Nickel content and the topological feature of SNC were investigated using Scanning Electron Microscope-Energy Dispersive X-ray spectroscopy (SEM-EDX)(JCM-7000, JEOL, Japan), meanwhile the X-ray diffraction (XRD) (D2-Phaser, Bruker, Germany) was used to investigate the structural and crystal phases of SNC.

Both precursors and nanostructured NiO powders were also characterized using XRD, Fourier Transformed Infra-Red Spectroscopy (FTIR), SEM, and SEM-EDX. The surface area and pore analysis were conducted using the N₂ isothermal adsorption-desorption method, performed at 77 K using Belsorp Mini X (Microtrac, Germany). Thermogravimetric Analysis/Differential Thermal Analysis (TG/DTA) was performed using Thermogravimetric Analyzers 60, Shimadzu, Japan. UV-visible spectra of nanostructured NiO powders were analyzed using Specord 200 Plus, Analytik Jena, Germany. The UV-Vis spectra were used to develop the Tauc-Plot. The Raman spectrum of nanostructured NiO powders were recorded by Raman Spectrometer (Horiba Scientific LabRam HR, Japan). The TEM imaging of nanostructured NiO samples were performed using Transmission Electron Microscopy (Thermofisher Scientific Fei Tecnai G2 Supertwin, USA).

The characterization of SNC can be seen in Fig. S1, proving the presence of NiO [31,32]. Due to the acid strength of HCl, the leaching reaction of NiO is explained in Eq. (4):

The effect of HCl concentration was examined by varying the concentration of HCl in the leaching process at 60 minutes, 60°C, and the S/L value of 100 g/L_lixiviant, with the result shown in Fig. 1a. The LEs increased along with HCl concentration, although the improvement at concentration above 3.75 M was not significant (from 89.02% to 90.78% and 92.9%). This showed that HCl quickly reacted with NiO, the stoichiometric value of HCl should at least be achieved, particularly when the S/L or pulp density value was high. Based on the Fig. 2a., 3.75 M HCl concentration was selected.

High pulp density or S/L significantly affected the product throughput and attracted industries’ attention as a technological feasible process. In this study, the effect of S/L ratio during the leaching process was examined by maintaining the HCl concentration at 3.75 M and a temperature of 60°C for 60 minutes. Based on Fig. 1b., LE was decreased with the increasing S/L ratio. Generally, the acid consumption of pure NiO is theoretically two mol_HCl/mol_NiO or approximately 139 g_NiO/L. At the S/L value above 100 g/L, LE dropped significantly due to acid depletion and the increasing slurry viscosity, leading to poor mass transfer. Therefore, the optimal S/L ratio to maintain the high product throughput was 100 g/L [33–35].

The temperature effect during the SNC leaching was investigated using 3.75 M HCl with an S/L ratio of 100 g/L and a leaching time of 60 minutes. As shown in Fig. 1c., the LE increased along with the leaching temperature, with the highest value of 98.8% obtained at 70°C. The increasing LE followed the Arrhenius law, where the reaction rate increased along with leaching. Furthermore, the high leaching temperature would lead to a decrease in solution viscosity and better mass transfer. Moreover, the required threshold is limited to 70°C, where a significant increase will pose a challenge to parameter control [36].

The leaching kinetics were initially investigated by measuring LE at 0–120 minutes and multiple temperatures of 30, 50, 60, and 70°Cat the pre-determined S/L and HCl concentrations. Based on the trend shown in Fig. 1d., high reaction rates were achieved in the first 25 minutes. Subsequently, the reaction tended to slow down based on the insignificant increase of LE over time, showing an initially high HCl reactivity. Continuous consumption of HCl caused the reduction of [H⁺], lower concentration gradient, and increase in viscosity. The increase in leaching efficiency after 120 minutes is negligible due to the reduced concentration gradient, which slows down the reaction. Therefore, a leaching time of 60 minutes (1 hour) was selected. A shorter reaction time also suggests a lower residence time. On a pilot or larger scale, this reduction in residence time would decrease the reactor volume, leading to lower capital costs. A heterogeneous reaction phenomenon was analyzed using the SCM, logarithmic rate law, and Avrami model to understand the mechanism of Ni leaching from SNC and determine the suitable leaching kinetic equation [37,38].

X-ray diffraction method was used to investigate the crystal structure of the as-prepared powders. Fig. 3a. shows the X-ray diffractogram of the Ni(OH)₂ samples obtained from the spent catalyst leaching solution and the commercial nickel chloride through ammoniacal precipitation. Based on the results, both samples showed a crystalline phase due to the sharp peaks that were well indexed to the JCPDS No. 14-0117 or PDF card number 14–117 with no impurities detected [42,43]. This showed that the leaching process of SNC with aluminum supported only selectively precipitates nickel at pH 11–12 [44]. Furthermore, the Al ions remained in the solution due to the formation of soluble aluminate ions at elevated pH. The peaks (100) and (110) did not show line broadening, indicating the anisotropic property of β-Ni(OH)₂ [42]. Using the Debye-Scherer equation, the crystallite size of Ni(OH)₂-Cat and Ni(OH)₂-NiCl₂ at (011) was 6.4 nm and 9.9 nm, respectively. The smaller crystallite size of the Ni(OH)₂-Cat compared to the Ni(OH)₂-NiCl₂ sample was due to the presence of Al ions. This prevented the formation of particle growth during the hydroxide precipitation through the disturbance of the Ostwald ripening mechanism [45]. Fig. 3(b-c). shows the morphology and elemental analysis of the as-prepared samples. The morphology of Ni(OH)₂-Cat in Fig. 3b. (inset) and Fig. 3c. showed that the Ni(OH)₂ particle had a quasi-spherical secondary particle with a diameter of 12 μm. The particle consisted of nano-sheets, similar to the previous study. The EDX mapping showed the homogenous distribution of Ni, Al, and O atoms, with Al atoms originating from the residue of Al₂O₃ support. Meanwhile, Fig. 3d. confirms the presence of a dense secondary particle and the absence of Al atoms. Fig. 3e. confirms the characteristic of the β-phase of precipitates through the infra-red spectra analysis with the presence of peaks at wavenumbers of 3600, 3400, and 1600 cm⁻¹, showing the presence of stretching mode of disturbed and free hydroxyl groups. The peaks at the wavenumber of 1300 and 1000 cm⁻¹ corresponded to the presence of carbonate species on the surface of the powders due to a side reaction between the hydroxyl group and CO₂ gas from the humid open atmosphere [46]. Furthermore, Ni(OH)₂-Cat showed a prominent broadening peak at 3425 cm⁻¹ due to trapped water molecules. Bands detected at <600 cm⁻¹ were correlated to the Ni-O stretching vibration mode, showing the presence of nickel hydroxide [47], while a band in <500 cm⁻¹ was related to the Al-O mode [48]. The TG/DTA curve of the Ni(OH)₂-Cat and Ni(OH)₂-NiCl₂ presented in Fig. 3f., showed two endothermic peaks attributed to the desorption of trapped water molecules at the temperature of below 200°C and the calcination temperature of 288°C which produced nanostructured NiO powder [41]. The results of qualitative and quantitative analysis in Table S2 are consistent with the XRD and FTIR.

The nanostructured NiO powders obtained from the sintering of Ni(OH)₂ samples were characterized Fig. 4 shows the characterization of NiO-Cat in comparison with NiO-NiCl₂ by XRD, FTIR, Raman spectroscopy, Tauc-Plot from UV-Visible DRS, surface area analysis using Brunauer-Emmet-Teller (BET) method, SEM-EDX, and TEM. Meanwhile, Fig. 4a shows the X-ray diffraction patterns of the as-prepared and commercial samples. NiO-Cat and NiO-NiCl₂ show a face-centered cubic structure of NiO, which is well indexed to the commercial NiO and the reference JCPDS or PDF No. 04-0835 [30,49]. Compared to commercial powder, NiO-Cat and NiO-NiCl₂ have line broadening, which can be attributed to the small crystallite size. In addition, the comparison of the as-prepared NiO-Cat and NiO-NiCl₂ provides strong evidence for product commercialization. Table S3. shows the structural parameters of the samples. The as-prepared nanostructured NiO powder crystallite sizes are significantly lower than in the commercial, showing the formation of nanostructure during hydroxide precipitation [50]. However, the lattice parameter does not show a significant difference between each sample.

Fig. 4b shows the infra-red spectra of the as-prepared NiO samples with Ni-O bond present in the wavenumber of <500 cm⁻¹ and surface impurities. These include carbonate and hydroxide groups due to the adsorption of water and CO₂ molecules from the atmosphere. NiO-Cat has a significant -OH group detected at 3420 cm⁻¹ due to a higher amount of H₂O adsorption than NiO-NiCl₂ sample, showing a high surface area [51]. The Raman spectra in Fig. 4c. show slight pattern similarity between NiO-Cat and NiO-NiCl₂, except with the peak broadening. A total of 4 modes can be identified from the nanosized NiO consisting of the first order of phonon modes at 300–700 cm⁻¹ (LO and TO) and the second order at 700–1200 cm⁻¹ (2LO and 2TO) [52–54]. The Raman spectra of NiO-Cat have higher second-order phonon modes than NiO-NiCl₂. UV-visible spectroscopy analysis of the as-prepared samples was conducted to obtain the Tauc-Plot presented in Fig. 4d. and Fig. 4e., respectively. Based on the calculation, the energy gap of NiO-Cat and NiO-NiCl₂ were 3.50 and 3.91 eV, respectively, showing the presence of p-type semiconductor materials and their possible application for UV irradiated photocatalytic [55,56]. The result of the surface area and pore analysis of NiO-Cat are shown in Fig. 4f. The N₂ adsorption and desorption isotherms behavior were related to the mesoporous characteristic of NiO-Cat with the BET surface area of 77.2 m²/g and the average pore radius of 3.18 nm, based on the Barret-Joyner-Halenda (BJH) method. Meanwhile, in Fig. 4g. NiO-NiCl₂ has a BET surface area of 38.747 m2/g and an average pore radius of 3.342 nm. The large surface area of NiO-Cat is beneficial for catalyst application due to the presence of large active sites and promoting reaction rates, especially in heterogenous photocatalysis [57,58]. The BET surface area analysis result of NiO-Commercial can be seen in Fig. S4.

The morphology and elemental analysis of NiO samples were examined using SEM, as shown in Fig. 4(g–k). In comparison, the morphology feature of NiO-Cat in Fig. 4a. and Fig. 4b. are different from the Ni(OH)₂ precursors in Fig. 3b. and Fig. 3d.. The decomposition of Ni(OH)₂ into NiO lead to the change of nanosheet particle shape into a needle-like nanoparticle. The elemental analysis by EDX shows the presence of evenly distributed Ni, Al, and O elements. Fig. 4i. exhibits the HRTEM analysis of NiO-Cat sample and confirms nanosized particles with diameters 2–12 nm and a d_spacing of 0.24 nm, as presented in Table S3. Moreover, the morphology and elemental analysis of NiO-NiCl₂ in Fig. 4j. show a sphere-like shape of particles with high agglomeration levels but no Al elements. These results suggest that the as-prepared NiO-Cat has comparable characteristics to NiO made using fresh raw materials, namely NiO-NiCl₂ [9]. Therefore, NiO-Cat is a promising material originating from SNC and can be used as a new catalyst for the photodegradation of pollutants.

The photocatalysis parameters were first investigated on 20 mg/L RhB solution. The effect of H₂O₂, UV-C irradiation, and catalyst loading can be seen in Fig. 5(a–b). Fig. 5a. shows that UV-C itself can decolorize the RhB through photolysis. The addition of H₂O₂ increased the decolorization rate of RhB under UV-C irradiation. However, using NiO-Cat photocatalyst, the decolorization rate increased significantly, which is indicated by the increasing slope. In Fig. 5b. the catalyst loading increased when the catalyst loading was 1.6 g/L as a result of increasing active sites and photogenerated reactive compounds. However, at a higher value, the DE significantly dropped due to light scattering and shading, as we can see that the catalyst can cause turbidity, thus reducing the intensity of the light [59,60].

Fig. 5(c–f). shows the photocatalytic result of 20 mg/L RhB and MB using 1.6 mg/mL NiO-Cat and 10 mM H₂O₂ under UV-C irradiation. Based on the results, the absorbance peaks of both dyes are decreasing over time. This shows the successful decomposition of organic synthetic dyes using the as-prepared photocatalyst with RhB and MB DE of 81% and 88% after 70 minutes of photocatalytic process under UV-C, respectively, and DE above 98% after 120 minutes.

Fig. 5(g–j). shows the photocatalytic result of 20 mg/L AO7 and MO using similar parameters, showing that absorbance peaks of both dyes are decreasing over time. This shows the successful decomposition of organic synthetic dyes using the as-prepared sample with AO7 and MO DE of 76% and 39% after 70 minutes of photocatalytic under UV-C, respectively. After 120 minutes, the DE for AO7 and MO is 92 and 83%, respectively.

NiO-Cat can behave as a photocatalyst due to the gap energy, serving as a semiconducting material. Moreover, the nanostructured morphology and large surface area exhibited by NiO-Cat also contribute to the photocatalytic performance. Generally, electrons are capable of exciting at the edge of the conduction band (c) and recombining with the holes at the edge of the valence band (v). When holes are generated, their reaction with water molecules produces oxidizing radicals of hydroxyl (•OH). Subsequently, the radicals attack dye molecules, leading to decomposition into simpler molecules while losing color in the process. The formation of hydroxyl radicals can be accelerated by additional reducing agents such as H₂O₂ [25,59].

In this study, radical trapping analysis was carried out by adding several radical scavengers during the photodegradation process of 20 mg/L RhB with 1.6 g NiO/L_RhBsolution and 10 mM H₂O₂ Generally, the active species for organic pollutant degradation consisted of photo-generated holes (h⁺), superoxide radicals (•O₂⁻), and hydroxyl radicals (•OH). However, isopropyl alcohol (IPA), disodium ethylenediaminetetraacetic acid (EDTA-2Na), and benzoquinone (BQ) were used during the RhB photodegradation process as the radical avengers for h⁺, •O₂, and •OH, respectively. The results of radical scavenger addition with each concentration 10 mM were shown in Fig. 6a., suggesting a severe inhibition effect due to •OH scavenging with a 76% reduction of DE. The suppressed photocatalyst activity was also detected in the presence of EDTA-2Na and IPA. Therefore, the decolorization was highly affected by hydroxyl radicals [10], as expressed in Eq. (16–20) and visualized in Fig. 6b.

The dye decomposition kinetic curves in Fig. 5(d,f,h,j). were constructed based on the PFO model (Eq. (21)) and half-time (Eq.(22)). The curves showed R² above 90%, indicating a proper model to study the phenomena [10]. The pseudo-second-order (PSO) model (Eq. (23–24)) result was shown in Fig. S5, while the degradation constant, R^2, and half-time were presented in Table 2. The PFO and PSO models were applied due to the simplicity of the reaction mechanism, which is only affected by the reaction between pollutant molecules with the photogenerated reactive compounds or radicals [59].

Ct is the dye concentration at various times, C₀ is the initial dye concentration, k_pfo and k_PSO is the pseudo-first-order and pseudo-second-order dye degradation constant (min⁻¹ and L/mg/min), and t_1/2pfo and t_1/2pso is the pseudo-first-order and pseudo-second-order reaction half-time (min). Based on the kinetic data in Table 2, the cationic dyes show large k and short t_1/2 due to hydroxyl radicals and hydroxyl ions. Cationic dyes degrade quickly under alkaline conditions, while anionic dye degrades more slowly under the same conditions[11]. The R² coefficient shows that the PFO kinetic model is suitable for dye degradation. The cycle performance of the photocatalyst shown in Fig. S6. provides a promising reusability of NiO-Cat.

Since radicals are heavily included in the process, the treated water was analyzed using atomic absorption spectroscopy to detect the presence of leached Ni in the sample. This was attributed to the harmful effects of Ni ions on the environment, particularly humans. Based on the analysis, the concentration of Ni was 4.2 ppb, which was significantly lower than the standardized water quality parameters determined by the Indonesian Ministry of Health (Kemenkes) of less than 70 μg/L or 70 ppb [61]. To further minimize the risk of Ni leaching, the addition of stable and eco-friendly coatings was suggested. Carbonaceous materials including graphene and reduced graphene oxide (rGO) have also shown promising potential to be applied as a co-doping or binary photocatalyst for future investigation. Based on the overall process, this study served as a step toward a new direction for sustainable technology. The effectiveness of the method used was comparable with the previous studies, as shown in Table S4. Moreover, the presence of H₂O₂ was compensated with the low-powered UV light used in the process [62].

The results showed the characterization and photocatalytic performance of the as-prepared NiO. Based on the overall process, the method showed promising potential to be considered suitable to simultaneously handle the chemical and textile industry waste, specifically SNC and dye-containing wastewater. The NiO-Cat is also promising to be applied during the natural organic matters (NOMs) content reduction in a wastewater treatment plant. On the other hand, the effect of free cations in the wastewater and the poisoning mechanism of the NiO-Cat still need to be further developed in the future. The economic aspect of the process was attractive for commercial development due to the inclusion of commercially available and cheap chemical reagents such as HCl, ammonium hydroxide, and sodium hydroxide. Based on the environmental impact, the conversion process generated less hazardous waste, such as catalyst support containing aluminosilicates and wastewater comprising ammonium and sodium chloride. The ammonium chloride in wastewater could be used as a fertilizer or converted into ammonium hydroxide solution through NaOH treatment.

A simple gate-to-gate analysis was performed, and the estimated specific energy consumption of the SNC conversion to NiO photocatalyst was 146 kWh/kg_product. The emission factor of a coal power plant in Indonesia was found to be approximately 0.75–0.85 kg CO₂e/kWh, leading to a carbon footprint of 106–120 kg CO₂e/kg_product. This value was significantly lower compared to the freshly prepared commercial NiO of ~369 kg CO₂e/kg [63]. Moreover, the complete cradle-to-grave life cycle analysis of the method used in this study was an interesting topic that could be investigated in the future.