Penghilangan Adsorptif Ion Tembaga (II) dari Larutan Berair Menggunakan Magnetit Nano-Adsorben dari Limbah Skala Pabrik:Sintesis, Karakterisasi, Adsorpsi, dan Pemodelan Kinetik Studi

Hasil dan Diskusi

Analisis Struktural dan Fase

Hasil pemeriksaan XRD timbangan limbah pabrik setelah MST dan CTST ditunjukkan pada Gambar 2a. Setelah proses MST (Gbr. 2ai), XRD mengkonfirmasi keberadaan wuestite (FeO) dan magnetit (Fe₃ O₄ ). Difraksi Bragg dari wuestite diamati pada 2θ dari 36,33° (111), 61,40 ° (044), 73,25 ° (113) sesuai dengan ICSD:98–001-2335. Fase magnetit diamati pada 2θ dari 35,61° (113) dan 43,28° (004), setuju untuk referensi Fe₃ O₄ Berkas ICSD 98–010-9826.

a Spektrum difraksi sinar-X skala pabrik setelah menjalani MST dan CTST, b spektrum magnetit yang diindeks pada waktu penggilingan yang berbeda dari 3, 5, dan 7 jam, c Lebar penuh pada setengah maksimum (FWHM) dan ukuran kristal dari MNA dengan waktu penggilingan

Untuk spektrum XRD setelah proses CTST (Gbr. 2aii), diamati adanya fase magnetit, hematit, dan wuestit. Sudut difraksi Bragg (2θ) dari semua puncak sangat penting untuk mengidentifikasi dan menyetujui file referensi ICSD 98–010-9826 untuk magnetit dengan puncak adalah 30,23° (022), 35,61° (113), 43,28° (004), 57,24°(333), 62,86° (044). Puncak hematit juga diamati pada 33,44° (104), 48,48° (202) sesuai dengan ICSD 98–004-6407); dan sudut difraksi wuestit 36,33° (111) dan 61,27° (022) yang dicocokkan dengan ICSD:98–001-2335. Hasilnya disepakati dan serupa seperti yang dilaporkan dalam literatur sebelumnya [84].

Gambar 2b mengilustrasikan pola difraksi XRD serbuk timbangan gilingan setelah proses penggilingan bola energi tinggi, pada interval waktu 3, 5, dan 7 jam yang berbeda. Spektrum difraksi sampel yang disintesis menunjukkan adanya magnetit (Fe₃ O₄ ) fase dengan diameter berukuran nano untuk semua waktu penggilingan. Sudut difraksi pada 2θ dari 30.24°, 36.66°, 36.57°, 42.45 °, 57.26°, 61.54° dan 62.76° dapat diindeks ke (022), (113), (222), (004), (224), (115) dan (044) mengkonfirmasikan puncak tanda tangan dari sel satuan kubik Fe₃ O₄ , masing-masing. Spektrum XRD dicocokkan dengan referensi ICSD 98–01-11,241 magnetit dengan grup ruang Fd -3 m dan parameter kisi (a = b = c ) dari 8.3440 Å. Adsorben nano-magnetit menunjukkan kemurnian tinggi dengan peningkatan waktu penggilingan seperti yang ditunjukkan pada Gambar. 2b. Energi tinggi yang dihasilkan oleh benturan bola baja di dalam vial bertanggung jawab untuk memutuskan ikatan oksigen dan mengurangi hematit (Fe₂ O₃ ) menjadi magnetit (Fe₃ O₄ ) fase. Seiring waktu penggilingan meningkat, diamati pembentukan magnetit nanokristalin ditentukan oleh pelebaran puncak XRD. Seiring bertambahnya waktu penggilingan, pelebaran puncak XRD diamati meningkat, menunjukkan penurunan ukuran partikel. Intensitas puncak XRD juga diamati menurun, dengan meningkatnya waktu penggilingan. Pola tersebut menunjukkan penurunan ukuran partikel sampel [61]. Ketika ukuran partikel menurun, regangan yang diinduksi selama proses penggilingan mengakibatkan penurunan intensitas puncak dan pelebaran puncak difraksi. Ukuran kristal rata-rata D sampel dihitung menggunakan rumus Debye-Scherrer seperti pada Persamaan. (6) [62].

dimana D adalah ukuran kristal rata-rata, λ adalah panjang gelombang sinar-X (0,1541 nm), β adalah lebar penuh pada setengah maksimum (FWHM), dan θ adalah sudut difraksi. Spektrum XRD dianalisis secara otomatis menggunakan perangkat lunak X'pert Highscore Plus. Hubungan FWHM dan ukuran kristal ditunjukkan pada Gambar 2c. Analisis menunjukkan bahwa perubahan serbuk MNA mikroskopis dengan peningkatan waktu penggilingan dari 3, 5 dan 7 jam dengan variasi perubahan FWHM dan ukuran kristal rata-rata MNA seperti yang ditunjukkan pada Gambar. 2c. Tren variasi FWHM menunjukkan bahwa dengan peningkatan waktu milling dari 3, 5 dan 7 jam, FWHM menunjukkan tren peningkatan. Dengan peningkatan waktu penggilingan dari 3, 5 dan 7 jam, ukuran kristal rata-rata menurun dari nilai minimum masing-masing 17,8 nm, 15,2 nm, dan 9,8 nm.

Komposisi Morfologi dan Mikrostruktur

Mikrograf HRTEM dari MNA yang digiling pada waktu penggilingan yang berbeda selama 3, 5 dan 7 jam disajikan pada Gambar 3. Mikrograf menunjukkan bahwa partikel MNA menunjukkan bentuk yang tidak beraturan selama tiga periode penggilingan seperti yang ditunjukkan pada Gambar 3. Juga, ukuran partikel MNA rata-rata 5,53 nm terlihat pada waktu penggilingan 3 jam dibandingkan dengan 14,45 nm (5 jam) dan 19,16 nm (7 jam). Ini menyiratkan bahwa ukuran partikel MNA yang lebih kecil dapat dicapai pada waktu penggilingan yang lebih pendek. Ukuran partikel rata-rata selama 3, 5 dan 7 jam diperoleh dalam kisaran 10 hingga 22 nm. Dengan bertambahnya waktu penggilingan, regangan mikro dalam sampel juga meningkat [63]. Oleh karena itu, jam penggilingan yang lama akan menghasilkan lebih banyak regangan pada sampel. Kenaikan regangan kisi dengan waktu penggilingan disebabkan oleh efek distorsi yang kuat yang disebabkan oleh dislokasi atom dan difusi dalam kisi yang diperkenalkan selama proses penggilingan. Namun, efek aglomerasi diamati dalam sampel MNA seperti yang ditunjukkan pada Gambar. 3, karena perilaku tarik-menarik magnet dari bubuk magnetit.

Gambar HRTEM dengan bilah skala 20 nm MNA di a 3 j b 5 j dan c 7 jam waktu penggilingan

Analisis Properti Magnetik

Sifat magnetik sampel diselidiki menggunakan VSM pada percobaan suhu kamar. Respons magnetisasi (M ) dengan medan magnet eksternal yang diterapkan (H ) sampel ditunjukkan pada Gambar. 4. Magnetisasi saturasi (M _s ), remanensi (M _r ) dan koersivitas (H _c ) sampel dirangkum dalam Tabel 1. Nilai koersivitas MNA berada dalam kisaran 200–270 G, remanensi antara 1,5 dan 6,6 emu/g, dan nilai magnetisasi saturasi antara 21 dan 27 emu/g. Karena diameter ukuran partikel yang kurang dari 20 nm, sampel memiliki sifat superparamagnetik. Dari parameter magnetisasi (Tabel 1) menunjukkan bahwa sampel terdiri dari campuran senyawa superparamagnetik dan feromagnetik yang berkontribusi terhadap peningkatan kapasitas adsorpsi. Sampel MNA-7 h menunjukkan parameter magnetik tertinggi (Gbr. 4) yang berkontribusi pada kapasitas adsorpsi tertinggi dari studi adsorpsi Cu (Gbr. 5).

M-H grafik histeresis sampel pada waktu penggilingan yang berbeda a 3 j, b 5 j dan c 7 j

a Bagan batang persentase penghapusan; b penyerapan logam/ kapasitas adsorpsi MNP ke Cu ²⁺ pada jam penggilingan yang berbeda

Pengaruh Parameter Adsorpsi

Analisis lebih lanjut pada studi adsorpsi batch MNA 3, 5 dan 7 jam telah diselidiki. Gambar 5 menunjukkan studi penyerapan dilakukan untuk MNA pada berbagai waktu penggilingan 3, 5 dan 7 jam. Grafik menunjukkan kapasitas adsorpsi tertinggi (penyerapan logam) (q _e ) dan persentase penghapusan tertinggi (%RE ). Jam penggilingan MNA-7 jam menunjukkan kapasitas adsorpsi tertinggi dan persentase penghilangan tertinggi dari larutan berair. Oleh karena itu, MNA-7 h dipilih sebagai nano-absorben MNA untuk analisis absorpsi batch lebih lanjut pada beberapa parameter waktu kontak, konsentrasi awal, dosis absorben, luas permukaan, pH, dan suhu.

Analisis Area Permukaan

Adsorpsi nitrogen menggunakan BET digunakan untuk menilai luas permukaan dan karakteristik pori-pori MNA selama 7 jam penggilingan. Gambar 6a menunjukkan hasil BET untuk MNA-7 jam dengan volume pori rata-rata 0,011 cm ³ g ⁻¹ dan luas permukaan spesifik 5,98 m ² g ⁻¹ . N₂ kurva adsorpsi-desorpsi MNA-7 h terletak pada kurva histeresis Tipe III (Gbr. 6b) seperti yang dilaporkan sebelumnya oleh Sing et. Al. (1985) [65]. Hasil BET menggambarkan interaksi adsorben-adsorbat dari molekul teradsorpsi yang berkerumun di sekitar permukaan MNA [66]. Oleh karena itu, adsorpsi pada tipe III mengungkapkan molekul gas secara fisik teradsorpsi ke MNA [67].

a Distribusi diameter pori 7 h MNA b Isoterm adsorpsi-desorpsi nitrogen

Analisis EDS

Konstituen unsur MNA-7 h diilustrasikan pada Gambar. 7. Analisis FESEM dan EDS mengungkapkan adanya unsur Fe dan O dengan persentase masing-masing 78,25% dan 21,75%, untuk MNA-7 jam sebelum proses adsorpsi (Gbr. 7a). Gambar 7b menggambarkan spektrum EDS bahwa keberadaan unsur Fe, Cu dan O setelah proses adsorpsi. Adanya tembaga pada spektrum menunjukkan adsorpsi Cu ²⁺ oleh MNA. Tren ini juga sesuai dengan spektrum yang ditampilkan dalam studi yang dilaporkan oleh Lingamdinne et al. (2016) [64] sebagai nanopartikel oksida besi digunakan untuk menghilangkan adsorptif logam berat.

Analisis spektrum EDS (a ) sebelum adsorpsi dan (b ) setelah adsorpsi ion Cu(II)

Analisis FTIR

Konfirmasi lebih lanjut dari adsorpsi Cu ke MNA-7 h juga ditentukan melalui analisis FTIR. FTIR diselidiki untuk mengidentifikasi gugus fungsi dan perlekatan Cu ²⁺ ke MNA. Gambar 8 menunjukkan spektrum FTIR MNA-7 jam sebelum dan sesudah adsorpsi ion Cu (II). Spektrum FTIR mengungkapkan pita puncak karakteristik yang kuat dari Fe₃ O₄ nanopartikel. Setelah adsorpsi, spektrum FTIR menunjukkan perubahan intensitas pita dalam kisaran 500–600 cm ⁻¹ dan 2800–3600 cm ⁻¹ yang mengarah ke Cu ²⁺ penyerapan. Selain itu, pita adsorpsi pada 525 cm ⁻¹ dan 576 cm ⁻¹ mewakili situs tetrahedral dan oktahedral dari nanopartikel magnetit pita Fe-O [68]. Spektrum adsorpsi yang kuat dan luas pada 3478 cm ⁻¹ sesuai dengan gugus hidroksil (− OH) dan jejak molekul air pada permukaan MNA [69]. Spektroskopi FTIR mengungkapkan bahwa MNA memiliki struktur kristal karena adanya beberapa zat kimia yang teradsorpsi pada permukaan MNA.

Spektrum inframerah transformasi Fourier dari MNA-7 jam sebelum dan sesudah adsorpsi ion Cu (II)

Analisis Potensi Zeta

Gambar 9 menunjukkan potensi zeta permukaan MNA-7 h. Potensi zeta diselidiki untuk mendapatkan penentuan yang akurat dan tepat dari muatan netralisasi yang terjadi dari proses adsorpsi. Data potensi zeta memberikan nilai yang terukur untuk memantau dosis adsorben yang optimal selama proses adsorpsi Cu. Hasil penelitian menunjukkan bahwa titik isoelektrik (pH_pzc ) terjadi pada pH 5,4, titik di mana Cu ²⁺ adsorpsi pada MNA terjadi secara optimum. Dalam larutan berair, permukaan oksida besi ditutupi dengan OH ⁻ sehingga FeOH pada permukaan dapat berubah menjadi gugus fungsi Fe lain seperti FeO atau FeOH₂ , due to the protonation or deprotonation process [70, 71]. The balance of protonation and deprotonation depend on the pH of the solution, and the pH_pzc of the absorbent. The zeta potential results suggest that the adsorption was efficient at pH of 5.4.

Zeta potential for pH values of 1, 2, 4, 6, 8, 10, and 12 for MNA-7 h

Batch Adsorption Analyses

Effect of Contact Time

Figure 10a shows the effect of contact time on adsorption capacity and rate of Cu ²⁺ uptake onto MNA after 250 min. It is evident that at longer contact time, the adsorption capacity reach equilibrium as the pH was kept at 5.4 and adsorbent dosage at 0.05 g. The maximum removal efficiency attained was 62.61% as shown in Fig. 10b. The Cu ²⁺ removal efficiency surges rapidly from the early first 5 min, and later slower and stable throughout the adsorption process. This is attributed to the fact that the rate of the adsorption capacity was high due to the abundant free binding and active sites of the Cu ²⁺ . Based on Fig. 10, it was noticed that the percentage removal and adsorption capacity increased rapidly with the increase in contact time at the initial stage. The contact time has a substantial influence on the efficacy of Cu ²⁺ removal and adsorption capacity. Increase in contact time from 0 to 240 min led to an increase in the removal efficiency of Cu ²⁺ from 0.81% to 62.61%. For contact time greater than 120 min, the removal efficiency of Cu ²⁺ remains steady, as the active sites has been saturated on the surface of the adsorbent. Similarly, the highest adsorption capacity of 4.41 mg/g was attained at 120 min of contact time. Thus, the equilibrium time was attained at 120 min.

a Adsorption capacity; b Copper removal efficiency under various contact times of MNA-7 h (metal solution:200 mL; temperature:25 °C; initial pH:5.4; initial concentration:50 mg/L; adsorbent dosage:0.05 g)

The results of copper removal efficiency follow a definite trend (Table 2). It shows that the higher the time, the more the removal efficiency. There were significant differences (p  < 0.05) among the removal efficiencies under varying contact times. Generally, as contact time progresses, the removal efficacy also improves. Table 3 shows the percentage of the copper removal at different adsorbent dosages. The result shows that as the time progresses, the removal percentage increases. 0.05 g adsorbent dosage recorded the highest copper removal efficiency (62.58 g) after 120 min contact time.

Effect of Initial Concentration

Figure 11 shows as the initial concentration increases, the equilibrium adsorption capacity also increases. Thus, the higher initial concentration due to 0.05 g adsorbent in the Cu ²⁺ solution to fill the active sites on the adsorbent and the quantity of copper adsorbed increases with the increase in Cu ²⁺ concentration [83]. The initial concentration of Cu ²⁺ increased from 10 mg/L to 50 mg/L with corresponding increase in adsorption capacity from 0.04 mg/g to 4.41 mg/g, which in turn provide a higher driving force for the ions from the solution to the adsorbents, resulting in more collisions between Cu ²⁺ and active sites on the MNA-7 h. Since nearly all the adsorption sites of MNA-7 h existed on their exterior, it is easy for the adsorbate to access these active sites, thereby facilitating a rapid attainment of equilibrium condition.

Copper removal efficiency under various initial concentration of MNA-7 h (metal solution:200 mL; temperature:25 °C; initial pH:5.4; adsorbent dosage:0.05 g)

Effect of MNA Dosage

Adsorbent dosage plays an important role during the adsorption process, as it controls the ability of the adsorbent for a given solution. The more the dosage, the more obtainable site for sorption to occur [67]. Figure 12a shows the adsorption capacity, q _e of Cu ²⁺ with respect to different dosages of MNA-7 h at 0.05 g, 0.2 g, 0.5 g, and 0.8 g, respectively. The adsorption capacity was observed to be dependent on adsorbent dosage, which determines the availability of the active sites and the amount of the surface area for adsorption. This is due to the increase in surface area and the probability of collision and interaction between the particles of nano-adsorbent and Cu ²⁺ [72]. As shown in Fig. 12b, at 0.05 g dosage, 62.61% copper removal efficiency and 4.41 mg/g of adsorption capacity were recorded. The Cu ²⁺ removal increases sharply and becomes stable as the adsorbent dosage increases. As the adsorbent dosage increases, the larger surface interaction and the agglomeration effects develop. Thus, it causes a decrease in free specific area per unit mass of MNA surface, causing a reduction in contact surface with the adsorbate surface. This will lead to the decrease in q _e and %RE. Besides, the decrease in q _e and %RE , perhaps was due to the saturation of Cu ²⁺ in solution with respect to available adsorption binding sites [73]. Thus, a higher amount of adsorbent causes an aggregation which decreases the total surface area of the MNA, thereby leading to a decrease in adsorption capacity [74–76]. The aggregation could result to a decrease in total surface area of the adsorbent and an increase in diffusion path length [75].

a Adsorption capacity; b Copper removal efficiency under various adsorbent dosage of MNA-7 h (metal solution:200 mL; initial concentration:50 mg/L; temperature:25 °C; initial pH:5.4)

Effect of pH

The removal of Cu ²⁺ from the aqueous solution through adsorption is highly dependent on the solution pH which determines the surface charge of the adsorbent and the adsorbate speciation [77]. Adsorption is regarded to be minimal at acidic state owing to higher concentration of H₃ O ⁺ which competes with the positively charged ions for the actively binding site on the adsorbent surface, and this usually led to low contaminant removal [77, 78]. The influence of pH on the adsorption of Cu ²⁺ on MNA was evaluated between the pH range of (2–12). Figure 13 shows the effect of pH on the adsorption capacity and removal efficiency of Cu ²⁺ . It was observed that increase in pH from 2 to 5.4, results in an increase in adsorption capacity from 0.58 mg/g to 4.408 mg/g and percentage removal of copper from 10.71% to 62.61%, respectively. However, the equilibrium adsorption capacity of Cu ²⁺ is low at a strong acidic condition recording 0.58 mg/g at pH 2 due to the presence of a high percentage of H₃ O ⁺ ion which competes with Cu ²⁺ at the sorption sites of MNA. Besides, when pH is higher than 5.4, the adsorption capacity decreases from 49.32% to 44.69%. At a higher pH, higher concentration of OH ⁻ causes a decrease in the adsorption rate. Figure 13a and b shows that pH has a significant impact on Cu ²⁺ adsorption capacity and removal percentage (%RE ) of Cu. The removal rate for Cu ²⁺ increases with an increase in pH, from 10.71% to 28.04% and to 62.61% when pH is at 2, 4 and 5.4, respectively, before declining to 49.32%, 42.56% and 44.69 at pH 8, 10 and 12, respectively (Fig. 13a).

a Adsorption capacity; b copper removal efficiency under various pH of MNA-7 h (metal solution:200 mL; initial concentration:50 mg/L; temperature:25 °C; adsorbent MNA dosage:0.05 g)

Similarly, the adsorption capacity also increases from 10.71% to 62.61% with the increase in pH from 2 to 5.4 and decreases until it reaches pH 12. At pH 2 and 4, the amount of protonation of the adsorbent surfaces results in a decrease in Cu ²⁺ adsorption. The results also are in agreement with zeta potential graph as indicated in Fig. 9. Also, Fig. 9 describes the net charge of the MNA adsorbent surface at different pHs, with the point of zero charges (pH_pzc ). As the pH increases, the H ⁺ ion is lower and causes the surface of the adsorbent to become negatively charged, with the increase in %RE of Cu ²⁺ , thereby increasing the electrostatic attraction force between the adsorbents in the solution [79]. Therefore, pH influences the surface zeta potential of MNA. The surface functionality of iron oxides varies depending on the nature of iron oxides and the pH value.

Copper Adsorption Kinetics

Kinetic studies are essential in the adsorption process to describe the uptake rate performance of MNA-7 h and influence the residual time for the entire adsorption process. The adsorption kinetics of Cu ²⁺ on MNA-7 h was determined using similar procedures to those used in the batch adsorption studies [55]. The Lagergren’s first-order kinetic model and second-order kinetic model for the removal of Cu ²⁺ at various initial concentrations from the aqueous solution using MNA at 0.05 g/L of the MNA dosage are shown in Fig. 14. The calculated q _e values are in agreement with the theoretical values, and the graph shows good linearity with R ² above 0.96. Therefore, the adsorption kinetics follows the pseudo-second-order model. The pseudo-second-order model represents the adsorption kinetics, involving donation or electron exchange between adsorbate and adsorbent. Table 4 shows the fitted parameter summary of Cu ²⁺ kinetics at different initial Cu ²⁺ concentrations (q _e :mg/g, k ₁ :min ⁻¹ , k ₂ :g/mg/min) of MNA-7 h. For the parameters of initial concentration, C _i (mg/L), adsorption capacity, q _e (mg.g ⁻¹ ), k ₂ is the rate constant of pseudo-first-order, k ₂ is rate constant of pseudo-second-order, and the R ² is the correlation coefficient. Two kinetic models:Lagergren’s first-order and pseudo-second-order order were applied to further study the rate of adsorption process for Cu ²⁺ . The kinetic parameters of pseudo-first-order and pseudo-second-order are presented in Table 4. According to Table 4, pseudo-second-order was best fitted for the adsorption of Cu ²⁺ . Pseudo-second-order revealed a higher correlation coefficient of R ²  = 0.999, for Cu ²⁺ removal.

a Lagergren’s first-order kinetic model; b Pseudo-second-order kinetic model for the removal of Cu (II) ions at various initial concentrations from the aqueous solution using MNA at 0.05 g/L dosage of MNA-7 h

Copper Adsorption Isotherms

The adsorption isotherms experimental data were investigated using Freundlich isotherm model (Eq. 7) and Temkin isotherm model (Eq. 8). The adsorption isotherm is illustrated in Fig. 15a and b. The Freundlich isotherm is expressed as:

where q _e is adsorbent capacity at equilibrium (mg/g); B :(RT /b ) is the Temkin constant related to heat of adsorption (J/mol) R is universal gas constant (8.314 J/mol K); T is absolute temperature (K); 1/b indicates the adsorption potential of the adsorbent; kt is equilibrium binding constant corresponding to the maximum binding energy (L/mg); and Ce is adsorbate concentration at equilibrium (mg/L). From the graph, the straight line emerged, and the values of q _m  and K _L  constants can be calculated using the slope and the intercept of the straight line. Freundlich and Temkin models was used to examine the relationship between the adsorbent and adsorbate. As shown in Fig. 15, the initial concentrate correlation coefficient with an R ² value of 0.914 and manifested Temkin adsorption isotherm to be more favourable for the removal of copper. The synopsis of the isotherm parameter of Freundlich, Temkin parameter with correlation coefficient, R ² for adsorption of Cu (II) on MNA-7 h at room temperature is depicted in Table 5. 1/n constant reciprocal implies natural sorption; therefore, the adsorption process is beneficial. The values of n, 1/n , K _F , and R ² for the current work are also presented in Table 5. Temkin isotherm adsorption plot shows maximum Cu ²⁺ removal by MNA-7 h at optimal conditions and reveals the feasibility of the process. The data prove that the Temkin model well fitted the experimental data than Freundlich based on the correlation coefficient, R ² (Table 5). This could be due to the fact that the Temkin isotherm model considers the effect of indirect adsorbate on the adsorption process and assumes that the heat of the adsorption of molecules decreases linearly in the adsorption layer [80].

a Linearized Freundlich; b Linearized Temkin isotherm models for Cu ²⁺ adsorption by MNA-7 h at various adsorbent dosages:contact time 240 min; initial pH 5.4; room temperature

Regeneration and Desorption Study

The reusability of adsorbent is a prime issue since periodically regenerating adsorbent is strongly desirable for industrial applications [81]. In the reusability tests, the adsorption and desorption cycle of Cu ²⁺ onto MNA was repeated three times. The adsorption capacity of MNA was recycled, and the regeneration of the MNA of the adsorption–desorption cycle of Cu ²⁺ was repeated three times using the same MNA. The desorption process was studied for 3 cycles of MNA-7 h. The results proved that the magnetic nanoparticles have higher sustainability for industrial applications. The results also revealed that Cu ²⁺ could be desorbed from the adsorbent in the presence of deionized water as the desorbing agent. For repeated use of MNA, the adsorbed Cu ²⁺ were desorbed under suitable conditions. In this work, the percentage of desorption by MNA was obtained by 0.1 M HCl as shown in Table 6. Hence, MNA exhibits an enhanced recovery efficiency of 70.87%. Figure 16 displays the desorption efficiency of MNA. The adsorption capacity decreased by 10% during three adsorption–desorption cycles which indicates the stability and reusability of MNA.

Desorption of copper onto MNA-7 h in three cycles