Characterization Analysis of Mudstone and Study of Its Pb (II) Adsorption Characteristics in Polluted Water Bodies

In order to explore a medium material for the efcient treatment of Pb(II) pollutants in groundwater, in this paper, mudstone is selected as the medium material, and the morphological structure of the mudstone is characterizes via X-ray difraction (XRD), scanning electron microscopy (SEM), and Brunauer–Emmett–Teller (BET) analyses to study the feasibility of the mudstone adsorbing Pb(II) ions. Ten, static adsorption experiments are carried out to investigate the removal efect of mudstone on Pb(II) in aqueous solutions under diferent conditions and to determine the optimal adsorption conditions. Finally, the results are ftted and analyzed using a thermodynamic model to explore the adsorption mechanism of the mudstone. Te main results of this study are as follows. Te main mineral composition of the mudstone used in the experiments includes CaCO 3 , SiO 2 , CaAl 2 O 4 · 10H 2 O, and CaFe 4 O 7 . Te specifc surface area of the mudstone is as high as 23.027m 2 · g − 1 , the pore size is 9.145nm, and its surface structure is rough, with pores and fssures developed. Te pore space and adsorption capacity of the mudstone were enhanced. When 1g · L − 1 of mudstone was added, the pH value of the solution was 6, the reaction time was 60min, and the initial concentration of Pb(II) was 30 mg · L − 1 . Te removal efciency of Pb reached 84.5%, and the adsorption amount was 25.352mg · g − 1 . For the removal of Pb(II) from the aqueous solution by the mudstone under diferent concentrations of Pb(II), the reaction was in accordance with the Langmuir adsorption isotherm model, and the maximum adsorption amount reached 54.975mg · g − 1 . Te relationship between the removal of Pb(II) and the reaction time was in accordance with the pseudo-second-order rate model. Te results of this study suggest that mudstone can be used for the removal of Pb(II) from aqueous media.


Introduction
Water is an indispensable natural resource in human life and is an integral part of the ecological and environmental system [1].Currently, due to the rapid development of agriculture and industry, the excessive use of chemical fertilizers and pesticides in the agricultural feld has caused serious pollution of water bodies with Pb(II) ions [2,3].Pb(II) ions in water bodies are difcult to degrade, and they have a high toxicity.Long-term consumption of water with a high lead content is likely to cause cancer.Te lead pollution cycle is long, and it is easily bioaccumulated.Trough its gradual accumulation in the food chain, it causes serious harm to the human living environment, biosphere, and human health [4][5][6][7].Terefore, the purifcation of polluted water bodies has become a research hotspot in the feld of environmental science, and it is of great signifcance to apply clay mineral materials to the study of the adsorption of heavy metals in water bodies [8,9].Tere are two treatment methods for water bodies polluted by Pb(II) ions: one is to reduce the bioavailability of the Pb(II) ions in the water body, and the other is to directly remove the Pb(II) ions from the water body [10,11].Terefore, some efcient water treatment technologies, for example, physical adsorption, electrochemical adsorption, ion exchange, chemical precipitation, and a series of other treatment methods and technologies, are used in the treatment of Pb(II) in water bodies [12].
In the method of absorbing Pb(II), the choice of the adsorbent is extremely important.Clay minerals have a super-self-purifcation capacity in the restoration of water bodies polluted by Pb(II) ions [13].Te common clay minerals used to restore water bodies heavily polluted by Pb(II) ions include montmorillonite, attapulgite, zeolite, kaolinite, sepiolite, vermiculite, and illite [14][15][16][17].As a mineral material with abundant reserves and a low price, mudstone is a composite clay mineral material composed of layered porous silicate-rich minerals.Mudstone is mainly composed of minerals such as illite, kaolinite, montmorillonite, quartz, feldspar, mica, epidote, chlorite, ferromanganese oxide, and organic matter [18].Tere are certain diferences in the mineral components contained in diferent regions [19].Mudstone has a strong viscoelasticity, is easily weathered into fne particles, is easy to crush, easy to mine, cheap, and easy to obtain.In addition, as a composite clay mineral material, mudstone is a good soil-forming parent material, and its basic characteristics determine that it will not generate secondary pollution when used as an adsorbent.In addition, the restoration cost of this method is low, it is simple and easy to operate, and it has broad application prospects.
In this study, the adsorption method was used to remove the Pb(II) in a water body.Te aim was to develop an environmentally friendly adsorption material with a high adsorption efciency, good performance, and no secondary pollution.Terefore, in this study, through the characterization analysis of mudstone, the possibility of its adsorption of the heavy metal Pb(II) was explored, and the adsorption efect of the mudstone on Pb(II) in a water body under diferent infuencing factors was investigated so as to provide a theoretical scientifc basis and technical support for the efcient use of mudstone material.

Materials.
Te mudstone used in this study was collected from Yaoqu Village, Yaoqu Town, Yaozhou District, Tongchuan City, Shaanxi Province, and the chemical agent used was purchased from the Aladdin Reagent Co., Ltd.(Shanghai, China).

Preparation of Mudstone Material.
First, the mudstone protolith collected was air dried, and the air-dried clay mineral material mudstone was refned.Ten, the refned mudstone was cleaned with deionized water for three times to remove the impurities in the sample.Te washed sample was air dried again and sieved through a 100-mesh screen to make the mudstone material required for the experiment.Te prepared mudstone material was placed in a plastic bag and stored under dry conditions for later use.Te basic physical and chemical properties of the mudstone material are presented in Table 1.Te Pb content of the mudstone was far lower than the soil pollution risk screening value, and it did not carry the target heavy metal when adsorbing the Pb(II) in the polluted water body.

Characterization of Mudstone.
Te surface morphology and elements composition of the mudstone were analyzed using X-ray difractometers (America -FEI-Quanta FEG 250 and Japan-Rigaku-Smart Lab 9KW models), and the specifc surface area and pore size of the mudstone were determined using an America -Mack -ASAP 2020HD8.

Adsorption Experiment.
A batch experiment was conducted to study the adsorption efect of the mudstone on Pb(II) in 100 mL of water contaminated with Pb(II) at a concentration of 30 mg•L −1 , Experiment 1: Diferent amounts of mudstone (0.25 g•L −1 , 0.5 g•L −1 , 0.75 g•L −1 , 1 g•L −1 , 1.25 g•L −1 , and 1.5 g•L −1 ) were added to the contaminated water under constant temperature shaking.Experiment 2: Te best adsorption amount of mudstone from experiment 1 was added to the polluted water, the pH of the solution was adjusted (2, 3, 4, 5, 6, 7, and 8), and the reaction time was 120 min.Adsorption experiments were conducted to determine the best pH value for heavy metal adsorption by the mudstone.Experiment 3: Te best adsorption amount of mudstone from experiment 1 was added, and the pH was adjusted to the ideal value obtained from experiment 2. Ten, experiments on Pb(II) adsorption by the mudstone were conducted to study the efect of the adsorption reaction time on the adsorption efect by collecting samples at different times (5 min, 10 min, 20 min, 30 min, 40 min, 50 min, 60 min, 70 min, 80 min, 90 min, 100 min); Experiment 4: Under the optimal conditions determined in Experiment 1, Experiment 2, and Experiment 3, mudstone was add to solutions with various initial Pb(II) concentration (10,20,30,40,50, and 60 mg•L −1 ) to observe the Pb(II) adsorption efect of the mudstone under diferent initial concentrations.Te supernatant of the adsorbed mixture was fltered, and the Pb(II) concentrations of the fltrate was determined via multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS).To ensure the accuracy of the experiments, each experiment was repeated three times.
Te removal rate and adsorption amount are important indicators of the performance of adsorbent materials.Te adsorption amount (q e ) and removal rate (R) of lead by the mudstone were calculated as follows: where q e is the unit adsorption capacity of the composite clay mineral material for Pb (II) after adsorption equilibrium (mg•L −1 ); C 0 is the initial concentration of Pb(II) (mg•L −1 ); C e is the concentration of Pb(II) in the adsorption equilibrium state (mg•L −1 ); V is the volume of the sewage solution poured into the conical bottle initially (mL); m is the amount of mudstone used (g) [20].present SEM photos of the mudstone collected from the Tongchuan area, Shaanxi, under magnifcations of 5000x, 10000x, 20000x, and 50000x, respectively.It can be seen from Figure 1 that the surface structure of the mudstone is rough and consists of aggregates of irregular fakes; the particles are connected by irregular surface-to-surface and surface-to-edge contacts; and the pore channels of the mudstone are unobstructed.Tis scattered structure helps in the generation of pores, interstices, voids, cracks, fssures, and the connections between them.Te pores, interstices, voids, cracks, and fssures are mostly formed via stacking of the irregular mineral particles contained in the mudstone, so there is a certain diference in the pore size and pore wall thickness.Te size of the space between the pores also varies, which not only increases the free adsorption group of the mudstone but also helps the mudstone to form a pore channel structure.Tere are pores and fssure structures of diferent sizes distributed on the surfaces of the mudstone particles, which are aggregated in a disorderly manner.Te well-developed pores and rough pore walls on the mudstone surface and the good connection between the pores enhance the efective space and adsorption capacity of the mudstone pore channels and improve the physical adsorption and chemisorption capacity of the mudstone.Phase analysis of the mudstone material was carried out.Te X-ray difraction (XRD) results for the mudstone are shown in Figure 2. Te results show that the minerals contained in mudstone mainly include CaCO 3 , SiO 2 , CaAl 2 O 4 •10H 2 O, and CaFe 4 O 7 .Tis is because clay minerals are the main rock forming minerals in mudstone, and the clay minerals are mainly aluminum-silicate minerals.Terefore, the Si and Al contents of the mudstone are large.In addition, the shapes and positions of all of the main difraction peaks of the mudstone are prominent, the XRD difraction peaks of the crystals are sharp and symmetrical, and the crystal structure is complete.Te strong binding capacity of the silicate minerals contained in the mudstone for Pb (II) can be used to improve the adsorption capacity of the mudstone for the heavy metal Pb (II) to a large extent.

Results and Discussion
Figure 3 shows the nitrogen adsorption curve of the mudstone.Based on analysis of the nitrogen adsorption curve, the adsorption isotherm belongs to the second category of the Brunauer-Emmett-Teller (BET) classifcation, with a small slope and slow rise in the frst half and a sharp rise in the second half.As the relative pressure increases, capillary condensation occurs simultaneously with multilayer adsorption.When P/P 0 � 0, the curve intersects with the ordinate (representing the adsorption volume) at a nonzero value, the intercepts are not equal, so there are micropores and the micropore volumes are not equal.Te average specifc surface area obtained using methods such as the BETmethod is 23.027 m 2 •g −1 , and the average pore size is 9.145 nm.

Adsorption of Pb(II)
Ions in Batch Systems.Te Pb(II) adsorption performance of the mudstone is not only related to its surface structure but is also infuenced by external factors.Mudstone was used as the adsorption material to investigate the efects of the pH, amount of mudstone addition, reaction time, and initial Pb(II) concentration of solution on the heavy metal ion adsorption performance of the mudstone.

Efect of Mudstone Dosage on Absorption Efect.
Te results of the efect of the mudstone dosage on the removal of Pb(II) are shown in Figure 4. Tis is mainly due to the fact that the Pb(II) concentration of the solution was fxed, and the removal efciency of Pb(II) increased gradually as the amount of mudstone increased, which caused the mudstone to reach adsorption equilibrium.Te removal efciency increased linearly as the amount of mudstone added increased from 0.25 g•L −1 to 1 g•L −1 .Te Pb(II) removal efciency of the mudstone was as high as 85% when the amount of mudstone was 1 g•L −1 , and it had reached the equilibrium at this time.Although the Pb(II) adsorption efciency of the mudstone exhibited a tendency toward equilibrium with increasing mudstone dosage, when the addition of mudstone was small, the specifc surface area and the number of adsorption sites on the mudstone were limited, and the adsorption sites were fully utilized, thus reaching adsorption saturation quickly and resulting in a high adsorption amount.However, the limited number of adsorption sites was not sufcient to adsorb a large amount of the Pb(II) in the solution, so the removal efciency was low [21].As the addition of mudstone gradually increased, the specifc surface area and the number of adsorption sites and functional groups of the mudstone also increased, which caused the Pb(II) content of the solution to decrease.However, this led to an excess of adsorption sites on the surface of the mudstone, and the phenomenon of Journal of Chemistry unsaturated utilization of the adsorption sites occurred.
Based on the analysis of the results presented in Figure 5, the optimum amount of mudstone addition was 1 g•L −1 for Pb(II) adsorption by mudstone.

Efect of Solution pH on the Adsorption Efect.
According to the above efect of the amount of mudstone addition on the Pb(II) adsorption efect of the mudstone, the amount of mudstone was set as 1 g•L −1 in the experiment conducted to determine the efect of the solution pH on the adsorption efect.Te removal efciencies were all very low at pH < 3, and the removal efciencies were signifcantly higher at pH � 3∼6.At pH � 6∼8, the curve leveled of and the removal efciency stabilized near the maximum value, and the removal efciency was above 85%.Terefore, mudstone is most suitable for Pb(II) removal under weakly acidic or neutral conditions.At pH 6-7, Pb(II) may combine with OH − to produce Pb(OH) + and Pb(OH) 2 complexes via precipitation, which reduces the unfavorable contact with the oxidized surface [22].For pH 6-7, the increase in the removal of Pb(II) by the mudstone was the result of the combined efect of physical adsorption and a chemical precipitation reaction; the adsorption amount reached the maximum value, and the adsorption efect gradually stabilized [23].Te mudstone contained Fe and Al oxides with variable charges, and as the pH increased, the variable negative charge in the solution increased, causing the gravitational force on the Pb(II) increase and the electrostatic adsorption of Pb(II) increase.Te pH of the solution not only afects the magnitude of the electricity charge on the surface of the clay mineral material and the activity of the adsorption sites, but also the form of the metal ions present in the solution, thus afecting its adsorption properties [24].Terefore, pH � 6 was determined to be the optimum pH value for Pb(II) adsorption on mudstone.

Efect of Reaction Time on the Adsorption Efect.
Based on the efects of the amount of mudstone addition and the pH of the solution on the adsorption efect, the optimum During the reaction, the removal efciency and adsorption amount did not change signifcantly as the reaction time increased, and they reached an equilibrium state, indicating that the adsorption of Pb(II) by the mudstone was characterized by rapid adsorption and dynamic equilibrium, which is generally stable and will not be resolved [25].In summary, in the experiment on the static adsorption of Pb(II) by mudstone, the adsorption time was set to 60 min.Te rapid adsorption characteristics enable the mudstone to give full play to the  adsorption performance in a short reaction time, which provides a new idea for solving the problem of the emergency treatment of water bodies polluted by heavy metal.

Efect of Pb(II) Ion Content on the Adsorption Efect.
Te experiments on the efect of diferent Pb(II) ion contents on the adsorption of Pb(II) were carried out using a mudstone addition of 1 g•L −1 , a solution pH of 6, and a reaction time of 90 min.Figure 7 shows the adsorption capacity and removal rate of the mudstone for diferent Pb(II) contents.It can be seen from Figure 7 that the Pb(II) adsorption capacity of the mudstone increased continuously as the Pb(II) ion content increased, while the removal efciency exhibited a decreasing trend.On the one hand, the active sites and exchangeable ions on the surface of a certain mass of mudstone material are limited, and when the ionic strength increases to a certain degree, the adsorption sites and functional groups on the surface of the mudstone are reduced.On the other hand, the active sites and exchangeable ions on the surface of a certain mass of mudstone material are limited, and when the ionic strength increases to a certain level, the adsorption sites and functional groups on the surface of mudstone are fully utilized, and the material reaches adsorption saturation, resulting in the excess Pb(II) not being adsorbed, thus reducing the Pb(II) removal rate [26][27][28].In addition, the Pb(II) removal efciency of the mudstone was greater than 80% when the Pb(II) ion content was less than 30 mg•L −1 , and it gradually decreased when the Pb(II) ion content was between 30 mg•L −1 and 50 mg•L −1 , but remained above 75%.Terefore, under the double consideration of the removal rate and adsorption capacity, the mudstone was more efective in removing Pb(II) ions from the solution with a lower Pb(II) ion content, but it also had some value in treating the solution with a high Pb(II) ion content [29,30].

Analysis of the Adsorption Isotherm Model.
Te adsorption isotherm is the equilibrium state of the adsorption reaction when the reaction proceeds sufciently at a specifc temperature.When the adsorption reaction reaches equilibrium, there is a certain relationship between the ion concentration of the solution and the adsorption capacity of the adsorbent, and this dependence curve is the adsorption isotherm.It is possible to understand the trend of the isotherm under each parameter and to determine the strength of the adsorption performance.Te equilibrium time is the time when the adsorption reaction reaches equilibrium, and the adsorption capacity corresponding to this time is the amount of equilibrium adsorption.Te experimental results for the Pb(II) adsorption by mudstone from a solution were ftted using the following two models [31].
(1) Langmuir adsorption isotherm model Te adsorption model proposed by Langmuir in 1916 based on the theory of molecular motion is a commonly used equation for adsorption isotherms and has been widely used in the feld of adsorption [32].It is based on the assumption that the adsorption process is a dynamic process and that the Journal of Chemistry desorption rate is the same as the adsorption rate when the adsorption reaction reaches equilibrium [33,34].It is applicable to adsorbents with a uniform volume distribution and when the adsorption reaction occurs between single molecular layers.Te expression of its adsorption model equation is as follows [35]: Where b is the adsorption equilibrium constant; q e is the adsorption amount at adsorption equilibrium (mg•g −1 ); q m is the theoretical saturation adsorption amount (mg•g −1 ); and c e is the equilibrium concentration of the liquid phase after adsorption equilibrium (mg•L −1 ).( 2) Freundlich adsorption isotherm model Te Freundlich adsorption isotherm assumes that the adsorption process is a nonhomogeneous process, i.e., the adsorption sites on the surface of the adsorbent are irregular and are not independent of each other, but there may be a synergistic efect between the adsorption sites, and it belongs to an empirical equation that is applicable to multicomponent layer adsorption and surface adsorption under nonideal conditions [36].Te Freundlich adsorption isotherm can be used to express diferent systems of bilayers and reversible adsorption processes by ftting the following equation to the adsorption model [35]: Where k and n are the adsorption constants; q e is the adsorption amount at adsorption equilibrium (mg•g −1 ); and c e is the equilibrium concentration of the liquid phase at adsorption reaction equilibrium (mg•L −1 ).
Based on the results presented in Figures 8 and 9 and Table 2, both adsorption isotherm models can represent the adsorption process of the heavy metal Pb(II) by mudstone, but the correlation coefcient of the Langmuir isotherm model is higher(0.991),and it has a higher degree of linearity.Tis indicates that the Langmuir isotherm model is more suitable for the adsorption process of the heavy metal Pb(II) by mudstone.Te correlation coefcient of the Freundlich adsorption isotherm model is 0.958 (i.e., >0.95), which indicates that the adsorption of the heavy metal Pb(II) by the mudstone occurred in a unimolecular layer structure, while the correlation coefcient of the Freundlich adsorption isotherm model is 0.958> (i.e., >0.95), which indicates that there is also a multimolecular layer reaction in the adsorption of the heavy metal Pb(II) by the mudstone [37].Te maximum adsorption capacity ftted using the Langmuir isotherm model was 54.975 mg•g −1 .Te empirical constant derived from the Freundlich adsorption isotherm model is n � 1.771, 1 < 1.771 < 10, indicating that the adsorption reaction easily proceeds and that the reaction is difcult when n < 0.5 [38].In summary, it can be concluded that the adsorption of the heavy metal Pb(II) by the mudstone is a relatively complex process, and single-molecular-layer In (q e -q t ) 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 t (min) Pseudo-frst-order kinetic

Conclusions
In this study, a mudstone composed of clay mineral material with abundant reserves in China was used as the research object, and the efects of the infuencing factors, such as the amount of mudstone addition, solution pH, reaction time, and initial Pb(II) concentration of the solution, on the adsorption of Pb(II) from water bodies by a composite clay mineral material (mudstone) were investigated through static adsorption experiments.Te main conclusions of this study are as follows.
Te mechanism of using mudstone as a remediation material for lead-contaminated water was analyzed.Te main clay minerals contained in the mudstone were CaCO 3 , SiO 2 , CaAl 2 O 4 •10H 2 O, and CaFe 4 O 7 , among which the silicate minerals were dominant.Its surface structure was rough, and pores and fssures were developed.Te clay minerals were abundant, and the cation exchange capacity, specifc surface area, large pore size, and rich content of clay minerals indicate that using this mudstone to remediate lead-contaminated water using the adsorption mechanism is feasible.Regarding the factors afecting the adsorption of the Pb(II) in the solution by mudstone, the best Pb(II) adsorption efect of the mudstone in the static adsorption experiments was achieved when the amount of mudstone addition was 1 g•L −1 and the pH was 6. Te best Pb(II) adsorption efect of the mudstone was achieved under these conditions, and the removal efciency can reached 85.5%.Regarding the adsorption reaction time, the Pb(II) removal efciency of the mudstone was close to 75% at 30 min, and the Pb(II) adsorption capacity and removal rate of the mudstone were close to the highest value at 60 min, with a removal efciency of close to 88.8%.After 60 min, the adsorption capacity and removal rate did not change signifcantly as the reaction time increased, and they tended to become stable.Te initial Pb(II) concentration of the solution also afected the adsorption of Pb(II) by the mudstone.Te higher the initial concentration was, the higher the adsorption capacity was, and the lower the removal efciency was.Te data obtained from the adsorption experiments on the adsorption of the heavy metal Pb(II) by the mudstone were linearly ftted, and it was concluded that the Langmuir isotherm model was more suitable for describing the adsorption of the heavy metal Pb(II) by the mudstone.In addition, the ftting results indicate that the adsorption of the heavy metal Pb(II) by the mudstone occurred in a single molecular layer structure; while the correlation coefcient of the Freundlich isotherm model was 0.958 (i,e.,>0.95).Tis also indicates that the adsorption process of the heavy metal Pb(II) by the mudstone also involved a reaction in multimolecular layers.Te R 2 value of the pseudo-secondary kinetic model is larger than that of the pseudo-frst-order kinetic model, so the pseudo-secondary kinetic model is more consistent with the adsorption of the heavy metal Pb(II) by the mudstone.Tis also indicates that the adsorption of the heavy metal Pb(II) by the mudstone was dominated by chemisorption.

Figure 4 :
Figure 4: Efect of mudstone dosage on the removal of Pb(II).

Figure 5 :Figure 6 :Figure 7 :
Figure 5: Efect of pH on the removal of Pb(II) by mudstone.

Table 1 :
Basic physical and chemical properties of the mudstone.