Abdullah A. Basaleh, Muhammad H. Al-Malack,*, Tawfik A. Saleh

1 Department of Civil and Environmental Engineering, King Fahd University of Petroleum and Mineral, Dhahran 31261, Saudi Arabia

2 Department of Chemistry, King Fahd University of Petroleum and Mineral, Dhahran 31261, Saudi Arabia

Keywords:Waste management Adsorption Low-cost adsorbents Nanomaterials Polymerization Composites

A B S T R A C T In this research,polyamide modified baghouse dust nanocomposite(PMBHD)was synthesized from steel industry waste using the interfacial polymerization technique.Adsorption capacities of the PMBHD were examined for the uptake of cadmium Cd (II), lead Pb (II), and methylene blue MB from simulated solutions.The effects of different operational factors of the adsorption,including contact time,pH,adsorbent dosage, initial concentration,and temperature,were investigated.The obtained results revealed that the equilibrium data of MB, Pb (II), and Cd (II) were best fitted to Dubinin-Radushkevich, Langmuir, and Freundlich isotherm. Maximum removal uptake was found to be 6.08, 119, and 234 mg·g-1, whereas maximum removal efficiencies of 90%, 99.8%, and 98% were achieved for MB, Pb (II), and Cd (II).Adsorption kinetics of MB and metals well-fitted to the pseudo-second-order kinetic. The characterization results showed the presence of polymeric chain on the surface of the PMBHD. The thermodynamic study revealed that the values of the free energy ΔG for Pb (II) and Cd (II) were found to be negative,which indicates spontaneous,energetic,and favorable adsorption.While for MB removal,positive values of (ΔG) were noticed, which implies that the adsorption was unfavorable. The proposed mechanism for the adsorption of MB and metals on the PMBHD showed that the dominating mechanism is physisorption. The adsorption/desorption results verified the high reusability of the PMBHD for adsorption of MB and metals.

1. Introduction

Recently,considerable growth in urban and industrial activities has been experienced,which resulted in a daily discharge of pollutants into water streams [1,2]. Organic and inorganic pollutants,such as dyes and metals, are generated from many manufacturing processes, including printing, textile, batteries, paint, food, pulp and paper, cosmetics, electroplating,electrical, car manufacturing,pesticides, and mining [3]. In addition to the non-biodegradability of metals and dyes,they are toxic and accumulate in human tissues and food chains, causing many adverse environmental and health issues[4-6].Therefore,it is an utmost priority to remove or reduce the concentrations of such pollutants to meet the respective regulations and save flora and fauna. Several conventional and nonconventional methods are currently available for the remediation of wastewater containing dyes or metal pollutants, including ion exchange,electrochemical treatment,coagulation and flocculation,disinfection, membrane filtration, reverse osmosis, photocatalysis,and adsorption [6,7]. Some of these conventional methods are either ineffective or expensive [8]. Treatment of wastewater contaminated with dyes or metals using the adsorption method is one of the most extensively used techniques thanks to its efficiency, applicability for treating organic and inorganic pollutants,and simplicity in operation and design [9]. Commercial activated charcoal is one of the most extensively used adsorbents for organic and inorganic pollutants removal from wastewater[4,7].However,the production cost still an obstacle to its application in many cases. Therefore, many investigations have been explored recently to produce low-cost alternative adsorbents.

The usage of industrial waste materials,which potentially cause adverse effects on humans and the environment,for the removal of organic and inorganic pollutants is a promising attractive solution.Two significant advantages would be achieved if those materials could be used efficiently for wastewater treatment; low-cost adsorbents and effective environmentally friendly waste management. The steelmaking industry generates a considerable amount of solid waste in the form of huge stockpiles, which include steel furnace slag SFS, baghouse dust BHD, and sludge. Steelmaking by-products have been utilized for various applications of the civil engineering sector,such as the cement industry,road construction,and hydraulic engineering [10]. However, the high content in free MgO and CaO limited their use in the civil engineering sector[11].Although the recycle and reuse of steel production waste is the regular practice[12,13],still considerable amount is dumped in landfills. However, a shortage of landfill locations, rising landfill fees,and stringent environmental standards encourage investigation for alternative potential utilization for other purposes such as wastewater treatment.

Several studies have been recently reported on the use of modified steel manufacturing waste as an adsorbent for wastewater treatment. For example, Bhatnagaret al. [14] investigated the use of activated dust, sludge, and furnace slag wastes from steel manufacturing plant for lead removal from aqueous solutions.Modified basic oxygen furnace slag was also investigated for dye removal by Xueet al. [15]. Furthermore, modified blast furnace slag from the steelmaking process was examined for the treatment of wastewater containing phosphorous by Gonget al.[16] Unmodified and modified steelmaking slags were investigated for cadmium removal from acidic solution by Duan and Su [17]. Thermally modified steel slag was also studied for phosphate removal from domestic wastewater by Yuet al[18].Besides, modified steel slag was utilized by Repoet al[19]. as a potential inexpensive alternative for the removal of metals from simulated solutions in the presence of chelating agents. Recently,many studies investigated the use of magnetic nano adsorbents for wastewater treatment due to their efficient removal and easy separation [20,21]. However, to provide magnetic properties, typically, iron oxides are loaded to the adsorbents [22,23], which requires extra cost due to the use of iron reagentsviacomplex methods. On the other hand, steel BHD already have magnetic properties due to the high content of iron oxides. The XRF(X-ray fluorescence) analysis of the collected BHD sample showed that it contains FeO(61%).Although BHD has magnetic properties,however, its adsorption efficiency for dyes and metals is limited.The reviewed literature revealed that many investigations that had been employed to enhance the adsorption efficiency of steelmaking waste. However, most of these studies focus on the development of a mesoporous structure that requires high temperatures and the use of different chemicals that incur high preparation costs. In this sense, the current study investigates the enhancement of BHD adsorption efficiency by attaching the polyamide polymer to the BHD, which does not require a high temperature. The polyamide modified baghouse dust PMBHD contains -NH and C＝O groups on its surface, which have a good attraction for cationic species such as MB and metals. Besides, the magnetic properties of the PMBHD will reduce the separation time and cost in large scale applications using magnetic separation [24-26].

This study reports the synthesis of PMBHD by grafting polyamide branches on baghouse dustviainterfacial polymerization,using 1,3-phenylenediamine in an aqueous phase and trimesoyl chloride in the organic phase. The adsorption performance of the PMBHD towards MB,Cd(II),and Pb(II)was investigated in a single and multi-component system. The influence of various conditions such as pH,concentration,time,and dosage on the uptake behavior of the PMBHD was also studied. The proposed interaction mechanism between the PMBHD and the targeted pollutants was also introduced. The results suggested that PMBHD can be used for the decontamination of industrial wastewater, including those from batteries and textiles.

2. Materials and Methods

2.1. Chemicals and reagents

Chemical reagents used in the current research are of analytical grade. Ultrapure double distilled water (Millipore, Milli-Q) was used for the preparation of solutions. Cadmium nitrate tetrahydrate(Cd(NO3)2·4H2O)and lead nitrate(Pb(NO3)2)were purchased from BDH Chemicals, Poole, England. Laboratory grade methylene blue was purchased from Fisher scientific company. One gram of methylene blue was dissolved in 1000 ml of water to obtain a 1000 mg·L-1stock solution. 1.6 g of Pb (NO3)2, or 2.744 g of Cd(NO3)2·4H2O was dissolved in deionized water to prepare metals stock solutions (1 L, 1000 mg·L-1). The values of pH were adjusted using diluted NaOH and HNO3.n-hexane,m-phenylenediamine,trimesoylchloride, and thionyl chloride were purchased from Sigma Aldrich.

2.2. Synthesis of baghouse dust nanocomposites

Steel manufacturing waste material,or baghouse dust,was collected from a steel plant. It was modified with 1,3 phenylenediamine (MPD) and trimesoyl chloride (TMC) to form polyamidemodified baghouse dust (PMBHD),viainterfacial polymerization,as shown in Fig. 1. In this regard, The BHD was dispersed in 100 ml of water with the help of a sonicator. MPD in the aqueous phase was prepared by adding 2 g of MPD to 100 ml of water,and the solution was maintained under sonication until MPD dissolved in water. Then, the dispersed BHD was added to the MPD solution under agitation, and the mixture was left for 10 min to allow the interaction between the amine group and the BHD. Besides that,TMC in an organic phase was prepared by adding 0.2 g of TMC to 100 ml of hexane under sonication. Then, the prepared solution was mixed in drops to the solution of BHD-MPD and kept under agitation for 24 h to have a homogenous composite. The obtained PMBHD was heated at 60℃ for around two hours; then, it was allowed to cool down to room temperature. Eventually, it was filtered, washed thoroughly using deionized water to remove any remaining chemicals, and dried.

Fig. 1. Synthesis of the polyamide modified baghouse dust (PMBHD).

Fig. 2. SEM/EDS (a, b) unmodified BHD, and (c, d) PMBHD.

3. Results and Discussion

3.1. Adsorption performance of the synthesized PMBHD

Adsorption performance of BHD before and after modification was conducted by adding 1 g·L-1to 15 and 50 mg·L-1of MB and metals, respectively, at pH 5 for 24 h. Removal efficiencies of the BHD for methylene blue,cadmium,and lead removal from aqueous solutions before and after modification are depicted in Table S1(Supplementary Material). Maximum enhancement of about 40%was noticed for MB and Cd(II)removal,while for lead removal,the improvement was found to be around 15%. The obtained results indicated that removal efficiencies of the PMBHD for MB and metals were found to increase due to the modification process.

3.2. Material characterization

Surface morphology of unmodified and the PMBHD visualized by scanning electron microscope (SEM) are shown in Fig. 2(a)and (c), respectively. Clusters of hexagonal-shaped particles with protrusions and sharp edges were observed on the surface of unmodified BHD, which might be ascribed to the presence of iron oxides nanoparticles.Similar morphology was reported by Nasrollahzadehet al[27], who synthesized Fe3O4-SiO2nanocomposite.On the other hand, groups of agglomerated particles with smooth edges were noticed on the surface of the PMBHD sample, as depicted in Fig. 2(c). The presence of agglomerated particles with a smooth surface on the PMBHD micrograph indicates that the polyamide chain was synthesized on the surface. The presence of the polyamide was also verified by the analysis of surface compositions detected by energy dispersive spectrometer (EDS), as shown in Fig. 2(b) and (d). The main elemental compositions of unmodified and the PMBHD are iron,carbon,oxygen,calcium,zinc,and magnesium. The EDS of the PMBHD shows the presence of nitrogen,which indicates the presence of the polyamide chain[28].

The X-ray diffraction(XRD)patterns of unmodified and PMBHD samples are depicted in Fig.3.It was noticed that the main peaks of unmodified and the PMBHD pattern are almost identical.However,the PMBHD has lower intensities, which implies that the crystal structure has not been significantly changed due to the synthesis process. However, low intensities of the PMBHD pattern indicates that the crystal size was found to decrease, which might be ascribed to less crystallinity of polymer chain due to its amorphous nature [29]. The primary component was found to be magnetite.The large characteristic peaks at 2θ of 35.407°, 30.07°, 42.926°,53.3°, 56.68°, 62.37°, and 73.67°are ascribed to cubic spinel magnetite [30]. Moreover, minor components of calcite and zinc oxide phases were also identified, where peaks at 2θ of 29.48°, 42.92°,47.67°, and 56.779°might be ascribed to the calcium compound,whereas the peaks at 2θ of 31.86°, 34.51°, 36.363°, 47.65°, 62.97°and 68.058°could be attributed to the zinc oxide. The presence of a new peak of the PMBHD diffraction at 2θ around 27°might belong to the polyamide chain.

Fig. 3. XRD pattern of unmodified BHD and the PMBHD.

The Fourier Transform Infrared (FTIR) spectroscopy of unmodified and the PMBHD was reported between 500 and 4000 cm-1,as depicted in Fig. 4. The FTIR spectroscopy of unmodified BHD exhibits a peak at 1471 cm-1, which could be attributed to the C-O asymmetric stretching group, while the band at 1418 cm-1could be attributed to Si-O valence vibrations[19].The peaks at around 990 and 873 cm-1might be assigned to the stretching vibration of the Al-O and Si-O. Moreover, the peak at 564 cm-1might be ascribed to the Fe-O bending vibration [30-33]. In addition to the bands previously shown in the unmodified BHD, the spectra of the PMBHD nanocomposite showed many new peaks related to the polyamide group. The new broad absorption at 3416 cm-1is assigned to the N-H stretching vibration. Moreover, the new band at 1625 cm-1was due to C＝O stretching vibration. The new absorption band at 1030 cm-1is attributed to C-N stretching vibrations. The obtained IR spectra of the PMBHD confirmed the presence of the amide group, indicating that the polymer chain is attached to the surface of the PMBHD, which agrees with the EDS analysis where nitrogen and carbon were reported.

Fig. 5(a) shows the nitrogen adsorption/desorption isotherm graph of the unmodified and PMBHD. Both isotherms follow type IV,which implies a mesoporous structure.The mesoporous characteristics of unmodified and PMBHD are shown in Table 1. Significant enhancement in the porous characteristics of the PMBHD was noticed. The Brunauer-Emmett-Teller (BET) surface area was enhanced from 6.82 to 12.94 m2·g-1, and the pore volume was found to increase from 0.025 to 0.05 cm3·g-1when compared to the unmodified BHD. The XRF analysis of BHD and the PMBHD are depicted in Fig. 5(b). The major components of BHD and PMBHD were iron and zinc oxides with minor oxides of Ca and Si and traces of Mn and Ti oxides. The magnetic property of the PMBHD is gained from the high content of iron oxides, as shown in Fig. 3.

Table 1 Porous characteristics of the unmodified BHD and PMBHD

The SEM/EDS was also conducted for the PMBHD after uptake of MB and metals,as shown in Fig.6.The presence and distribution of lead ions on the PMBHD surface were confirmed, as depicted in Fig. 6(a) and (b). Similarly, the presence and the distribution of cadmium ions on the PMBHD surface were confirmed, as illustrated in Fig. 6(c) and (d). In addition to carbon and nitrogen, the structure of MB contains sulfur,therefore,the presence and distribution of sulfur ions depicted in Fig.6(e)and(f)indicated that MB molecules are present on the adsorbent surface.

Fig. 4. FTIR spectra of unmodified BHD and PMBHD.

3.3. Adsorption study

3.3.1. Effect of pH

The variation in the uptake efficiencies of Pb(II),Cd(II),and MB was investigated at different pH values at room temperature(25℃). The adsorption efficiency of MB was studied at pH values from 2 to 12, while for Pb (II) and Cd (II) removal, pH values were varied from 2 to 7 and from 2 to 10.For MB,the experiments were conducted at an adsorbent dosage of 1 g·L-1,a contact time of 3 h,and an initial concentration of 15 mg·L-1. Fig. 7(a) depicts the MB removal efficiencies at various pH values. An increase in removal efficiency from 27% to 65% was noticed when pH was increased from 2 to 12.For Pb(II)and Cd(II)removal,experiments were performed at an initial concentration of 50 mg·L-1, an adsorbent dosage of 0.15 and 0.25 g·L-1, and a contact time of 3 h. The removal efficiencies of Pb (II) and Cd (II) at different pH values are shown in Fig.7(b) and(c). Similar trends for both metals were observed, where removal enhancement was noticed by increasing pH values. The removal efficiency of Pb (II) was found to enhance from 10% to 100% when pH was increased from 2 to 7, whereas for Cd (II) removal, an increase in pH from 2 to 10 resulted in removal enhancement from 2%to 90%.Removal enhancement due to pH increase could be attributed to the speciation of different ions at different pH values and adsorbent-adsorbate electrostatic interactions. At low pH values, intense competition takes place between H+and the cationic species of MB or metal ions for adsorbent sites. However, by increasing pH values, the H+decrease,which reduces competition. Furthermore, a significant increase in removal efficiencies for both metals were observed at high pH values. The removal efficiency of 100% and 90% were noticed at pH values of 7 and 10 for Pb (II) and Cd(II). This might be attributed to both adsorption and hydroxide precipitations, which occur at pH values more than 8 and 6 for Cd (II) and Pb (II), as confirmed by blank samples obtained at different pH values.

Fig. 5. (a) N2 adsorption/desorption isotherms plot, (b) XRF analysis of BHD and PMBHD.

Fig. 6. Surface compositions (EDS) and mapping of the PMBHD after adsorption of (a and b) Pb (II), (c and d) Cd (II) and (e and f) MB.

The obtained results can also be attributed to adsorbent surface charge change at different pH values. The negative charge of the adsorbent surface increases as pH increases, which increases the attraction between adsorbent surface and the cationic species of MB or metal ions, thus increasing the removal ability. The change of the PMBHD surface charge was investigated by measuring the zeta potential of the PMBHD at various pH values, as presented in Fig. 7(d). As shown in the results, the zeta potential value was found to change from positive values (+26.5, +6.96, and+3.64 mV) at acidic pH range to negative values (-6.83, -12.3,and -24.7 mV) at basic pH range. This explains the variation in removal efficiencies at different pH, where low removal was reported at low pH values due to the repulsion between the adsorbent surface positive charge and cationic species of MB or metals,while high removal efficiencies were noted at high pH because of the attraction between the cationic MB or metals species and the adsorbent surface negative charge, as depicted in Fig. 7(d).

Fig.7. Effect of pH on removal efficiency of(a)MB(b)Pb(II)and(c)Cd(II)using PMBHD,(d)Zeta potential of the PMBHD.Conditions;(a)Co=15 mg·L-1,dosage=1.0 g·L-1,t = 3 h, (b) Co = 50 mg·L-1, dosage = 0.15 g·L-1 and t = 3 h (c) Co = 50 mg·L-1, dosage = 0.25 g·L-1 and t = 3 h.

3.3.2. Effect of initial concentration

The effect of Cd (II), Pb (II), and MB concentrations on removal efficiencies was investigated at room temperature (25℃). Batch experiments of MB removal were performed at an adsorbent dosage of 1 g·L-1, a contact time of 3 h, an initial pH value of 5,and initial concentrations between 3 and 25 mg·L-1, as shown in Fig. S1(a) (Supplementary material). It was observed that by increasing the initial concentration from 3 to 25 mg·L-1, a gradual reduction of the removal from 65% to 35% was noticed. The effect of metal concentrations on the removal efficiencies is depicted in Fig. S1 (Supplementary material). The concentrations of Cd (II)and Pb (II) were varied between 12.5 and 100 mg·L-1, at pH 5,and mixed for three hours with a constant dosage of 1 and 0.15 g·L-1.Similarly,the increase in concentration led to a decrease in uptake efficiency. A decrement in Pb (II) removal from 100% to 55% was noticed by increasing the concentration from 12.5 to 100 mg·L-1.In contrast,for Cd(II)removal,an insignificant change in removal was observed by increasing concentration from 12.5 to 50 mg·L-1.In comparison,by increasing initial concentration from 50 to 100 mg·L-1, a gradual decrease in removal efficiency from 90% to 55% was noticed. The reduction in removal efficiency due to the increase in initial concentrations could be attributed to the fact that a finite number of high-energy sites are available on the adsorbent surface, which is enough to remove pollutant cations at low concentration. However, at high concentrations, those sites become saturated, and insignificant removal takes place.

3.3.3. Effect of the PMBHD dosage

The influence of the PMBHD amount on uptake efficiencies of MB and metals are presented in Fig. S2 (Supplementary material).Batch experiments of MB and metals were performed at room temperature (25℃), an initial concentration of 15 and 50 mg·L-1, pH value of 5, and contact time of 3 h.

Fig. 8. Pseudo-first order plot; (a) MB (b) Pb (II) and (c) Cd (II). Pseudo-second order plot; (d) MB (e) Pb (II) and (f) Cd (II).

The obtained results revealed that the increase in the PMBHD dosage enhanced the uptake rate of metals and MB dramatically.However, after a certain amount of PMBHD, the increase becomes insignificant. For example, by increasing the adsorbent dosage from 1 to 4 g·L-1, the removal efficiency of MB was found to increase from 36% to 85%. At high dosages, more active sites are available on the adsorbent surface, which increases the removal efficiency. However, insignificant enhancement (3.5 percent) was noticed by further increasing the dosage to 6 g·L-1, which could be ascribed to the overlapping of adsorbent surface sites at a high dosage. Similarly, apparent enhancement in Pb (II) and Cd (II)removal efficiencies were noticed when dosages were increased from 0.05 to 0.25 and from 0.15 to 1 g·L-1.

3.3.4. Effect of temperature

The effect of temperature on the removal efficiency of MB and metals was investigated at temperatures of 25, 40, and 60℃, as depicted in Fig. S3 (Supplementary material). Batch experiments of MB were performed at a dosage of 1 g·L-1, pH value of 5, and 15 mg·L-1initial concentration, whereas for metals, experiments were performed by mixing 0.15 and 0.5 g·L-1of PMBHD with 50 mg·L-1of Pb (II) and Cd (II). As temperature increases, the uptake rate of MB and metals also increases(Fig.S3).An enhancement in adsorption efficiency of MB from 27 to 45 was noticed when the temperature was increased from 25 to 60℃,as depicted in Fig.S3(a).A slight increase in the removal of metals was noticed at higher temperatures. For example, as depicted in Fig. S3(c), the uptake efficiency of Cd (II) was observed to increase slightly from 66% to 75% with increasing the temperature from 25 to 60℃.The enhancement in removal due to the increase in temperature might be ascribed to the fact that pore sizes might be enlarged at a higher temperature,which allows more molecules of the targeted pollutant to be adsorbed. Furthermore, it is known that the molecule diffusion rate of the targeted pollutant across the external boundary layers increases with increasing the temperature[34,35]. Also, the adsorbent equilibrium capacity for a particular adsorbate might be changed at different temperatures [36]. Moreover,enhancement in metals adsorption might be attributed to the endothermic uptake of divalent cations [37].

3.4. Kinetic studies

Kinetic parameters for the uptake of MB and metals were evaluated by applying the linear form of the Pseudo-first and secondorder models, as described in the Supplementary material. The experimental kinetics of MB and metals presented in Fig. S4 (Supplementary material) involved two stages; rapid and slow rate.Depending on the dosage and pollutant, the rapid uptake occurs in the first 15-60 min, and then it continues at a slow rate until equilibrium is reached in 60-90 min. The fitted kinetic models are illustrated in Fig. 8. Table 2 shows the calculated parameters of each model. Higher correlation coefficients and better agreement between the experimental and computed values of qe were observed for the second-order model. Therefore, the kinetics of MB and metals follow the pseudo-second-order model.

Table 2 Pseudo first and second-order parameters for MB and metal removal on the PMBHD

Table 3 Parameters of equilibrium isotherms for MB, Cd (II) and Pb (II) adsorption onto the PMBHD

3.5. Equilibrium isotherms

The linear form of Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich models for MB, Cd (II), and Pb (II) were depicted in Fig. S5 (Supplementary Material). The linear and non-linear variables for all models are given in Table 3. Details of all models’parameters computed using the linear forms are shown in Supplementary material. The non-linear fitting of the experimental data of MB and metals to various isotherm models were depicted in Fig. 9, and the calculated parameters were presented in Table 3.

Fig. 9. Non-linear equilibrium adsorption isotherms for MB, Cd (II) and Pb (II) removal on the PMBHD; (a) Langmuir, (b) Freundlich, (c) Temkin and (d) Dubinin-Radushkevich.

Fig. 10. Simultaneous removal of MB and metal from simulated industrial wastewater on the PMBHD.

Fig. 11. Proposed adsorption mechanisms for (a) MB and (b) Metals on the PMBHD.

Fig. 12. FTIR spectra after adsorption of MB, Cd (II) and Pb (II) on the PMBHD.

The results of linear and non-linear equilibrium isotherms revealed that the equilibrium data of MB was reasonably fitted to D-R and Langmuir isotherms, as indicated by the correlation coefficientR2. However, maximum adsorption capacity obtained from the experimental data were in better agreement with that calculated from D-R isotherm. Therefore, the experimental data of MB follow the D-R isotherm model. Moreover, the non-linear fitting of Cd (II) experimental data showed that Freundlich isotherm was best fitted, which also consistent with the linear fitting. The experimental data of Pb (II) was well fitted to the linear and non-linear isotherms of Langmuir and D-R. However, Langmuir isotherm showed higherR2values and better agreement with the experimental data. Therefore, Langmuir isotherm can be followed for Pb(II)experimental data.Accordingly,the experimental data of MB, Cd (II), and Pb (II) adsorption on the PMBHD were appropriately described with D-R, Freundlich, and Langmuir isotherm,respectively.Freundlich isotherm suggests a multilayer adsorption process on heterogeneous active cites on the adsorbent surface[38]. Langmuir assumes that the adsorption occurs as one layer on a homogeneous adsorbent surface [39], and D-R suggests the type of adsorption [40]. Therefore, the MB and Pb (II) ions are adsorbed as a monolayer, while Cd (II) ions are adsorbed as multilayers on the surface of the PMBHD.The values of the separation factor of LangmuirRLand 1/nof Freundlich for MB and metals were found to be between 0 and 1, which implies the favorability of the adsorption process[41].Moreover,mean energyE(J·mol-1）of the D-R isotherm indicates the type of adsorption, where physical adsorption is reported atEvalues below 8 kJ·mol-1,whereas E values more than 16 kJ·mol-1indicate chemisorption[42,43].TheEvalues obtained in the current study for MB,Cd(II),and Pb(II)were below eight kJ·mol-1, which implies that the removal of MB, and metals onto the PMBHD is physical adsorption. Comparative uptake capacities of different modified materials for MB and metals are summarized in Table 4,where comparable capacities of the PMBHD for adsorption of MB and metals were observed.

Table 4 Comparison of adsorption capacities of the PMBHD with various adsorbents for MB and metals

Table 5 Thermodynamic parameters of Pb (II), Cd (II) and MB adsorption on the PMBHD

Table 6 Properties of the investigated metals

3.6. Thermodynamic investigations

Nature and favorability uptake of MB and metals on the PMBHD were studied by calculating parameters of thermodynamics.Enthalpy (ΔH), entropy (ΔS), and free energy (ΔG) were determined using equations given in Supplementary Material. Table 5 shows the thermodynamic variables of MB, Pb (II), and Cd (II).The free energy (ΔG) indicates favorability and adsorption degree of spontaneity. The results of thermodynamics showed that for metals, negative values of (ΔG) were reported, and by increasing the temperature, the negativity also increases, which implies that the adsorption is favorable and energetic. While for MB uptake,positive values of(ΔG)were noticed,which means that the adsorption is unfavorable. Moreover, the adsorption nature of MB and metals was endothermic, as confirmed by the positive values of(ΔH).Furthermore,randomness increase during the uptake of metals and MB was indicated due to the positive values of(ΔS).Moreover, the magnitude of the enthalpies (ΔH) and the activation energies (Ea) were below 40 kJ·mol-1, which suggests that the uptake mechanism of metals and MB on the PMBHD is a physisorption process [57].

3.7. Simultaneous removal of MB and metals

Industrial wastewater might contain both organic and inorganic contaminants,including dyes and metals.Therefore,to investigate the efficiency of the produced adsorbent for industrial wastewater treatment, adsorption experiments were conducted on simulated industrial wastewater that contains a mixture of MB and metals including Pb,Cd,Cr,Cu,As,Ni,Zn,Co,and Al.The removal efficiencies of MB and various metals are illustrated in Fig. 10. The reported findings suggested that the PMBHD could be used efficiently for industrial wastewater treatment, where removal efficiencies of 64.17%, 85%, 96.15%, 96.35%, 97%, 99.83%, 99.80%,99.55%, and 98.14% were reported for As, Co, Cd, Zn, Ni, Cr, Cu,Pb, and MB, respectively.

3.8. Proposed adsorption mechanism of MB and metals onto the PMBHD

Although MB blue is soluble in water, it has N(CH3)2in both ends of its structure, which might cause a hydrophobic behavior[58].Adsorption mechanism of MB onto the PMBHD might include electrostatic and non-electrostatic interactions between the active sites on the PMBHD surface and MB molecule. Possible electrostatic interaction could evolve between the partial negative charge of the oxygen and nitrogen on the surface of the PMBHD and the cationic sulfur of the dye molecule. Moreover, the interaction of hydrogen bonding might evolve between the nitrogen atom on the MB and the hydrogen atom of the amide group. Adsorbentadsorbate non-electrostatic interaction could involve π-π stacking,van der Waals forces, and hydrophobic interactions [59,60]. The interaction of π-π stacking could be developed between the benzyne ring of the MB molecule and the benzyne ring on the surface of the PMBHD.The proposed interactions between the PMBHD and MB molecule is depicted in Fig. 11(a).

Uptake of metals on the PMBHD is attributed to several interaction mechanisms, including electrostatic and non-electrostatic attraction, ion exchange, and metal complexation. The proposed adsorption mechanism of metals using the PMBHD is depicted in Fig.11(b).The possible electrostatic attraction takes place between the metals positive charge and the partial negative charge of the oxygen and nitrogen atoms of the amide group on the PMBHD surface. Non-electrostatic forces may involve van der Waals forces.Furthermore, a cationic exchange might be taking place between the calcium present in the BHD in the form of calcite and the metal cations.Moreover,metal complexation might be involved between cationic metal species and the NH group on the PMBHD surface[30].

The thermodynamic study of MB, Pb(II), and Cd (II) adsorption on the PMBHD suggested that physisorption is the dominating process,where the activation energiesEaand the enthalpy were below 40 kJ·mol-1[28].

The affinities of metals and MB toward the PMBHD were ordered as Pb(II)＞Cd(II)＞MB This might be ascribed to the properties of the investigated pollutants,as shown in Table 6.The active sites on the PMBHD might have a higher physical affinity for Pb2+ions due to its bigger ionic radius, higher electronegativity, and smaller hydrated ionic radius when compared to Cd2+ions. Moreover,the low adsorption capacity of MB when comparing to metals might be ascribed to its molecular nature;therefore,it has the lowest affinity.

The proposed mechanisms were confirmed by the FTIR spectra of the PMBHD after adsorption,as illustrated in Fig.12.The FTIR of the PMBHD after MB adsorption showed that the C＝O group was found to be shifted from 1625 to 1603 cm-1, while the peak of the N-H group shown at 3416 cm-1in the spectra of PMBHD disappeared after adsorption. This change confirms the significant role of the amide group in the adsorption process.Similar findings were noticed for the FTIR spectra of the PMBHD after the uptake of Pb (II) and Cd (II), where the band of NH group at 3416 cm-1disappeared, while C＝O group shifted to 1633.5 and 1631 cm-1.

3.9. Regeneration of the PMBHD

The reuse of a spent adsorbent is a significant aspect of practical applications. The PMBHD was regenerated after adsorption of MB and metals using different eluents,as shown in Fig.13.The PMBHD was regenerated for MB using different eluents, including deionized water at pH value of 2 and 10 and 0.1 mol·L-1concentrations of HCl, HNO3,and NaOH, as depicted in Fig. 13(a). The highest adsorption capacity of PMBHD for MB was noticed in the second cycle for the sample that was regenerated using NaOH. Therefore,the PMBHD was regenerated five times using NaOH, where an insignificant drop of the adsorption capacity was observed, as shown in Fig. 13(b). The regeneration of the PMBHD for cadmium at acidic solutions such as deionized water at a pH value of 2 and 0.1 mol·L-1concentrations of HCl and HNO3is shown in Fig. 13(c). It was observed that the best regeneration was achieved for the sample that was desorbed using 0.1 mol·L-1of HCl. Therefore,further regeneration of the PMBHD for cadmium and lead was conducted using 0.1 mol·L-1concentration of HCl.The reduction of the adsorption capacities of the PMBHD for Pb (II) and Cd (II) was around 15% after five cycles, as shown in Fig. 13(d). The obtained results confirmed the high reusability of the PMBHD for adsorption of MB and metals.

Fig. 13. Regeneration of PMBHD for (a) MB using different eluents, (b) MB using NaOH, (c) Cd (II) using different eluents and (d) Cd (II) and Pb (II) using HCl.

4. Conclusions

Efficient polymeric magnetic nanocomposite adsorbent was synthesized from readily available steel manufacturing waste baghouse dust(BHD).In-situinterfacial polymerization was employed for the synthesizing process, where the baghouse dust waste was modified with the polyamide chain to produce polyamide modified baghouse dust nanocomposite (PMBHD). The PMBHD is ecofriendly sustainable nanocomposite, industrial waste (BHD) was converted to an efficient adsorbent that can be used for environment protection by the elimination of harmful pollutants such as metals and dyes. Therefore, two benefits were gained; waste minimization and pollution prevention. Adsorption performance for the synthesized nanocomposite was tested for single and simultaneous uptake of metals and MB from aqueous solutions.Maximum removal efficiencies of MB, Cd (II), and Pb (II) onto the PMBHD were 90%, 98%, and 99.8%, whereas maximum adsorption uptake of 6.08, 119, and 234 mg·g-1was reported. The equilibrium isotherm study revealed that Dubinin-Radushkevich, Freundlich,and Langmuir isotherms described the experimental results of MB,Cd(II),and Pb(II).Moreover,kinetic studies of MB and metals were quantitively described by the pseudo-second-order kinetic model.The thermodynamic investigations indicated that the dominant mechanism for MB, Pb (II), and Cd (II) adsorption onto the PMBHD is physisorption. The adsorption of MB and metals on the PMBHD were fast, and the equilibrium was attained within 50 min. Moreover, excellent adsorption behavior was noted in the presence of many co-existing ions. This makes the PMBHD promises adsorbent for real wastewater treatment. The regeneration studies proved that the PMBHD is recyclable and can be used efficiently for five cycles.The synthesized nano-adsorbent is rich in ferrous oxides, which are environmentally friendly, abundantly available on Earth’s crust. Therefore, it may be utilized for in-situ remediation of contaminated water with organic or inorganic pollutants with zero or the low possibility of secondary contamination. Moreover, PMBHD is advantageous over other adsorbents due to its magnetic properties, which reduces the separation time and cost in large scale applications using magnetic separation.

Acknowledgements

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully thank King Fahad University of Petroleum and Minerals (Saudi Arabia, Dhahran) for the valuable support provided during this study.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.08.050.