray diffractometer (Cupertino, CA, USA), model PAD X, equipped with a Cu-Kα photon source (45 kV, 40 mA), at a scanning rate of 3◦/min. The thermogravimetry differential thermal analysis

(TG/DTA) was performed using a Perkin Elmer Diamond TG/DTA instrument (Waltham, MA, USA) at a 10 ◦C/min heating rate. Transmission electron microscopy (TEM) images and X-ray energy-dispersive spectroscopy (EDS) were perceived by a Hitachi H-8100 microscope (Tokyo, Japan). The X-ray photoelectron spectra (XPS) were acquired by a Perkin Elmer PHI 5600 ci X-ray photoelectron spectrometer (Waltham, MA, USA). The Cd(II) concentration was estimated using a Varian SpectrAA 220FS atomic absorption spectrometer (Lake Forest, CA, USA).

## **4. Conclusions**

The facile hydrothermal process produced an efficient adsorbent and photocatalyst, CNTs-Ag2S. The ATR-FTIR, XRD, EDS, and XPS confirmed the formation of CNTs-Ag2S in the right structure and phase. The TEM explored the deposition of Ag2S nanoparticles over the surface of CNTs. The TG/DTA revealed the high thermal stability of CNTs-Ag2S. The dual-tasking CNTs-Ag2S could accomplish the complete Cd(II) adsorption and the degradation of AYR in water. The agreement of second-order kinetics for Cd(II) adsorption reveals that chemisorption is the rate-determining step of the adsorption process. The Weber−Morris intraparticle pore diffusion model represented that intraparticle diffusion could not be the sole rate-limiting step in Cd(II) adsorption, instead, it occurred through multiple phases. The validation of the Langmuir model illustrates that the Cd(II) adsorption takes place with monolayer molecular covering and chemisorption. Not limiting to Cd(II) adsorption, the CNTs-Ag2S could also be an excellent adsorbent for adsorption of other toxic heavy metals. Apart from excellent adsorbent, CNTs-Ag2S could also be an exceptional photocatalyst as reveled by degradation of AYR. The elevated degradation of AYR demonstrated that CNTs-Ag2S is the strong sunlight-active photocatalyst that could be applied in the degradation of other toxic dyes. The linearity of the L-H plots depicted that the degradation of AYR occurred through pseudo-first-order kinetics. The CNTs-Ag2S could be easily recovered and used for several times without losing its activity. Overall, CNTs-Ag2S is a robust adsorbent as well as photocatalyst that could be employed in the adsorption of different heavy metals and the photodegradation of alternative dyes.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/catal13030476/s1. Table S1: Parameters calculated for Cd(II) adsorption over CNTs-Ag2S from the intra-particle diffusion plot; Figure S1: EDS spectrum of CNTs-Ag2S; Figure S2: Plot perceived qt as a function of time for Cd(II) adsorption over CNTs- Ag2S; Figure S3: Intraparticle diffusion model for Cd(II) adsorption over CNTs- Ag2S; Figure S4: Freundlich isotherm plot for Cd(II) adsorption over CNTs- Ag2S; Figure S5: Temkin isotherm plot for Cd(II) adsorption over CNTs- Ag2S; Figure S6: Degradation of alizarin yellow R in presence of CNTs-Ag2S for successive three cycles under illumination to sunlight; Figure S7: XRD of CNTs- Ag2S (a) before and (b) after using in successive three cycles of photodegradation of alizarin yellow R under illumination to sunlight.

**Author Contributions:** G.M.N.: data curation, writing original draft, review, and editing funding acquisition. S.F.A.: investigation, data curation. E.A.J.: investigation, data curation. R.L.R.: writing, review, and editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** The author (GMN) acknowledges the support of the National Academy of Sciences of the U.S.-Egypt Science and Technology Joint Fund and the Welch Foundation, Texas, United States, for departmental grant L-0002-20181021.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

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