Study of Humic Acid Adsorption Character on Natural Maifan Stone: Characterization, Kinetics, Adsorption Isotherm, and Thermodynamics.

In this paper, the adsorption of humic acid (HA) on natural maifan stone (MS) in aqueous medium was investigated. The changes in MS after adsorption have been characterized explicitly. The adsorption behavior was studied by varying the factors of pH (5-10), reaction time (10-180 min), initial HA concentration (5-50 mg/L), adsorbent dosage (0.1-1.2 g), and temperature (25-45 °C). The kinetics of the adsorption process of HA was fitted well with the pseudo-second-order model (R 2 = 0.99). The isothermal results revealed that the adsorption process is favorable, and highly fitting Langmuir models (R 2 > 0.99) were used. Additionally, the obtained maximum adsorption capacity of MS for HA was approximately 1 mg/g. The adsorption process of HA onto MS was endothermic according to the thermodynamic study. The changes in the excitation-emission-matrix of HA and the X-ray diffraction of MS after adsorption indicate the interaction of HA and MS. However, the reason for these changes is still unclear. Thus, the results show that the natural MS exhibited a certain adsorption capacity for HA. It is promising to develop novel natural MS-based materials for adsorption of HA.

Introduction

Natural organic matter (NOM) is a mixture of organic chemicals, including humic acid (HA), folic acid, amino acids, and so forth.[1] The presence of NOM in water can cause serious environmental problems because NOM are prone to produce carcinogenic byproducts during the disinfection of water.[2] They can also combine with heavy metals present in water to form dangerous and carcinogenic substances.[3] In addition, recent research has shown that NOM is an important cause of constructed wetland blockage.[4] Therefore, it is meaningful to remove NOM substances from wastewater.

Currently, various biological and physicochemical methods have been employed for pollutant removal, including advanced oxidation, biodegradation, adsorption, and flocculation.[5−28] The complete degradation and oxidation of HA is difficult because of the refractory character. Among these methods, adsorption is the most common method used for contaminant removal in the wastewater treatment process with the advantages of no potential secondary pollutants, efficient removal rate, and simple technical requirement.[29−43]

Adsorption of HA present in wastewater has been reported previously. The main adsorbents consisted of natural and artificial materials, including activated carbon, coal ash, agriculture wastes, zeolite composite, TiO2 nanoparticles, and so forth.[44−49] Apart from these adsorbents, natural stones were investigated (apart from zeolite), which have a promising potential for HA removal.

Maifan stone (MS), a natural inorganic alum-inosilicate mineral, widely distributed in China with the advantages of low cost and extensive availability, has attracted the attention of researchers and has been employed in the field of pollutant removal.[50,51] Ou et al. have studied the in situ immobilization of toxic metals using MS and illite/smectite clay stone to restore the soil,[52] and the results show that MS has promising amendments for contaminated soil remediation. Guan et al. have found the fast adsorption of lead(II) on MS.[53] However, the adsorption of NOM using MS is less reported. In this study, HA was employed as the targeted object.

The aims of this paper are to (1) characterize the changes in the physical and chemical properties of MS after adsorption explicitly, (2) utilize MS as an adsorbent for HA removal from aqueous solution, (3) investigate the effects of varying factors and mechanisms for adsorption, including pH, dosage of MS, kinetics, isotherm, and thermodynamics, and (4) study the change in the fluorescence property of HA after adsorption using excitation–emission–matrix (EEM). This is the first paper to explore the potential application of MS as an adsorbent for NOM.

Results and Discussion

Batch Experiments

Figure a indicates the effect of dosage of MS on HA adsorption. The results show the following: (i) the unit removal rate increased and decreased consequently with increasing MS dosage; (ii) the unit adsorption quantity decreased with the increase in dosage; and (iii) the degradation rate reached the peak, 68%, when the dosage was 0.6 g. This behavior was attributed to the number of enhanced adsorption sites with increasing dosage, which resulted in an increase in the removal of HA.[54] However, the overload MS dosage would block the adsorption site on the MS, which hinders the adsorption of HA on MS. Therefore, a dosage of 0.6 g was selected as the experimental parameter for the remaining experiments.

Figure 1

Effects of (a) dosage of MS, (b) pH, and (c) adsorption cycle number on HA adsorption.

Figure b indicates the effect of pH on HA adsorption. The results show that the degradation rate and unit adsorption quantity decreased with increasing pH and had peak values, 76% and 0.31 mg/g, respectively, when pH was 5. This is due to the following two factors: (i) HA molecules belong to aromatic carboxylic acid ions, which are negatively charged. (ii) The zeta potential characterization shows the pHpzc value of MS to be 2. Therefore, the MS has net negative charges on their surfaces with pH > 2 in solution. On the contrary, the MS will have a positive charge with pH < 2 in solution. Therefore, the adsorption of HA on MS was driven by electrostatic attraction at a low pH value. Similar results have been reported during the removal of HA using other adsorbents such as montmorillonite.[55]

Figure c shows the regeneration rate of saturated MS using NaOH solution as the regeneration reagent. The results reveal that the regeneration rate decreased with increasing regeneration times, and it reached 34% in the fourth regeneration. The possible factor is the quantity loss and blocking of adsorption point after adsorption and regeneration.

Figure shows the pseudo-first-order (a), pseudo-second-order (b), and intraparticle diffusivity (c) kinetic plots, respectively. Table provides the kinetic model parameters. The experimental data fitted well with the pseudo-second-order (b) model.

Figure 2

Pseudo-first-order (a), pseudo-second-order (b), and intraparticle diffusivity (c) model plots for HA adsorption.

Table 1

Kinetic Model Parameters

Pseudo-first-order			pseudo-second-order
k1 (min–1)	qm (mg/g)	R2	qe (mg/g)	k2 (g/mg/min)	R2
0.20	0.28	0.90	0.3	9.75	0.99

The pseudo-second-order kinetic model was employed to explain the adsorption process of pollutants in aqueous solutions and the adsorption site on the adsorbent.[54]Figure b shows the pseudo-second-order kinetic fitting line. The theoretical equilibrium adsorption capacity qe obtained by the equation fitting was very close to the actual equilibrium adsorption capacity qe, which indicates that the adsorption process follows the pseudo-second-order model.

Figure shows the Langmuir (a) and Freundlich (b) plots for the adsorption isotherm. The adsorption isotherm parameters obtained from the models are given in Table . The Langmuir model was more suitable for HA adsorption.

Figure 3

Langmuir (a) and Freundlich (b) model plots for HA adsorption.

Table 2

Isotherm Parameters

Freundlich		Langmuir
kf	0.18	kl (L/mg)	0.091
n	0.49	qm (mg/g)	1.26
R2	0.96	R2	0.99

The Langmuir model assumes the homogeneous adsorption and monolayer adsorption on the surface of the adsorbent. The parameter kl obtained indicates that the adsorption process belongs to favorable adsorption (when kl is between 0 and 1). The maximum adsorption capacity qm, 1.26 mg/g, was calculated from the Langmuir equation.

Table shows the thermodynamic parameters of HA adsorption; the values of ΔH and ΔS are negative, whereas the value of ΔG is positive, indicating that the HA adsorption process is a feasible, spontaneous, endothermic physical adsorption.[56] Besides, randomness decreased at the solid/liquid interface during the adsorption process.

Table 3

Thermodynamic Parameters

ΔG
298 K	308 K	318 K	ΔH	ΔS
–17	–22	–34	0.42	–24

Comparison with Other Adsorbents

Table shows the comparison of various adsorbents for HA. The results in Table indicate that the adsorption capacity using MS was not very high compared with other adsorbents. The possible reason for the relatively low adsorption capacity is due to the different kinds of HA and the different adsorption conditions in the adsorption of HA. However, the MS used in this study is purely natural, which could be modified further to enhance its adsorption capacity for HA. In addition, the MS is widely distributed and easily available in China. Therefore, MS is a promising and economic adsorbent for HA.

Table 4

Comparison with Other Adsorbents for HA Adsorption

adsorbent	adsorption capacity (mg/g)	references
MS	1	this study
palygorskite	17	(57)
activated Greek bentonite	10	(58)
TiO2/GAC	1	(59)
chitosan-coated granules	0.4	(60)
talc	2	(61)

Characterization of MS and HA

Scanning electron microscopy (SEM) was used to observe the morphology change of MS after adsorption. The SEM results at different scales are shown in Figure . The initial MS has a relative smooth surface and an obvious layered structure, which correspond to the typical quartz morphology.[62] After adsorption for HA, the MS shows a loose spongelike shape, and broken debris appeared on the surface of MS. In addition, the surface of MS became rougher with channels. The possible reason is the strong stirring during adsorption and the adsorbed HA on the surface of MS. Fourier transform infrared (FTIR) was employed to observe the functional group change on the surface of MS after adsorption. The FTIR results are shown in Figure . Before and after adsorption, no obvious peak change appeared for MS. The strong band at 3438 cm–1 is ascribed to the −O–H stretching vibration peak. The weak band at 1630 cm–1 corresponds to the C=C (sp2) stretching vibration peak. The band at 1008 cm–1 is due to the −C–N stretching vibration peak. The other bands from 900 to 400 cm–1 are linked with the C–H or C–C stretching vibration peak. X-ray diffraction (XRD) was employed to analyze the phase composition change on the surface of MS after adsorption. The XRD results are shown in Figure . The MS has three obvious diffraction peaks approximately at 20.8, 26.6, and 36.5°, which correspond to the (100), (011), and (110) diffraction planes (PDF: 79-1906) of quartz, respectively.[63] The crystallite size of the particles is calculated to be 2.4–4.2 nm according to the Debeye–Scherrer equation. In addition, a diffraction peak that appeared around 12.4° is ascribed to the (002) diffraction plane (PDF: 79-1270) of clinochlore. After adsorption of HA, the intensity of identification peaks for quartz and clinochlore increased. There is an interesting phenomenon which is possible due to the complex reaction of HA with quartz and clinochlore to enhance its crystallinity.

Figure 4

SEM of MS before (a,c,e) and after (b,d,f) adsorption.

Figure 5

FTIR of MS before (upper) and after (down) adsorption.

Figure 6

XRD of MS before (upper) and after (down) adsorption.

Thermal gravimetric analysis (TGA) was used to detect the temperature–mass relationship of a substance. The TGA results are shown in Figure . The mass loss at 26–167 °C can be attributed to the removal of adsorbed water or pore water. The mass loss at 167–583 °C can be attributed to the removal of crystal water in MS. The mass loss at 583–1100 °C can be attributed possibly to the collapse and degeneration of the metal crystal in MS. There is no more than whole mass loss of 3% at 1100 °C, indicating excellent thermal stability of MS.

Figure 7

TGA of MS.

Brunauer–Emmett–Teller (BET) analysis was used to measure the adsorption capacity and pore structure of a material. The N2 isotherm, pore distribution, and BET parameter results are shown in Figure and Table . According to the BDDT classification,[64] the N2 adsorption isotherms of MS exhibited in Figure a is type III. Figure b indicates the obvious mesoporous structure. The calculated BET surface area and average pore diameter of MS is 2.5 m2/g and 33.7 nm, respectively.

Figure 8

BET analysis of MS, (a) N2 isotherm of MS, and (b) pore distribution of MS.

Table 5

BET Parameters of MS

BET surface area (m2/g)	microporous volume (cm3/g)	average pore diameter (nm)
2.5814	0.000267	33.7154

Zeta potential was used to test the colloidal dispersion stability and calculate pHpzc of the material. The zeta potential results are shown in Figure . The calculated pHpzc value of MS is 2.0. In addition, the value of zeta potential shows the stability of MS under natural conditions. Contact angle is an important parameter to measure the wetting performance of the liquid on the material surface. The contact angle results are shown in Figure . The contact surface of MS is approximately 23.0°, less than 90°, indicating the hydrophilic character of MS.

Figure 9

Zeta potential measurement of MS under different pH values.

Figure 10

Contact angle measurement of MS.

EEM and ultraviolet–visible (UV–vis) spectra show the important optical properties of HA. The EEM and UV–vis results are shown in Figures and 12, respectively. HA in this paper has the typical EEM spectrum with EX > 250 nm and EM > 380 nm.[65] An interesting result was found; the EEM intensity was stronger after adsorption, which is possibly due to the dissolution of complexation of MS with HA, leading to the higher fluorescence intensity. Similar results are found in the XRD pattern of MS. After adsorption, the UV–vis spectrum from 210 to 600 nm of solution including HA decreased, which indicates the reduction of HA concentration in solution.

Figure 11

EEM spectra measurement of HA solution before adsorption (a) and after adsorption (b).

Figure 12

UV–vis spectra measurement of MS.

Conclusions

In this paper, MS was characterized explicitly and used as an adsorbent for HA for the first time. The batch experiment results show that the dosage of MS and pH value play an important role for HA removal. The pseudo-second-order model kinetics and Langmuir isothermal models are fitted to adsorption of HA on MS with the theoretical adsorption capacity of appropriate 1 mg/g. The thermodynamic results indicate that the adsorption process of the HA onto MS was endothermic.

The characterization of MS indicates the mesoporous property of MS. In addition, after adsorption, the characterization shows an increase in the peak intensity in the XRD of MS and in the fluorescence intensity of HA solution, which is an interesting phenomenon and should be further investigated.

Materials and Instruments

Materials

The raw MS was collected from Tongliao City, China in March 2018. The collected MS was ground and sieved through a 200-mesh screen. The composition of MS powder was determined using X-ray fluorescence (XRF) analysis, as shown in Table . Other reagents used include HA, sulfuric acid, and sodium hydroxide. All agents were of analytical grade and bought from Aladdin Company. Aqueous HA solution was prepared using distilled water.

Table 6

Composition of MS Using XRF Analysis

compound name	SiO2	Al2O3	Fe2O3	CaO	Na2O
concentration (%)	56.87	18.557	5.717	4.749	4.493
compound name	K2O	CO2	MgO	TiO2	P2O5
concentration (%)	2.855	2.786	2.252	0.85	0.335

Instruments

The spectrophotometric measurements of HA concentration were performed using a UV–vis spectrometer (UNICOWFUV-2). The BET analysis was performed by an automatic surface area and porosity analyzer (ASAP 2020M). TGA was performed by a NETZSCH TG thermal gravimetric analyzer. The zeta potential analysis was performed by a NanoPlus zeta potential analyzer. The contact angle analysis was performed by a NanoPlus contact angle meter. The XRF mapping analysis was performed using a Zetium XRF analyzer. The XRD analysis was performed on a D8 ADVANCE X-ray diffractometer. The SEM analysis were carried out using a field emission scanning electron microscope (JSM-IT300). FTIR spectra was obtained using a Nexus FTIR spectrometer. The UV–vis spectrum of HA solution was characterized using a UV–vis–near-infrared spectrophotometer (LAMBDA 750 S). The EEM spectra was measured using a three-dimensional fluorescence analyzer (Fluo-Imager). All general instruments employed include a centrifuge (TG16-II), a drying box (DGG-9123A) and a shaker (SHZ-82A).

Experiments and Models

Experimental Work

The batch experiments were carried out in a 50 mL flask containing 30 mL of HA solution. The pH value of solution was adjusted using 0.1 mol/L H2SO4 and 0.1 mol/L NaOH. The flask was shaken at 200 rpm and 298 K. The aqueous samples were filtrated using a filter paper, and the concentrations of HA in the solution were analyzed. In the process of cycle experiment of saturated MS, 1 mol/L NaOH was used to regenerate the MS after adsorption. The saturated MS was immersed in the NaOH solution (at the ratio of 1 g: 50 mL) for 12 h. Then, the regenerated MS was collected by filtering and drying in an oven for 24 h at 105 °C. The details of the experimental conditions are presented in Table .

Table 7

Experiment Conditions

set	aim of experiment	HA concentration (mg/L)	adsorbent dosage (g)	reaction time (min)	pH	temperature (°C)
1	effect of adsorbent	10	0.1/0.2/0.4/0.6/0.8/1.2	120	7	25
2	effect of pH	10	0.6	120	5/6/7/8/9/10	25
3	recycle number	10	0.6	120	7	25
4	adsorption kinetics	10	0.6	10/20/30/60/90/120/180	7	25
5	adsorption isotherm	5/10/15/20/30/40/50	0.6	120	7	25
6	adsorption thermodynamics	5/10/15/20/30/40/50	0.6	120	7	25/35/45

Model Fitting Formulas

Pseudo-first-order, pseudo-second-order, and intraparticle diffusivity kinetic equations were used for fitting the kinetic model[66−69] (eqs –3). Langmuir and Freundlich models were selected to fit the isotherm models[70,71] (eqs and 5). The thermodynamic data was fitted by the Gibbs model (eq ).

Among the constants, qe and q are the adsorption capacities (mg/g) at equilibrium and at time t, respectively. k1 and k2 are pseudo-first-order and pseudo-second-order constants, respectively. The constant qm is the maximum adsorption capacity (mg/g). The constants kl and kf are Langmuir and Freundlich constants, respectively. kw and b are intraparticle diffusivity constants. The constants K, R, and T are adsorption equilibrium constant (L·mg–1), gas Moore constant [8.3145 J·(mol·K)−1], and absolute temperature (K), respectively. ΔG, ΔH, and ΔS are the standard free-energy change of adsorption (J·mol–1), standard adsorption heat (J·mol–1), and adsorption quasi entropy variable [J·(mol·K)−1], respectively.

Determination of HA Concentration

The concentration of HA was determined by a UV spectrometer at λmax value of HA (254 nm wavelength), and the amount of HA adsorbed (mg/g) was calculated based on a mass balance equation as given in eq

Among the constants, qe is the equilibrium adsorption capacity per gram dry weight of the adsorbent (mg/g); co is the initial concentration of HA in the solution (mg/L); ce is the final or equilibrium concentration of HA in the solution (mg/L); V is the volume of the solution (L); and W is the weight of the adsorbent (g).