使用来自轧制废料的磁铁矿纳米吸附剂从水溶液中吸附去除铜 (II) 离子：合成、表征、吸附和动力学建模研究

结果与讨论

结构和相分析

MST 和 CTST 后废料氧化皮的 XRD 检查结果如图 2a 所示。在 MST 工艺之后（图 2ai），XRD 证实了方铁矿 (FeO) 和磁铁矿 (Fe3O4) 的存在。在 36.33° (111)、61.40° (044)、73.25° (113) 的 2θ 处观察到方铁矿的布拉格衍射，与 ICSD 匹配：98–001-2335。在2θ为35.61°（113）和43.28°（004）处观察到磁铁矿相，同意参考Fe3O4 ICSD文件98-010-9826。

一 经过MST和CTST后氧化皮的X射线衍射图谱，b 在 3、5 和 7 小时的不同研磨时间下磁铁矿的索引光谱，c MNA的半峰全宽(FWHM)和微晶尺寸随球磨时间的变化

对于 CTST 工艺后的 XRD 谱（图 2aii），观察到磁铁矿、赤铁矿和方铁矿相的存在。所有峰的布拉格衍射角 (2θ) 是必不可少的识别和同意参考 ICSD 文件 98-010-9826 磁铁矿的峰是 30.23° (022), 35.61° (113), 43.28° (004), 57.24°（333），62.86°（044）。赤铁矿峰也在 33.44° (104)、48.48° (202) 处观察到，与 ICSD 98–004-6407 匹配）；和 36.33° (111) 和 61.27° (022) 的方铁矿衍射角与 ICSD 匹配：98–001-2335。结果与前人文献[84]报道的结果一致。

图 2b 显示了在 3、5 和 7 小时的不同时间间隔内高能球磨过程后氧化皮粉末的 XRD 衍射图。合成样品的衍射光谱表明在所有研磨时间中都存在直径为纳米的磁铁矿 (Fe3O4) 相。 30.24°、36.66°、36.57°、42.45°、57.26°、61.54°和62.76°的2θ衍射角可索引为（022）、（113）、（222）、（004）、（224） (115) 和 (044) 分别证实了立方晶胞 Fe3O4 的特征峰。 XRD 谱与参考 ICSD 98-01-11,241 的磁铁矿相匹配，空间群为 Fd -3 米 和晶格参数 (a =b =c ) 的 8.3440 Å。如图 2b 所示，随着研磨时间的增加，纳米磁铁矿吸附剂表现出高纯度。小瓶中钢球碰撞产生的高能量负责破坏氧键并将赤铁矿 (Fe2O3) 还原为磁铁矿 (Fe3O4) 相。随着研磨时间的增加，观察到纳米晶磁铁矿的形成是通过 XRD 峰的加宽来确定的。随着研磨时间的增加，观察到 XRD 峰展宽增加，表明粒径减小。还观察到 XRD 峰强度随着研磨时间的增加而降低。该图案表明样品的粒径减小 [61]。随着粒径的减小，研磨过程中引起的应变导致峰强度降低和衍射峰变宽。平均晶粒尺寸D 使用 Debye-Scherrer 公式计算样品的数量，如等式。 (6) [62]。

其中 D 是平均晶粒尺寸，λ 是 X 射线的波长 (0.1541 nm)，β 是半高宽 (FWHM)，θ 是衍射角。使用 X’pert Highscore Plus 软件自动分析 XRD 谱。 FWHM 和微晶尺寸的关系如图 2c 所示。分析表明，随着研磨时间从 3、5 和 7 小时的增加，微观 MNA 粉末的变化以及 FWHM 和 MNA 平均微晶尺寸的变化，如图 2c 所示。 FWHM 的变化趋势表明，随着研磨时间从 3、5 和 7 小时的增加，FWHM 呈现增加趋势。随着球磨时间从 3、5 和 7 小时增加，平均晶粒尺寸分别从最小值 17.8 nm、15.2 nm 和 9.8 nm 减小。

形态和微观结构组成

在 3、5 和 7 小时的不同研磨时间研磨的 MNA 的 HRTEM 显微照片如图 3 所示。显微照片显示 MNA 颗粒在三个研磨期间呈现出不规则的形状，如图 3 所示。此外，与 14.45 纳米（5 小时）和 19.16 纳米（7 小时）相比，在 3 小时研磨时间观察到平均 MNA 粒径为 5.53 纳米。这意味着可以在更短的研磨时间内获得更小的 MNA 粒径。 3、5 和 7 小时的平均粒径范围为 10 至 22 nm。随着研磨时间的增加，样品中的微应变也增加 [63]。因此，延长研磨时间会对样品产生更大的应变。晶格应变随研磨时间的增加是由于研磨过程中引入的晶格中的原子位错和扩散引起的强烈畸变效应。然而，由于磁铁矿粉末的磁吸行为，在 MNA 样品中观察到团聚效应，如图 3 所示。

在 a 处具有 20 纳米 MNA 比例尺的 HRTEM 图像 3 h b 5 h 和 c 7 小时研磨时间

磁特性分析

在室温下使用 VSM 实验研究样品的磁性能。磁化响应（M ) 与外加磁场 (H ) 的样品如图 4 所示。饱和磁化强度 (M s), 剩磁 (M r) 和矫顽力 (H c) 样品汇总在表 1 中。 MNA 的矫顽力值在 200-270 G 的范围内，剩磁在 1.5 和 6.6 emu/g 之间，饱和磁化强度值在 21 和 27 emu/g 之间。由于粒径小于 20 nm，样品具有超顺磁性。从磁化参数（表 1）来看，这表明样品由有助于增加吸附容量的超顺磁性和铁磁性化合物的混合物组成。 MNA-7 小时样品显示出最高的磁参数（图 4），这有助于铜吸附研究（图 5）的最高吸附容量。

M-H 不同球磨时间样品的滞后曲线a 3 h，b 5 h 和 c 7 小时

一 去除百分比条形图； b MNP对Cu ²⁺ 的金属吸收/吸附能力 在不同的研磨时间

吸附参数的影响

已经研究了对 MNA 3、5 和 7 小时的批量吸附研究的进一步分析。图 5 显示了在 3、5 和 7 小时的不同研磨时间对 MNA 进行的吸收研究。该图显示了最高的吸附容量（金属吸收）（q e) 和最高去除率 (%RE ）。 MNA-7 小时研磨时间显示出最高的吸附能力和最高的水溶液去除百分比。因此，选择MNA-7 h作为MNA纳米吸收剂，对接触时间、初始浓度、吸收剂用量、表面积、pH值和温度等几个参数进行批量吸收分析。

表面积分析

使用 BET 的氮吸附用于评估 MNA 7 小时研磨的表面积和孔隙特征。图 6a 显示了 MNA-7 小时的 BET 结果，平均孔体积为 0.011 cm ³ g ⁻¹ 比表面积为 5.98 m ² g ⁻¹ . MNA-7 h 的 N2 吸附-解吸曲线位于 III 型滞后曲线（图 6b）中，与 Sing 等人先前报道的一致。阿尔。 (1985) [65]。 BET 结果描述了吸附分子聚集在 MNA 表面周围的吸附剂 - 被吸附物相互作用 [66]。因此，III型吸附表明气体分子被物理吸附在MNA上[67]。

一 7 h MNA 的孔径分布b 氮吸附-解吸等温线

EDS 分析

MNA-7 h 的元素成分如图 7 所示。FESEM 和 EDS 分析显示，吸附过程前 MNA-7 h 元素 Fe 和 O 的存在百分比分别为 78.25% 和 21.75%（图 7）。 7a)。图 7b 描绘了吸附过程后元素 Fe、Cu 和 O 的 EDS 光谱。谱图中铜的存在表明对Cu ²⁺ 的吸附 由 MNA。这种趋势也与 Lingamdinne 等人报告的研究中显示的光谱一致。 (2016) [64] 氧化铁纳米颗粒用于吸附去除重金属。

EDS 频谱分析 (a ) 在吸附之前和 (b )吸附Cu(II)离子后

FTIR 分析

还通过 FTIR 分析进一步确认了 MNA-7 h 上的 Cu 吸附。 FTIR 用于鉴定 Cu ²⁺ 的官能团和连接 到 MNA。图 8 显示了吸附 Cu (II) 离子前后 MNA-7 小时的 FTIR 光谱。 FTIR 光谱揭示了 Fe3O4 纳米颗粒的强特征峰带。吸附后的 FTIR 光谱显示谱带强度在 500-600 cm ^-1 范围内发生变化 和 2800–3600 厘米 ⁻¹ 导致 Cu ²⁺ 吸附。此外，525 cm ⁻¹ 处的吸附带 和 576 厘米 ⁻¹ 代表 Fe-O 带磁铁矿纳米粒子的四面体和八面体位点 [68]。在 3478 cm ⁻¹ 处的强而宽的吸附光谱 对应于羟基 (- OH) 和 MNA 表面的微量水分子 [69]。 FTIR光谱表明，由于MNA表面吸附了少量化学物质，因此MNA具有晶体结构。

MNA-7 h吸附Cu(II)离子前后的傅里叶变换红外光谱

Zeta 电位分析

图 9 显示了 MNA-7 h 的表面 zeta 电位。对 Zeta 电位进行了研究，以准确准确地确定吸附过程中发生的中和电荷。 Zeta 电位数据为监测 Cu 吸附过程中的最佳吸附剂剂量提供了一个可测量的值。结果表明，等电点 (pHpzc) 出现在 pH 5.4，即 Cu ²⁺ 发生在 MNA 上的吸附处于最佳状态。在水溶液中，氧化铁的表面被OH ^- 覆盖 由于质子化或去质子化过程，表面上的 FeOH 可以变成其他 Fe 官能团，如 FeO 或 FeOH2 [70, 71]。质子化和去质子化的平衡取决于溶液的 pH 值和吸收剂的 pHpzc。 zeta电位结果表明在pH为5.4时吸附有效。

MNA-7 h 的 pH 值为 1、2、4、6、8、10 和 12 时的 Zeta 电位

批量吸附分析

接触时间的影响

图 10a 显示了接触时间对 Cu ²⁺ 吸附容量和速率的影响 250 分钟后摄取到 MNA。很明显，在较长的接触时间，吸附容量达到平衡，因为 pH 值保持在 5.4，吸附剂剂量保持在 0.05 g。如图 10b 所示，获得的最大去除效率为 62.61%。 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.

一 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].

一 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 H3O ⁺ 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 H3O ⁺ 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).

一 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⁺ 吸附。 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 (pHpzc). 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.

一 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].

一 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

Conclusions

Novel magnetite nano adsorbent (MNA) from mill scale waste has been successfully synthesized via conventional milling technique and impacted by high-energy ball milling procedure at varying milling time. The high-energy ball milling (HEBM) at 3, 5 and 7 h successfully produced the MNA in the range of 10–25 nm, as confirmed by HRTEM. The HEBM technique was used to reduce the microcrystalline size to nano-sized particles showing the potentials of MNA as an efficient precursor for Cu ²⁺ removal in an aqueous solution.

VSM results showing the MNA-7 h possess the highest magnetization property and indicate the best absorbent, with the specific surface area of 5.98 m ² g ⁻¹ and the average pore size of 8.01 nm thereby showing the better adsorption capacity. The adsorption of Cu ²⁺ on MNA-7 h was confirmed by EDS and FTIR analysis. For the adsorption studies, pH at 5.4, dosage of 0.05 g and 240 min of contact time, the highest adsorption capacity, q e and removal efficiency of 4.408 mgg ⁻¹ and 62.61% were achieved. Also, at the initial concentration of 50 mgL ⁻¹ Cu ²⁺ , the q e of 4.41 mgg ⁻¹ was recorded. The reusability efficiency of 70.87% was attained even after three cycles of reapplications and desorption. The Temkin adsorption isotherm fits best with a correlation coefficient, R ² of 0.91. Based on these findings, it can be inferred that MNA is a promising precursor for Cu ²⁺ removal.

Availability of data and materials

The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.