Adsorption of Heavy Metals on Soil Collected from Lixisol of Typical Karst Areas in the Presence of Caco3 and Soil Clay and Their Competition Behavior

sustainability

Article Adsorption of Heavy Metals on Soil Collected from Lixisol of Typical Karst Areas in the Presence of CaCO3 and Soil Clay and Their Competition Behavior

1, 2, 2 3 4, Guandi He †, Zhenming Zhang † , Xianliang Wu , Mingyang Cui , Jiachun Zhang * and Xianfei Huang 5,* 1 The Key Laboratory of Plant Resources Conservation and Germplasm Innovation in Mountainous Region (Ministry of Education), Institute of Agro-Bioengineering and College of Life Sciences, Guizhou University, Guiyang 550025, China; [email protected] 2 Guizhou Institute of Biology, Guizhou Academy of Sciences, Guiyang 550009, China; [email protected] (Z.Z.); [email protected] (X.W.) 3 College of Life Science, Guizhou University, Guiyang 550025, China; [email protected] 4 Guizhou Botanical Garden, Guizhou Academy of Sciences, Guiyang 550004, China 5 Guizhou Provincial Key Laboratory for Information Systems of Mountainous Areas and Protection of Ecological Environment, Guizhou Normal University, Guiyang 550001, China * Correspondence: [email protected] (J.Z.); [email protected] (X.H.) These authors contributed equally to this work. † Received: 8 July 2020; Accepted: 1 September 2020; Published: 7 September 2020

Abstract: The content of heavy metals in the soil in Guizhou Province, which is a high-risk area for heavy metal exposure, is significantly higher than that in other areas in China. Therefore, the objective of this study was to evaluate the ability of CaCO3 and clay to accumulate heavy metals in topsoil sample collected from Lixisol using the method of indoor simulation. The results showed that the contents of Cu, Zn, Cd, Cr, Pb, Hg and As in the soil sample were 10.8 mg/kg, 125 mg/kg, 0.489 mg/kg, 23.5 mg/kg, 22.7 mg/kg, 58.3 mg/kg and 45.4 mg/kg, respectively. The soil pH values increased with the CaCO3 concentration in the soil, and the fluctuation of the soil pH values was weak after the CaCO3 concentrations reached 100 g/kg. The adsorption capacity of lime soil increased by approximately 10 mg/kg on average, and the desorption capacity decreased by approximately 300 mg/kg on average. The desorption of all heavy metals in this study did not change with increasing clay content. Pseudo-second-order kinetics were more suitable for describing the adsorption kinetics of heavy metals on the soil material, as evidenced by the higher R2 value. The Freundlich model can better describe the adsorption process of As on lime soil. The process of As, Cr, Cd and Hg adsorption on the soil sample was spontaneous and entropy-driven. Additionally, the process of Cu and Pb adsorption on the soil materials was spontaneous and enthalpy-driven. Generally, the adsorption and desorption of heavy metals in polluted soil increased and decreased, respectively, with increasing CaCO3 content. The effect of calcium carbonate on the accumulation of heavy metals in soil was greater than that of clay. In summary, CaCO3 and pH values in soil can be appropriately added in several areas polluted by heavy metals to enhance the crop yield and reduce the adsorption of heavy metals in soils.

Keywords: heavy metals; Guizhou Province; CaCO3; clay; pseudo-second-order kinetic; Freundlich model

1. Introduction There are two main soil types, i.e., Lixisols and Acrisols, in Guizhou Province, of which the former are characterized by high content of calcium and magnesium carbonate (CaCO3, MgCO3), and carbonate

Sustainability 2020, 12, 7315; doi:10.3390/su12187315 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 7315 2 of 19 plays an important role in the biogeochemical process of heavy metals [1,2]. The parent material, carbonate rock, consists mainly of calcite, dolomite and other carbonate minerals [3]. The major chemical components of Lixisols are CaCO3 and MgCO3 (more than 90%), and the other components are mainly SiO2, Al2O3, Fe2O3, etc. [4]. However, excessive heavy metals in the soil sample will damage the ecosystem and harm human health because of their refractory accumulation through the food chain [5]. Ning et al. [6] measured the concentrations of heavy metals (Cu, Zn, Mn, Cd, Ni, Pb and Co) in limestone forests in Huaxi district, Guiyang, and the heavy metal pollution was assessed by comparing our results with the background values of heavy metals in limestone soils in Guizhou Province, China. The results showed that the measured contents of Cu, Mn, Cd, Pb, Ni and Co were higher than or similar to these background values. Sun et al. [7] found that the concentrations of heavy metals in calcareous soil profiles remarkably exceeded the national standards in the study area, especially the concentrations of Cd and As. Therefore, it is necessary to understand the adsorption of heavy metals in topsoil samples collected from Lixisol. The content of CaCO3 in soil can significantly affect the total content and activities of heavy metals, thereby enhancing or restraining their absorption by plants. Especially, under the higher acidity and lower CaCO3 content in soil, heavy metals in soil are of higher activity, rendering them easy to absorb by plants [8]. In contrast, in alkaline and CaCO3-rich soils, heavy metals easily form hydrated hydroxides and insoluble carbonates or are adsorbed and fixed by soil colloids, which has a certain buffer effect [9]. The reason for this finding may be that calcium carbonate has a certain influence on the buffering performance of acids. Liu et al. [10] studied the buffer effect of different types of soils in the Changchun region and the influence of the physical and chemical characteristics of soil on the soil buffer action in a simulated experiment. According to the experiment, the average acid buffer capacity of calcareous soil containing calcium carbonate was 1.8 times that of non-calcareous meadow soil, black soil, albic soil and dark brown soil. Similarly, Huang et al. [11] investigated the effect of phosphate and carbonate on the immobilization of multiple heavy metals (Pb, Cu and Cd) in contaminated soils. The results showed that the application of calcium carbonate can significantly reduce exchangeable and carbonate-bound metals. In addition, calcium carbonate is an important cement for microaggregates and can form humic substances with soil organic matter, which can increase the surface area and negative charge of the soil, thus enhancing the Cr3+, Cu2+, Pb2+, Cd2+, Zn2+, etc. [12,13]. Unfortunately, the stability of the aggregates formed with calcium carbonate as a binder was poor due to the hydrolysis effect in a water environment [14]. Generally, the concentration of calcium carbonate in soil significantly affects the content of heavy metals. The adsorption and desorption mechanism of heavy metals is closely related to soil components [15,16]. The behavior of heavy metals in soil mainly depends on the interaction between them and different soil components. Clay (less than 2 µm) includes crystalline aluminate, amorphous aluminosilicate and hydrated metal oxide. It is of large surface area, surface charge characteristics and cation exchange characteristics, which means it is a relatively active adsorption component of soil [17,18]. Wu et al. [19] reported that the amount of total organic carbon (TOC) and heavy metals in different size fractions of the fine particles increases as the particle size decreases. Qin et al. [20] exhibited that the clay positively correlated with Cd, As, Hg, and the sand negatively correlated with Hg and As. Hence, it is necessary to study the adsorption of heavy metal ions by clay particles with soil. The residence time of the adsorbate on the surface of the adsorbent was an important factor in the evaluation of whether the adsorption process was completed and the calculation of the total amount of solute adsorption [21]. The adsorption kinetics can be evaluated by the adsorption rate, equilibrium time and adsorption capacity or by discussing the limit of the total amount of heavy metals adsorbed by soil. Furthermore, the adsorption isotherm model was also a major method for investigating the adsorption characteristics. The main objectives in this study were to (1) explore the adsorption and desorption behavior of heavy metals onto topsoil sample collected from Lixisol, considering calcium carbonate and soil clay; (2) understand the adsorption nature of heavy metals adsorbed by topsoil Sustainability 2020, 12, 7315 3 of 19 sample collected from Lixisol using kinetics, adsorption isotherms and thermodynamic study; and (3) investigate the effect of pH and electrolytes on adsorption capacity.

2. Methods and Materials

2.1. Materials Lixisols, a typical soil in Guizhou Province, Southwest China, was selected in this study. All soil samples used in the experiment were collected from the topsoil with a stainless-steel shovel and packed back with cloth bags. After natural air drying at room temperature and removal of plants, gravel and other impurities in the soil, the soil was ground with a porcelain mortar, a container for crushing experimental materials in an experiment, and sifted to 100 mesh. After sifting, all the soil was collected in plastic bags for future use.

2.2. Analysis Method The collected soil was divided into many parts, which was used to analyze the several indexes. For one of the soil samples, the pH of the soil was determined by potentiometry (soil-to-water ratio = 1:2.5) [22,23]. Soil organic matter was measured by the high-temperature external heat potassium dichromate oxidation volumetric method [24,25]. The calcium carbonate content in the soil was calculated using the volumetric titration method [26]. The soil particle size distribution was gauged by the hydrometer method [27]. The soil samples were digested by a microwave digester using HNO3-H2O2-HF (5:2:5), and acid was removed by a graphite furnace [28]. Hg and As were determined by an atomic ﬂuorescence photometer (AFS-2100, Beijing Haiguang Instrument Co., Ltd., Beijing, China). Pb, Cu, Zn, Cd and Cr of soils were determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES) (Prodigy XP, Leeman Labs Inc., Hudson, NY, USA).

2.3. Eﬀect of Diﬀerent Calcium Carbonate Contents on Adsorption and Desorption Eleven soil samples of 0.5 g (accurate to 0.0001 g) were accurately weighed and added to 100 mL centrifuge tubes. Super pure CaCO3 was added to each sample at 0, 0.25, 0.5, 1, 3, 5, 7, 10, 15, 20 and 25 g/kg. The standard heavy metal (Cd, Pb, Cr, Zn, As and Hg) mixed solutions (nitric acid medium), which were purchased from the National Center of Analysis and Testing for Non-ferrous Metals and Electronic Materials, of 50 mL with a concentration of 20 mg/L, were added to the above mixture. These compounds were oscillated on a constant-temperature oscillator (25 1 C). After ± ◦ 2 h, the cells were removed and cultured in a constant-temperature incubator (25 1 C) for 22 h and ± ◦ then centrifuged for 30 min (4000 r/min). The supernatant was taken and measured with the aid of a computer, and the formula used to calculate the adsorption capacity is as follows:

(Ci Ce) v q = − × (1) e m where qe (mg/kg) is the capacity of soil adsorption/desorption in the solution at equilibrium; Ci (mg/L) is the initial concentration of heavy metals in the supernatant; Ce (mg/L) is the equilibrium concentration of heavy metals in the supernatant; v (mL) is the volume of the total mixed solution used; and m (g) is the weight of the added soil material. The supernatants of the above soil samples were removed, and 20 mL of 0.1 mol/L NaNO3 was added. The mixtures were oscillated on a constant temperature oscillator (25 1 C) for 2 h. The ± ◦ solutions were cultured in a constant-temperature incubator (25 1 C) for 22 h after transfer to a ± ◦ volumetric ﬂask and then centrifuged at 4000 r/min for 30 min. Finally, the concentration of heavy metals in the supernatant was determined using ICP-OES. The desorption capacity was the concentration Sustainability 2020, 12, 7315 4 of 19

diﬀerence of heavy metals in the supernatant before and after adding NaNO3. The formula for calculation is as follows: (Ci Ce) v S= − × (2) m where S (mg/kg) is the desorption capacity of soil in the solution at equilibrium; C1 and C2 are the concentrations of heavy metals in the supernatant before and after adding NaNO3 (mg/L), respectively; v (mL) is the volume of the total mixed solution used; and m (g) is the capacity of the added soils. Figure1 shows the adsorption /desorption ﬂow chart.

Figure 1. The adsorption/desorption flow chart. Figure 1. The adsorption/desorption flow chart. 2.4. Particle Size Distribution of Soil The particle size distribution of soil was measured by the hydrometer method. A soil sample of 50 g passing a 2 mm sieve was weighed into a triangular flask. The appropriate dispersant, 0.5 mol/L NaOH solution to acidic soil of 40 mL or sodium hexametaphosphate solution to alkaline soil of 60 mL, 250 mL pure water were successively added to the above triangular flask. After the mixture soaked overnight, the mixtures in flasks were boiled on the heating plate for 1 h (using the defoamer to prevent overflow). After cooling, the sieve was washed through 0.2 mm, and the suspension was washed into a 1 L measuring cylinder. After all the suspension had passed, the mixtures were sieved through 0.2 mm and the suspension was washed into a measuring cylinder of 1 L. After all the suspension had passed through, the sieve was washed with pure water. The particles left on the screen were washed into the aluminum box, dried and weighed. The part washed into the measuring cylinder was added with water to a constant volume of 1 L. The solution in the measuring cylinder was stirred up and down repeatedly for one minute with an agitator. The time was counted at the end of mixing, which were respectively 30 s, 1 min, 2 min, 4 min, 8 min, 15 min, 30 min, 1 h, 2 h, 4 h, 8 h, 24 h, 48 h, and put it into the hydrometer to read, and measure and record the water temperature before reading. The particle size content was calculated according to the following equation,

(ρ1 + ρ2 + ρ3 + ρ0) v %= × 100 (3) m × where ρ1 is the hydrometer reading, g/L; ρ2 is the correction value of hydrometer calibration curve, g/L; ρ3 is the calibration temperature value of hydrometer, g/L; ρ0 is the corrected value of dispersant Sustainability 2020, 12, 7315 5 of 19 for hydrometer reading, g/L; v is the volume of suspension, mL; m is the weight of soil sample, g. The diameter (d) of soil is calculated by the following Equation (4): s 1800ηL d = (4) g(ρs ρ f )t − where d is the diameter of soil sample, mm; η is the viscosity coeﬃcient of water; L is the settlement depth of soil particles, g is the acceleration of gravity, cm/s; ρs is the soil particle density, g/cm3; ρf is the density of water, g/cm3; t is settlement time, s.

2.5. Adsorption Kinetic

Prepared solutions of 1 L contained 20 mg/L heavy metals and 0.01 mol/L NaNO3 (as supporting electrolyte) in a beaker. A 25.00 g aliquot of soil material was added to each prepared solution of 1 L. At room temperature, the mixtures were stirred at a constant speed for 1 min, 2 min, 4 min, 6 min, 8 min, 10 min, 15 min, 20 min, 30 min, 45 min, 1 h, 1.5 h, 2 h, 3 h, 4 h, 6 h, 8 h, 10 h, 12 h and 24 h, and an aliquot (20 mL) was selected and centrifuged at 4000 r/min for 3 min. Pseudo-first-order kinetics, pseudo-second-order kinetics and particle internal diffusion were used to fit the data of the adsorption process of heavy metals on the soil sample [29,30],

k1 lg(qe qt)= lgqe t (5) − − 2.303 t 1 1 = + t (6) 2 qt k2qe qe 0.5 qt = kintt + C (7) where qe (mg/g) and qt (mg/g) are the adsorption capacity at equilibrium and after time t (min), respectively; k is the kinetic adsorption rate constant. k (mg/g min 0.5) is the diﬀusion rate constant int · − in particles.

2.6. Adsorption Isotherm Six soil samples (0.5 g, accurate to 0.0001 g) were weighed into a 100 mL centrifuge tube, and then 20 mL of heavy metals (Cd, Pb, Cr, Zn, As, Hg) with concentrations of 0.00, 10.00, 20.00, 30.00, 40.00, and 50.00 mg/L were added to the centrifuge tube. NaNO3 (0.01 mol/L) as the supporting electrolyte was dropped into these mixtures, which were oscillated at 25 ◦C for 24 h to reach adsorption equilibrium. After centrifugation and ﬁltration, the supernatant was used to determine the concentration of heavy metals. The Langmuir and Freundlich models were frequently utilized to describe the isotherm adsorption eﬀect at the soil/water interface [31,32]. Their equations are as follows:

kQmCe qe = (8) 1+kCe

1/n qe = kF C (9) × e where qe (mg/g) is the adsorption capacity of adsorbate on the adsorbent surface at equilibrium; k (L/mg) is the Langmuir constant; Qm (mg/g) is the maximum adsorption capacity of adsorbate on the adsorbent surface; Ce (mg/L) is the concentration of adsorbate in solution at equilibrium; kF (mg/g) is the Freundlich constant; and n is the index of anisotropy, which is usually greater than 1. A larger value indicates the signiﬁcance of the nonlinear characteristic of the adsorption isotherm. Sustainability 2020, 12, 7315 6 of 19

2.7. Estimation of Thermodynamic Parameters The change in Gibbs free energy associated with a chemical reaction can be used to evaluate the spontaneity of the adsorption reaction [33]. Thermodynamic parameters can be obtained as follows [34]: Cs KT = (10) Ce

∆G◦ = RTlnKT (11)

∆G◦ = ∆H◦ T∆S◦ (12) − where Cs is the initial concentration of heavy metals on the soil material; Ce is the concentration of heavy metals in solution at equilibrium; and R is the gas constant (8.31 J mol 1 K 1). · − · − 2.8. Eﬀect of Initial pH on Adsorption Eight soil samples (0.5 g, accurate to 0.0001 g) were weighed into a centrifuge tube of 100 mL, A 20 mL aliquot of 20 mg/L heavy metal (0.01 mol/L NaNO3 as supporting electrolyte) was added into the above centrifuge tube, and the pH values of the mixture were adjusted between 3.00 and 10.00 using 0.1 mol/L NaOH and HNO3. The pH values in this experiment were set as 8 gradients (3, 4, 5, 6, 7, 8, 9 and 10). After shaking, centrifuging and ﬁltering, the supernatant was used to determine the concentration of heavy metals. Their adsorption capacity was calculated as in Section 2.3.

2.9. Eﬀect of Temperature on Adsorption The processing steps were the same as those described in Section 2.3, and the adsorption isothermal experiments were carried out at 50 ◦C, 45 ◦C, 40 ◦C, 35 ◦C and 30 ◦C. Their adsorption capacity was calculated as in Section 2.3.

2.10. Inﬂuence of Electrolyte on Adsorption Five soil samples (0.5 g, accurate to 0.0001 g) were weighed into 100 mL centrifuge tubes. Diﬀerent 1 2 3 4 5 concentrations of NaNO3 (10− , 10− , 10− , 10− and 10− mol/L) were added to the above centrifuge tubes. The treatment process and adsorption capacity were calculated as in Section 2.3.

3. Results and Discussion

3.1. Physical and Chemical Indexes of Topsoil Sample Collected from Lixisol The pH of topsoil in Guizhou Province was 7.64 in this study, which corresponds to weak alkaline soil (Figure2a). The contents of organic matter and calcium carbonate were 70.3 g /kg and 50.9 g/kg, respectively, which exhibited that the organic matter in soil of study area was rich according to Nutrient grading standard of the second national soil survey (China). The soil organic matter can inhibit or promote the adsorption of heavy metals in soil through a series of reactions, such as ion exchange adsorption, complexation and chelation between heavy metals and other functional groups, which aﬀect the precipitation, migration, transformation and bioavailability of heavy metals [35]. The particle size distribution of the soil sample in our study was 37.78% sand, 26.79% powder and 35.44% clay, which pertained to loamy clay. The moisture content of calcareous soil was 3.58%. Sustainability 2020, 12, x FOR PEER REVIEW 7 of 18

2a, the soil pH values increased with CaCO3 concentration in the soil, and the fluctuation of the soil pH values was weak after the CaCO3 concentrations reached 100 g/kg. In soil, the effect of CaCO3 on 2+ - - pH was due to the hydrolysis of carbonate (MCO3 + H2O ⇋ M + HCO3 + OH ) [36]. Liu et al. [37] reported that CaCO3 concentrations slightly affected the soil pH when the content of calcium carbonate was 50–100 g/kg, which is consistent with our study. The different clay contents caused a difference in soil pH values, and the maximum and minimum pH values of the soil were 6.44 and Sustainability 2020, 12, 7315 7 of 19 6.39, respectively.

8.00 7.95 a 7.90 7.85

pH 7.80 7.75 7.70 7.65 7.60 40 60 80 100 120 140 160 180 200 220 240 6.44 CaCO3 concentration (g/kg) 6.43 b 6.42

pH 6.41 6.40 6.39

20 40 60 80 100 120 Clay content(g/kg)

Figure 2. Effect of CaCO (a) and clay (b) contents on pH of topsoil sample collected from Lixisol. Figure 2. Effect of CaCO33 (a) and clay (b) contents on pH of topsoil sample collected from Lixisol. The contents of Cu, Zn, Cd, Cr, Pb, Hg and As in the soil material were 10.8 mg/kg, 125 mg/kg, 3.2. Effect of Calcium Carbonate and Soil Clay on Adsorption and Desorption of Heavy Metals in Topsoil 0.489 mg/kg, 23.5 mg/kg, 22.7 mg/kg, 58.3 mg/kg and 45.4 mg/kg, respectively. According to the “soil Sample Collected from Lixisol environmental quality: risk control standard for soil contamination of agricultural land” (GB 15618-2018) releasedGenerally, by Ministry the ofadsorption Ecology andcapacity Environment of heavy of themetals People’s in soil Republic was higher of China, than Cd the and desorption As exceed theircapacity, control which values implied which that are these 0.3 mg heavy/kg and metals 30 mg co/kg,uld respectively.be fixed by the As soil shown sample, in Figure and2 fewa, the heavy soil pHmetals values were increased desorbed; with thus, CaCO the 3migrationconcentration capacity in the of soil,fixed and heavy the metals fluctuation will ofalso the be soil reduced. pH values The wassoil weakmaterial after is theextensively CaCO3 concentrations distributed in reachedGuizhou 100 Province, g/kg. In soil,which the is e ffa ectreason of CaCO for the3 on higher pH was heavy due metal concentrations in the soil in Guizhou Province2+. As shown in Figure 3 and Table 1, the order of to the hydrolysis of carbonate (MCO3 + H2O M + HCO3− + OH−)[36]. Liu et al. [37] reported thatthe maximum CaCO3 concentrations adsorption capacities slightly a forffected heavy the metals soil pH is whenCu > Pb the > content Zn > Cd of > calcium Hg > Cr carbonate > As. For wasCu, 50–100the maximum g/kg, which adsorption is consistent capacity with was our 807 study. mg/kg, The diandfferent the minimum clay contents adsorption caused a capacity difference was in soil806 pHmg/kg. values, The and maximum the maximum desorption and minimumcapacity was pH values172 mg/kg, of the and soil th weree minimum 6.44 and desorp 6.39, respectively.tion capacity was 18.1 mg/kg. The maximum and minimum adsorption capacities of Pb were 802 mg/kg and 799 3.2.mg/kg, Effect respectively. of Calcium CarbonateThe maximum and Soil and Clay minimum on Adsorption desorption and Desorption capacities of of Heavy Pb were Metals 3.45 in Topsoilmg/kg and Sample−2.60 mg/kg, Collected respectively. from Lixisol The maximum and minimum adsorption capacities of Zn were 802 mg/kg and 800Generally, mg/kg, therespectively. adsorption The capacity maximum of heavy desorption metals incapacity soil was of higher Zn is 674 than mg/kg the desorption and the minimum capacity, whichdesorption implied capacity that theseis 282 heavymg/kg. metals The maximum could be adsorption fixed by the and soil desorption sample, and capacities few heavy of Cd metals were were795 mg/kg desorbed; and 661 thus, mg/kg, the migration respectively. capacity The minimum of fixed heavy adsorption metals and will desorption also be reduced. capacities The of soilCd materialwere 790 ismg/kg extensively and 409 distributed mg/kg, respectively. in Guizhou For Province, Hg, the which maximum is a reason and minimum for the higher adsorption heavy metalcapacities concentrations were 771 mg/kg in the soiland in758 Guizhou mg/kg, Province.respectively. As shownThe maximum in Figure desorption3 and Table capacity1, the order was of−0.436 the maximummg/kg, and adsorption the minimum capacities desorpti foron heavy capacity metals was is −7.70 Cu > mg/kg.Pb > ZnThe> maximumCd > Hg >adsorptionCr > As. Forcapacity Cu, theof Cr maximum was 767 mg/kg, adsorption and the capacity minimum was 807adsorption mg/kg, capaci and thety minimumwas 760 mg/kg. adsorption The maximum capacity wasdesorption 806 mg capacity/kg. The of maximum Cr was 510 desorption mg/kg, and capacity the minimum was 172 desorption mg/kg, and capa thecity minimum was 268 mg/kg. desorption The capacitymaximum was adsorption 18.1 mg/kg. capa Thecity maximum of As was and 763 minimum mg/kg,adsorption and its minimum capacities value of Pb was were 666 802 mg/kg. mg/kg andThe 799maximum mg/kg, respectively.and minimum The desorp maximumtion andcapacities minimum of desorptionAs were 8.90 capacities mg/kg of Pband were −2.36 3.45 mg/kg. mg/kg andThe desorption2.60 mg/kg, efficiency respectively. for Pb, The Cd, maximum Zn, Cu, Cr, and Hg minimum and As is adsorption 0.43%, 83.09%, capacities 83.98%, of 20.83%, Zn were 35.14%, 802 mg /notkg − anddetected 800 mg and/kg, 0.13%, respectively. respectively. The maximum desorption capacity of Zn is 674 mg/kg and the minimum desorption capacity is 282 mg/kg. The maximum adsorption and desorption capacities of Cd were 795 mg/kg and 661 mg/kg, respectively. The minimum adsorption and desorption capacities of Cd were 790 mg/kg and 409 mg/kg, respectively. For Hg, the maximum and minimum adsorption capacities were 771 mg/kg and 758 mg/kg, respectively. The maximum desorption capacity was 0.436 mg/kg, − and the minimum desorption capacity was 7.70 mg/kg. The maximum adsorption capacity of Cr was − 767 mg/kg, and the minimum adsorption capacity was 760 mg/kg. The maximum desorption capacity of Cr was 510 mg/kg, and the minimum desorption capacity was 268 mg/kg. The maximum adsorption Sustainability 2020, 12, 7315 8 of 19 capacity of As was 763 mg/kg, and its minimum value was 666 mg/kg. The maximum and minimum desorption capacities of As were 8.90 mg/kg and 2.36 mg/kg. The desorption efficiency for Pb, Cd, − SustainabilityZn, Cu, Cr, 2020 Hg, and12, x FOR Asis PEER 0.43%, REVIEW 83.09%, 83.98%, 20.83%, 35.14%, not detected and 0.13%, respectively.9 of 18

800 800 800 750 700 700 600 Pb adsorption quality Cd adsorption quality 700 Adsorption quality 650 Zn 500 desorption quality desorption quality 600 desorption quality 600 400 300 550 500 500 200 400 100 450

Adsorption capacity (mg/kg) 0 400 300 60 80 100 120 140 160 60 80 100 120 140 160 60 80 100 120 140 160

800 800 800 700 700 700 adsorption quality Cu adsorption quality Cr adsorption quality 600 Hg 600 desorption quality desorption quality 600 desorption quality 500 500 400 400 500 300 300 400 200 200 300 100 100 0 Adsorption capacity (mg/kg) capacity Adsorption 0 200 60 80 100 120 140 160 60 80 100 120 140 160 60 80 100 120 140 160

800 CaCO3 contents(mg/kg) CaCO3 contents(mg/kg) 700 600 As 500 adsorption quality 400 desorption quality 300 200 100

Adsorption capacity (mg/kg) 0 60 80 100 120 140 160

CaCO contents(mg/kg) 3 Figure 3.3. TheThe adsorptionadsorption and and desorption desorption capacities capacities of Pb,of Pb, Cd, Cd, Zn, Cu,Zn, Cr,Cu, Hg Cr, and Hg As and at diAsﬀerent at different CaCO3

CaCOcontents3 contents on soil on sample. soil sample.

Table 1. Relationship between different CaCO3 contents and adsorption or desorption of heavy metals Asin soil shown sample. in Table 2, the order of the maximum adsorption capacities for heavy metals is Pb > Cu > Hg > Zn > As > Cd > Cr. Firstly, the maximum and minimum adsorption capacities of Pb were CaCO 434 mg/kg3 and 399Pb mg/kg, (mg/kg) respectively. Cd (mg/kg) The Zn maximum (mg/kg) Cu desorption (mg/kg) Cr capacity (mg/kg) of HgPb (mgwas/kg) 328 mg/kg, As (mg/ kg)and Contents(g/kg) the minimum desorption capacity was 293 mg/kg. The maximum adsorption capacity of Cu was 431 AQ DQ AQ DQ AQ DQ AQ DQ AQ DQ AQ DQ AQ DQ mg/kg,58.8 and the minimum 802 3.45 adsorpti 795on 661 capacity 802 673was 378 807 mg/kg. 168 The 762 maximum 268 767 desorption7.07 752 capacity 0.970 − was 41259.8 mg/kg, and 800 the0.492 minimum790 653desorption 800 674capacity 807 was 172 375 767mg/kg. 270 The 758 maximum7.70 adsorption763 2.36 − − − 60.8 801 0.830 795 614 802 596 807 109 760 300 763 4.36 746 1.14 capacity of Hg was 415 −mg/kg, and its minimum value was 389 mg/kg. The maximum− and minimum− 62.8 802 0.332 795 623 801 590 807 120 763 290 762 4.84 750 2.16 − − desorption70.8 capacities 799 were2.24 98.7792 mg/kg 596 and 801 62.9 565 mg/kg, 807 respectively. 103 759 For 319 the 760adsorption7.24 capacity746 1.31 of − − 78.8 802 1.75 793 575 801 542 807 81.6 760 357 763 4.06 738 0.146 Zn, the maximum value− was 372 mg/kg, and the minimum value was 288 mg/kg.− The maximum 86.8 800 2.17 792 596 801 591 807 118 760 341 763 3.71 743 5.61 desorption capacity of Zn− was 391 mg/kg, and the minimum desorption capacity was− 344 mg/kg. The 98.8 800 2.12 794 549 802 488 807 70.1 764 372 768 3.18 724 0.864 − − maximum119 and minimum 802 2.03 adsorption793 507 capacities 802 442of As 806were 46.6368 mg/kg 762 448and 353 770 mg/kg,2.64 respectively.697 0.842 − − 139 801 1.92 791 462 801 349 806 33.5 762 478 771 0.436 686 8.90 The maximum desorption− capacity was 288 mg/kg, and the minimum desorption −capacity was 272 159 801 2.60 791 409 801 282 806 18.1 761 510 767 4.04 666 2.22 mg/kg. The maximum− adsorption capacity of Cd was 292 mg/kg, and the minimum− adsorption capacity was 236 mg/kg.Note: AQ The and maximum DQ represent desorption adsorption andcapacity desorption of Cd capacity, was 413 respectively. mg/kg, and the minimum desorption capacity was 378.1 mg/kg. For the adsorption capacity of Cr, its maximum and minimum The adsorption capacity of Cu, Zn, Cd and Pb did not obviously change with the increase in CaCO3 valuescontent were in soil. 231 However, mg/kg and the 145 adsorption mg/kg, respecti capacitiesvely. of HgThe and maximum Cr slightly desorp increasedtion capacity with increasing was 448 mg/kg, and the minimum desorption capacity was 434 mg/kg. However, the adsorption CaCO3 content in the soil sample. A negative correlation between the adsorption capacities of As and concentration of heavy metals in soil increased with the contain of clay (from 382 g/kg to 454 g/kg). the CaCO3 content was observed because the adsorption capacity of As decreased significantly as the The phenomenon may be that the clay in the soil had a large surface energy, which made its CaCO3 content of the soil material increased. Zhong et al. [38] reported effects of CaCO3 addition adsorptionon uptake ofcapacity heavy metalsrelatively and strong. arsenic Theref in paddyore, the fields, migration and the of exchangeable pollutants was arsenic limited. content in the Figure 4 shows that the effect of clay on heavy metals in lime soil was not as significant as that soils decreased significantly by adding CaCO3. The reason may be that Ca in soil can form insoluble precipitationof CaCO3. Their with adsorption As, which capacities was conducive gradually to the increase fixationd with of As. increasing The desorption clay content. capacities However, of Cu, the Cd adsorption capacity of As was slightly decreased with increasing clay content. Generally, the desorption of all heavy metals in this study did not change with increasing clay content. The desorption efficiencies for Pb, Cd, Zn, Cu, Cr, Hg and As are 93.79%, 79.63%, 20.12%, 99.65%, 94.88%, 54.55% and 69.44%, respectively. Sustainability 2020, 12, 7315 9 of 19

and Zn were decreased with increasing CaCO3 content in the soil sample. However, with the increase in CaCO3 content in the soil material, the desorption capacities of As and Pb did not significantly change. The desorption capacities of Cr were increased with increasing CaCO3 content in soil sample. Therefore, with the increase in the CaCO3 content in the soil sample, the adsorption and fixation of Cu, Cd and Zn were enhanced, and the adsorption and fixation of Cr and As were weakened, but Pb had no obvious effect. With the increase in CaCO3 content, the adsorption capacity of soil material was increased by approximately 10 mg/kg on average, and the desorption capacity decreased by approximately 300 mg/kg on average. The pH value of soil increased after adding calcium carbonate, which promoted the precipitation reaction of exchangeable Cd2+, Cu2+, Pb2+ and Zn2+ with carbonate and hydroxide in soil. Meanwhile, the coprecipitation of Ca2+ and metal ions is also conducive to the transformation of the exchange state to other forms. Additionally, the increase of soil pH can enhance the chelating capacity of clay and organic matter, promote the adsorption capacity of soil, and reduce the desorption of heavy metals, thus decreasing the solubility of heavy metals in soil [39]. In contrast, the adsorption capacity of the soil materials was continuously increased with the decrease in CaCO3 content. This may be because with the decrease of pH value, the negative charges on the surface of clay minerals, hydrated oxides and organic matter in the soil decreased, and the adsorption capacity of heavy metals weakened. Meanwhile, the specific adsorption of heavy metals on the surface of oxides and the stability of soil organic matter metal complexes decreased with the decrease of pH value. With the decrease of pH, the concentrations of H+ and Fe2+ in soil increase, and the competitive adsorption with heavy metals increases, which is not conducive to the adsorption of heavy metals. The decrease of pH is not conducive to the formation of metal hydrate ions, and the affinity of hydrated ions on soil adsorption sites is significantly lower than that of heavy metal ions, which is not conducive to the adsorption of heavy metals on soil [40]. However, the effects on different elements are different, such as inhibiting the transport of nitrate nitrogen from the underground of wheat to the aboveground, promoting the uptake of chromium in the aerial parts of Chinese cabbage, and enhancing the enrichment of lead in lettuce stems and leaves. As shown in Table2, the order of the maximum adsorption capacities for heavy metals is Pb > Cu > Hg > Zn > As > Cd > Cr. Firstly, the maximum and minimum adsorption capacities of Pb were 434 mg/kg and 399 mg/kg, respectively. The maximum desorption capacity of Pb was 328 mg/kg, and the minimum desorption capacity was 293 mg/kg. The maximum adsorption capacity of Cu was 431 mg/kg, and the minimum adsorption capacity was 378 mg/kg. The maximum desorption capacity was 412 mg/kg, and the minimum desorption capacity was 375 mg/kg. The maximum adsorption capacity of Hg was 415 mg/kg, and its minimum value was 389 mg/kg. The maximum and minimum desorption capacities were 98.7 mg/kg and 62.9 mg/kg, respectively. For the adsorption capacity of Zn, the maximum value was 372 mg/kg, and the minimum value was 288 mg/kg. The maximum desorption capacity of Zn was 391 mg/kg, and the minimum desorption capacity was 344 mg/kg. The maximum and minimum adsorption capacities of As were 368 mg/kg and 353 mg/kg, respectively. The maximum desorption capacity was 288 mg/kg, and the minimum desorption capacity was 272 mg/kg. The maximum adsorption capacity of Cd was 292 mg/kg, and the minimum adsorption capacity was 236 mg/kg. The maximum desorption capacity of Cd was 413 mg/kg, and the minimum desorption capacity was 378.1 mg/kg. For the adsorption capacity of Cr, its maximum and minimum values were 231 mg/kg and 145 mg/kg, respectively. The maximum desorption capacity was 448 mg/kg, and the minimum desorption capacity was 434 mg/kg. However, the adsorption concentration of heavy metals in soil increased with the contain of clay (from 382 g/kg to 454 g/kg). The phenomenon may be that the clay in the soil had a large surface energy, which made its adsorption capacity relatively strong. Therefore, the migration of pollutants was limited. Sustainability 2020, 12, 7315 10 of 19

Table 2. Relationship between clay content and adsorption or desorption of heavy metals in soil sample.

Clay Particle Pb (mg/kg) Cd (mg/kg) Zn (mg/kg) Cu (mg/kg) Cr (mg/kg) Hg (mg/kg) As (mg/kg) (g/kg) AQ DQ AQ DQ AQ DQ AQ DQ AQ DQ AQ DQ AQ DQ Sustainability354 2020 400, 12, x 304 FOR PEER 292 REVIEW 413 352 391 404 408 168 448 394 66.9 36510 286 of 18 355 400 309 281 407 346 388 403 411 145 436 391 63.7 357 272 356Table 2. 402 Relationship 310 280between 403 clay 337content 379 and adsorpti 401 407on or 171desorption 443 of 389heavy 65.6 metals 368 in soil 288 358sample. 400 315 280 401 341 376 401 404 173 434 389 62.9 365 273 366 400 328 257 391 315 367 395 410 167 431 391 87.7 358 279 374Clay Particle 399 337Pb 240 385Cd 288 355Zn 378Cu 407 188Cr 441 393Hg 98.7 364As 284 382(g/kg) 400 315(mg/kg) 277 (mg/kg) 404 335(mg/kg) 381 397(mg/kg) 411 190(mg/kg) 437 (mg/kg) 39581.2 (mg/kg) 360 277 394 401 319 AQ DQ 280 AQ 405 DQ 341 AQ382 DQ 402AQ 412DQ 200AQ DQ 434 AQ 389 DQ 67.3 AQ 367 DQ 283 414354 417 293 400 304 236 292 378 413 353 352 344391 425404 375408 212168 448 439 394 400 66.9 77.5 365 353 286 274 434355 434 301 400 309 252 281 388 407 361 346 360388 420403 394411 228145 436 443 391 412 63.7 82.8 357 355 272 283 454356 431 306 402 310 289 280 412 403 372 337 389379 431401 408407 231171 443 439 389 415 65.6 84.4 368 343 288 282 358 Note: 400 AQ315 and 280 DQ represent401 341 adsorption 376 and401 desorption 404 173 capacity, 434 respectively. 389 62.9 365 273 366 400 328 257 391 315 367 395 410 167 431 391 87.7 358 279 Figure3744 shows 399 that 337 the e240ff ect385 of clay288 on355 heavy378 metals407 188 in lime441 soil393 was 98.7 not 364as significant 284 382 400 315 277 404 335 381 397 411 190 437 395 81.2 360 277 as that of CaCO3. Their adsorption capacities gradually increased with increasing clay content. 394 401 319 280 405 341 382 402 412 200 434 389 67.3 367 283 However,414 the adsorption 417 293 capacity 236 of378 As was353slightly 344 decreased425 375 with212 increasing439 400 clay 77.5 content. 353 Generally, 274 the desorption434 of all 434 heavy 301 metals252 388 in this361 study 360 did420 not 394 change 228 with443 increasing 412 82.8 clay 355 content. 283 The desorption454 e fficiencies 431 for306 Pb, Cd,289 Zn,412 Cu, 372 Cr, Hg389 and431 As are408 93.79%,231 439 79.63%, 415 20.12%, 84.4 99.65%, 343 28294.88%, 54.55% and 69.44%,Note: respectively.AQ and DQ represent adsorption and desorption capacity, respectively.

440 a 420 400 420 400 b 380 380 400 360 360 c 380 adsorption quality adsorption quality 340 desorption quality 360 desorption quality 320 340 340 300 320 adsorption quality 320 280 desorption quality 300 300 260

Adsorption capacity (mg/kg) 240 280 280 340 360 380 400 420 440 460 340 360 380 400 420 440 460 340 360 380 400 420 440 460

440 adsorption quality 450 d e 400 430 desorption quality 400 350 f 420 350 300 adsorption quality 410 adsorption quality 300 desorption quality 250 desorption quality 400 250 200 390 150 200 380 100 150 370 50

Adsorption capacity (mg/kg) 340 360 380 400 420 440 460 340 360 380 400 420 440 460 340 360 380 400 420 440 460

Clay particle (g/kg) Clay particle (g/kg) 360 g 340 adsorption quality 320 desorption quality

300

280

Adsorption capacity (mg/kg) 340 360 380 400 420 440 460 Clay particle (g/kg) Figure 4. The adsorption and desorption capacities of Pb ( a),), Cd Cd ( (b),), Zn ( c), Cu ( d), Cr ( e), Hg ( f) and As (g) at didifferentfferent clayclay onon soilsoil material.material. 3.3. Adsorption Kinetics of Heavy Metals Adsorbed on Topsoil Sample Collected from Lixisol 3.3. Adsorption Kinetics of Heavy Metals Adsorbed on Topsoil Sample Collected from Lixisol The rate of metal ions moving in solution and fixing on the surface of the soil determined the The rate of metal ions moving in solution and fixing on the surface of the soil determined the efficiency of the adsorption process. The residence time of a solute on the surface of the soil adsorbent efficiency of the adsorption process. The residence time of a solute on the surface of the soil adsorbent was an important factor to judge whether the adsorption process was completed and to calculate the total amount of solute adsorption. The study of adsorption dynamic characteristics can be used to explore the possible adsorption mechanism from the perspective of the adsorption path. The adsorption capacities of heavy metals on the soil sample dramatically increased with increasing contact time (Figure 5). When the adsorption capacity reached a certain value, it weakly increased or decreased with increasing contact time. The adsorption rate of the soil material can reach 50% in 5 min, 80% in 15 min, 95% in 90 min and 99% after 720 min. The adsorption process of heavy metals by the soil sample was mainly concentrated in the first 90 min, and the subsequent adsorption Sustainability 2020, 12, x FOR PEER REVIEW 11 of 18

was relatively slow. The reason for this difference may be physical adsorption (fast adsorption stage) and chemical adsorption (slow adsorption stage). Generally, physical adsorption can achieve local equilibrium in a few milliseconds to a few seconds; however, chemical adsorption requires a certain activation energy to adsorb, resulting in a slow adsorption process. The pseudo-second-order kinetic equation was more suitable to describe the adsorption kinetics of heavy metals on the soil sample, as evidenced by the higher R2 value (Table 3). The maximum adsorption capacities of As, Cu, Zn, Pb, Cr, Cd and Hg were 0.777 mg/g, 0.793 mg/g, 0.391 mg/g, 0.808 mg/g, 0.378 mg/g, 0.325 mg/g and 0.804 mg/g, respectively, implying Pb > Hg > Cu > As > Zn > Cr > Cd. The adsorption rates of As, Cu, Zn, Pb, Cr, Cd and Hg were 0.159 g·mg−1min−1, 0.0476 g·mg−1min−1, 0.0382 g·mg−1min−1, 0.303 g·mg−1min−1, 0.256 mg−1min−1, 0.079 g·mg−1min−1 and 0.650 g·mg−1min−1, respectively, signifying Hg > Pb > Cr > As > Cd > Cu > Zn. In addition, the fitting degree of the intraparticle diffusion model was also relatively good. For this kind of porous adsorbate, intraparticle diffusion was also an important process of heavy metal adsorption. Sustainability 2020, 12, 7315 11 of 19 Table 3. Kinetics parameters of heavy metals adsorbed on the soil sample. was an important factor to judge whetherAs theCu adsorption Zn process Pb was completedCr andCd to calculateHgthe totalPseudo- amount ofQ solutee (mg/g) adsorption. 0.703 The study 0.065 of adsorption 0.003 dynamic0.116 characteristics0.387 0.026 can be used0.367 to explorefirst-order the possiblek1 (10−3 adsorptionmin−1) mechanism0.925 0.056 from the perspective38.200 3.100 of the adsorption1.572 path.40.225 0.697 kineticThe adsorptionR capacities of heavy0.281 metals0.056 on the soil−0.056 sample dramatically−0.056 0.070 increased −0.056 with increasing0.190 contactPseudo- time (FigureQe(mg/g)5). When the0.787 adsorption 0.056 capacity reached0.020 a certain0.075 value,0.465 it weakly 0.018 increased 0.480 or second- k2 (10−3 g decreased with increasing contact0.343 time. The adsorption4.025 1.181 rate of the4.399 soil material0.171 can reach3.808 50% in0.101 5 min, order mg−1min−1) 80% in 15 min, 95% in 90 min and 99% after 720 min. The adsorption process of heavy metals by the kinetic R 0.999 0.975 0.973 0.948 0.999 0.970 0.997 soil sample was mainly concentrated in the ﬁrst 90 min, and the subsequent adsorption was relatively k3 (mg 0.000 0.007 0.005 0.045 0.004 0.009 0.007 slow.Particle The reasong−1 formim this−0.5) diﬀerence may be physical adsorption(0.0004) (fast adsorption stage) and chemical adsorptiondiffusion (slow adsorption stage). Generally, physical0.000 adsorption can achieve local equilibrium in a C 0.603 0.065 0.115 0.334 0.026 0.281 fewmodel milliseconds to a few seconds; however, chemical(0.0001) adsorption requires a certain activation energy to adsorb, resultingR in a slow adsorption0.950 process.0.928 0.916 0.924 0.990 0.989 0.980

0.8 a b 0.6 Pb（mg/kg) Cd（mg/kg） 0.4 Zn（mg/kg） Cu（mg/kg） Cr（mg/kg） 0.2 Hg（mg/kg） As（mg/kg）

0.0 Adsorption capacity (mg/kg) capacity Adsorption

-0.2 -200 0 200 400 600 800 1000 1200 1400 1600 -200 0 200 400 600 800 1000 1200 1400 1600 I 1.0 c d Pb（mg/kg) 0.8 Cd（mg/kg） Zn（mg/kg） Cu（mg/kg） 0.6 Cr（mg/kg） Hg（mg/kg） As（mg/kg） 0.4

0.2 Adsorption capacity (mg/kg)

0.0 -200 0 200 400 600 800 1000 1200 1400 1600 -200 0 200 400 600 800 1000 1200 1400 1600 Contact time (min) Contact time (min)

Figure 5. a Figure 5. The fittingfitting of heavy metals adsorbed on the soil material by pseudo-first-orderpseudo-first-order kinetics ( a), pseudo-second-order kinetics (b), intraparticle diffusion (c) and experimental value (d). pseudo-second-order kinetics (b), intraparticle diffusion (c) and experimental value (d). The pseudo-second-order kinetic equation was more suitable to describe the adsorption kinetics of heavy metals on the soil sample, as evidenced by the higher R2 value (Table3). The maximum adsorption capacities of As, Cu, Zn, Pb, Cr, Cd and Hg were 0.777 mg/g, 0.793 mg/g, 0.391 mg/g, 0.808 mg/g, 0.378 mg/g, 0.325 mg/g and 0.804 mg/g, respectively, implying Pb > Hg > Cu > As > Zn > Cr > Cd. The adsorption rates of As, Cu, Zn, Pb, Cr, Cd and Hg were 0.159 g mg 1min 1, 0.0476 g mg 1min 1, · − − · − − 0.0382 g mg 1min 1, 0.303 g mg 1min 1, 0.256 mg 1min 1, 0.079 g mg 1min 1 and 0.650 g mg 1min 1, · − − · − − − − · − − · − − respectively, signifying Hg > Pb > Cr > As > Cd > Cu > Zn. In addition, the fitting degree of the intraparticle diffusion model was also relatively good. For this kind of porous adsorbate, intraparticle diffusion was also an important process of heavy metal adsorption. Sustainability 2020, 12, 7315 12 of 19

Table 3. Kinetics parameters of heavy metals adsorbed on the soil sample.

As Cu Zn Pb Cr Cd Hg

Qe (mg/g) 0.703 0.065 0.003 0.116 0.387 0.026 0.367 Pseudo-ﬁrst- k (10 3 min 1) 0.925 0.056 38.200 3.100 1.572 40.225 0.697 order kinetic 1 − − R 0.281 0.056 0.056 0.056 0.070 0.056 0.190 − − − Qe(mg/g) 0.787 0.056 0.020 0.075 0.465 0.018 0.480 Pseudo-second- k (10 3 g mg 1min 1) 0.343 4.025 1.181 4.399 0.171 3.808 0.101 order kinetic 2 − − − R 0.999 0.975 0.973 0.948 0.999 0.970 0.997 0.000 Particle k (mg g 1mim 0.5) 0.007 0.005 0.045 0.004 0.009 0.007 3 − − (0.0004) diﬀusion 0.000 model C 0.603 0.065 0.115 0.334 0.026 0.281 (0.0001) Sustainability 2020, 12, x FOR PEERR REVIEW 0.950 0.928 0.916 0.924 0.990 0.989 0.98012 of 18

3.4.3.4. AdsorptionAdsorption IsothermsIsotherms of of Heavy Heavy Metals Metals Adsorbed Adsorbed on on Topsoil Topsoil Sample Sample Collected Collected from from Lixisol Lixisol TheThe shapeshape andand changechange rulerule ofof the the adsorption adsorption isothermisotherm cancan bebe usedused toto understand understand thethe actionaction intensityintensity of of the the adsorbate adsorbate and and adsorbent, adsorbent, the the state state of of the the adsorbate adsorbate on on the the interface interface and and the the structure structure ofof the the adsorption adsorption layer. layer. All All heavy heavy metals metals exhibit exhibit a a larger larger slope slope at at a a lower lower equilibrium equilibrium concentration concentration (the(the heavy heavy metal metal concentration concentration which which was no was longer no improved longer afterimproved the di ffaftererent initialthe different concentrations initial ofconcentrations heavy metals of adsorbed heavy metals by the adsorbed soil sample) by the (Figure soil sample)6). The adsorption(Figure 6). The capacities adsorption of heavy capacities metals of wereheavy added metals with were increasing added with initial increasing heavy metal initial concentrations; heavy metal concentrations; however, the slopehowever, of the the isotherm slope of adsorptionthe isotherm curve adsorption was gentle curve at was higher gentle equilibrium at higher concentrations. equilibrium concentrations. The slope of The Aswas slope the oflargest, As was followedthe largest, by followed those of Hg,by those Cr and of Pb,Hg, whichCr and were Pb, which dramatically were dramatically adsorbed and adsorbed then slowly and then adsorbed. slowly Theadsorbed. slopes ofThe Cu, slopes Zn and of CdCu, slowly Zn and increased Cd slowly with incr increasingeased with initial increasing concentration. initial Theconcentration. reason may The be thatreason the may concentration be that the of concentration adsorbate in theof adsorbate solution was in the enhanced solution with was increasingenhanced with initial increasing concentration, initial whichconcentration, was more favorablewhich was for more the combination favorable withfor the adsorbate combination and adsorbent. with adsorbate However, and the capacityadsorbent. of eachHowever, heavy the metal capacity combined of each with heavy the soil metal material combined was di withfferent the since soil material these heavy was metalsdifferent had since different these properties.heavy metals Hence, had different the maximum properties. adsorption Hence, capacitythe maximum of each adsorption heavy metal capaci hastya of higher each heavy difference, metal ofhas which a higher As was difference, the most obvious of which with As the was increase the inmost initial obvious As concentration. with the increase In this study, in initial As was As selectedconcentration. for specific In this discussion study, As due was to selected its sensitivity for spec toific the discussion initial concentration due to its ofsensitivity heavy metals to the in initial soil sampleconcentration (Figure 7of). heavy metals in soil sample (Figure 7).

1600 As 1400 Cu Zn 1200 Pb Cr 1000 Cd Hg 800

600

400 Adsorption capacity (mg/kg) 200

0 10203040

Equilibrium concentration (mg/L) FigureFigure 6. 6.Adsorption Adsorption capacities capacities of heavy of metalsheavy adsorbedmetals adsorbed on soil material on soil at equilibrium material at concentration. equilibrium concentration.

Four types of adsorption isotherm lines, i.e., “S”, “L”, “H” and “C”, were used to discuss the adsorption isotherm in a study by Giles et al. [41]. The isotherm adsorption curve of As on lime soil had both “S” and “L” types. S-type adsorption was equivalent to a single multilayer reversible adsorption process occurring on the surface of a nonporous solid or macroporous solid, which was also consistent with the physical structure of soil [42]. The curve was steep first and then gentle with an increase in concentration at low concentrations, reaching the first inflection point to realize the single-layer adsorption saturation on the surface, which was the single-layer saturated adsorption capacity. The adsorbate diffused into the micropores of the adsorbent with increasing concentration. Because the surface area of the adsorbent was smaller than the internal surface area, the adsorption curve was concave upward. The adsorption capacity will not increase when the pores are completely filled and the adsorption sites are completely occupied, and the adsorption capacity will be controlled by the micropore volume, whose capillary diameter is slightly larger than the size of the adsorbate [43]. The adsorption process was mainly the result of volume filling (the adsorption capacity increased with increasing adsorbate concentration). When the binding sites or micropores on the adsorbent surface were completely filled, the adsorption reached saturation, and the adsorption Sustainability 2020, 12, x FOR PEER REVIEW 13 of 18

capacity no longer increased with increasing adsorbate concentration. The L-type curve may be caused by monolayer adsorption or micropore filling of a small amount of silica gel in the clay. Figure 8 shows the Langmuir and Freundlich isotherm model fitting of As adsorption on lime soil. The isotherm adsorption parameters obtained by the fitting are shown in Table 4. The correlation coefficients (R2) of the Langmuir and Freundlich models were 0.817 and 0.920, respectively, which implied that the Freundlich model can better describe the adsorption process of As on lime soil. In Sustainability 2020, 12, 7315 13 of 19 addition, the k and n values of the Freundlich model were 166 L/mg and 2.37.

1200 30℃ 35℃ 40℃ 1000 45℃ 50℃ 800

600

400 Adsorption capacities (mg/kg)

200

10 20 30 40 50 Initial As concentration (mg/L) Figure 7. Adsorption capacities of heavy metals adsorbed on the soil at didifferentﬀerent AsAs concentrations.concentrations.

Four types of adsorption1200 isotherm lines, i.e., “S”, “L”, “H” and “C”, were used to discuss the adsorption isotherm in a study by Giles et al. [41]. The isotherm adsorption curve of As on lime soil had both “S” and “L”1000 types. S-type adsorption was equivalent to a single multilayer reversible adsorption process occurring on the surface of a nonporous solid or macroporous solid, which was also consistent with the physical800 structure of soil [42]. The curve was steep first and then gentle with an increase in concentration at low concentrations, reaching the first inflection point to realize the single-layer adsorption saturation600 on the surface, which was the single-layer saturated adsorption capacity. The adsorbate diffused into the micropores of the adsorbent with increasing concentration.

Because the surface area400 of the adsorbent was smaller than the internal surface area, the adsorption curve was concave upward. The adsorption capacity will not increase when the pores are completely Langmuir adsorption isotherm filled and the adsorption (mg/kg) capacities Adsorption sites are completely occupied, and the adsorption capacity will be controlled 200 Freundlich adsorption isotherm by the micropore volume, whose capillary diameter Experimental is slightly values larger than the size of the adsorbate [43]. The adsorption process was mainly the result of volume filling (the adsorption capacity increased with 0 increasing adsorbate concentration).0 When 5 the binding 10 sites 15 or micropores 20 on 25 the adsorbent surface were completely filled, the adsorptionEquilibrium reached saturation,concentration and(mg/L) the adsorption capacity no longer increased with increasing adsorbate concentration. The L-type curve may be caused by monolayer Figure 8. Fitting values of Langmuir and Freundlich models of As adsorbed on the soil material. adsorption or micropore filling of a small amount of silica gel in the clay. Figure8 showsTable 4. the Adsorption Langmuir isotherm and Freundlich fitting parameters isotherm for modelAs adsorption fitting onto of As lime adsorption soil. on lime soil. The isotherm adsorption parameters obtained by the fitting are shown in Table4. The correlation coefficients (R2) of the Langmuir andLangmuir Freundlich modelsFreundlich were 0.817 and 0.920, respectively, which implied that the Freundlich modelqm canKL betterR2 describeKF the1/n adsorption R2 process of As on lime soil. In addition, the k and n values757 of the 0.161 Freundlich 0.817 model 166 were2.370 166 L0.920/mg and 2.37.

3.5. ThermodynamicTable 4.StudyAdsorption isotherm ﬁtting parameters for As adsorption onto lime soil.

The thermodynamics of theLangmuir adsorption of heavy metals Freundlich on lime soil can be used to study the exothermic, spontaneous or endothermic nature2 of heavy metal adsorption 2on the interface between qm KL R KF 1/n R the adsorbate and adsorbent757 [44]. 0.161 The positive 0.817 values 166of ΔS° suggested 2.370 an 0.920 increase in randomness at Sustainability 2020, 12, x FOR PEER REVIEW 13 of 18

capacity no longer increased with increasing adsorbate concentration. The L-type curve may be caused by monolayer adsorption or micropore filling of a small amount of silica gel in the clay. Figure 8 shows the Langmuir and Freundlich isotherm model fitting of As adsorption on lime soil. The isotherm adsorption parameters obtained by the fitting are shown in Table 4. The correlation coefficients (R2) of the Langmuir and Freundlich models were 0.817 and 0.920, respectively, which implied that the Freundlich model can better describe the adsorption process of As on lime soil. In addition, the k and n values of the Freundlich model were 166 L/mg and 2.37.

1200 30℃ 35℃ 40℃ 1000 45℃ 50℃ 800

600

400 Adsorption capacities (mg/kg)

200

10 20 30 40 50 Initial As concentration (mg/L) Sustainability 2020, 12, 7315 14 of 19 Figure 7. Adsorption capacities of heavy metals adsorbed on the soil at different As concentrations.

1200

1000

800

600

400

Langmuir adsorption isotherm Adsorption capacities (mg/kg) capacities Adsorption 200 Freundlich adsorption isotherm Experimental values

0 0 5 10 15 20 25 Equilibrium concentration (mg/L) FigureFigure 8.8. Fitting values of Langmuir and Freundlich modelsmodels ofof AsAs adsorbedadsorbed onon thethe soilsoil material.material. 3.5. Thermodynamic Study Table 4. Adsorption isotherm fitting parameters for As adsorption onto lime soil. The thermodynamics of the adsorption of heavy metals on lime soil can be used to study the Langmuir Freundlich exothermic, spontaneous or endothermic nature of heavy metal adsorption on the interface between 2 2 the adsorbate and adsorbent [44q].m TheK positiveL R values KF of ∆S1/n0 suggested R an increase in randomness at the solid/solution interface [45].757 The positive0.161 0.817 value obtained166 2.370 for ∆H0.9200 indicates the endothermic nature of heavy metal adsorption, which demonstrates that this process consumes energy [46]. Therefore, 3.5. Thermodynamic Study the process was a spontaneous and entropy-driven process. The negative value of ∆G0 showed the feasibilityThe thermodynamics and spontaneity of the of adsorption heavy metal of heavy adsorption metals onto on lime the soil can sample be used [47]. to Therefore,study the theexothermic, process of spontaneous As, Cr, Cd and or Hgendothermic adsorption nature onto the of soilheavy material metal wasadsorption spontaneous on the and interface entropy-driven between (Tablethe adsorbate5). Additionally, and adsorbent the process [44]. The of positive Cu and values Pb adsorbed of ΔS° suggested by the soil an sample increase was in randomness spontaneous at and enthalpy-driven. The process of Zn adsorption onto the soil material may be doubly enthalpy- and entropy-driven.

Table 5. Thermodynamic parameters for adsorption of heavy metals on soil sample.

∆S0 ∆H0 (kJ mol 1) ∆G0 (kJ/mol) · − (J mol 1K 1) · − − 303.150 K 308.150 K 313.150 K 318.150 K 323.150 K As 40.172 136.474 0.571 1.160 0.566 0.688 0.757 − − − − − Cu 1.309 1.792 0.397 0.678 0.552 0.303 0.093 − − − − − − − Zn 6.377 18.945 0.898 2.099 0.937 1.974 0.577 − − − − Pb 30.558 97.157 1.168 1.941 1.074 1.051 1.752 − − − − − − − Cr 2.188 5.157 0.434 0.559 0.521 0.605 0.358 − − − − − Cd 1.081 10 5 5.081 4.212 3.563 3.420 2.347 3.424 × − − − − − − Hg 2.153 10 4 78.417 0.571 1.160 0.566 0.688 0.757 × − − − − − −

The adsorption capacities were enhanced with increasing temperature, which may be due to the endothermic reaction. The adsorption capacity of heavy metals in soil was more easily adsorbed on the surface of soil particles with an increase in temperature, and there was no obvious desorption phenomenon due to the increase in temperature, which may be caused by stable chemical adsorption during the whole process. The adsorption capacities of heavy metals in the soil material fluctuated with increasing temperature; however, there was no significant increase or decrease. This may be because the adsorption has reached a balance (the binding sites of the surface and inner pores of the soil particles have been filled). Both adsorption and desorption were promoted with increasing temperature, which showed that the adsorption amount of heavy metals in lime soil did not obvious Sustainability 2020, 12, 7315 15 of 19 change with increasing temperature. The reason why the adsorption capacity of heavy metals in the soil material decreased with increasing temperature may be that adsorption was an exothermic reaction. The increase in temperature was an inhibiting factor for the total amount of heavy metals adsorbed in the soil sample, or it may be that after the adsorption reached a certain temperature, the adsorption process was changed from chemical adsorption to physical adsorption (the desorption rate of heavy metals from soil was greater than the adsorption rate).

3.6. Effect of pH and Electrolyte on Adsorption of Heavy Metals by Lime Soil The adsorption capacities of Cr decreased with increasing pH, which may be caused by the unique physical properties of Cr. The adsorption capacities of Cu, Zn, Hg and Cd increased with increasing pH, which may be because the increase in pH effectively reduced the concentration of H+ in the solution. However, soil colloids with a negative charge and heavy metal ions with a positive charge can more effectively increase the adsorption capacity of soil colloids for heavy metal ions; therefore, the adsorption capacities of heavy metal ions increased with increasing pH value. Surprisingly, the capacities of Pb and As were saturated at the beginning of adsorption (Figure9a). Generally, an increase in pH can promote the adsorption of several heavy metals and inhibit the adsorption of Cr; however, its mechanism still needs to be further investigated. The maximum adsorption capacities of heavy metals in the soil sample were reached at pH = 8.00, which may be related to the physical and chemical properties of the soil material. It was found that the slow adsorption processes emerged at pH < 7.00, implying that the adsorption process of heavy metals in the soil sample was slow under acidic conditions. In contrast, the fast adsorption stage occurred at pH > 7.00, indicating that the adsorption process of heavy metals in the soil was fast under alkaline conditions. The influence of pH on the adsorption of heavy metals by soil particles was generally shown in the following four aspects: the influence on the solubility of heavy metals in solution [48], the influence on the adsorption characteristics of the natural colloid surface in solid particles [49], control of various adsorption reactions on the surface of solid particles, and the increase in pH, which promoted an increase in the adsorption point of the soil colloid since soil colloids generally have a negative charge [50]. Figure9b exhibits the change in adsorption capacities followed by the di fferent electrolyte 5 4 3 2 1 concentrations (10− , 10− , 10− , 10− and 10− mol/L). The adsorption capacities of heavy metals increased with increasing electrolyte concentrations, which may be due to the promotion of the ion activity of heavy metals in the electrolyte. Several heavy metals showed the phenomenon of decreasing first and increasing later with the increase in NaNO3 concentration, which may be because the inert electrolyte was neutralized by the external negative charge of the soil, reducing the affinity towards heavy metals and the adsorption capacity of heavy metals. Generally, there are three approaches for a soil to adsorb heavy metals: the activity of free heavy metal ions will change because of the effect of ions or the pH by forming ions [51]; inert electrolytes will dissociate into anions and cations, and cations will compete with excessive heavy metal ions or heavy metal ions for adsorption [52]; inert electrolytes will affect the electrostatic point on the surface of the soil colloidal particles since the soil colloid is charged [53]. Sustainability 2020, 12, x FOR PEER REVIEW 15 of 18 the influence on the adsorption characteristics of the natural colloid surface in solid particles [49], control of various adsorption reactions on the surface of solid particles, and the increase in pH, which promoted an increase in the adsorption point of the soil colloid since soil colloids generally have a negative charge [50]. Figure 9b exhibits the change in adsorption capacities followed by the different electrolyte concentrations (10−5, 10−4, 10−3, 10−2 and 10−1 mol/L). The adsorption capacities of heavy metals increased with increasing electrolyte concentrations, which may be due to the promotion of the ion activity of heavy metals in the electrolyte. Several heavy metals showed the phenomenon of decreasing first and increasing later with the increase in NaNO3 concentration, which may be because the inert electrolyte was neutralized by the external negative charge of the soil, reducing the affinity towards heavy metals and the adsorption capacity of heavy metals. Generally, there are three approaches for a soil to adsorb heavy metals: the activity of free heavy metal ions will change because of the effect of ions or the pH by forming ions [51]; inert electrolytes will dissociate into anions and cations, and cations will compete with excessive heavy metal ions or heavy metal ions for adsorption Sustainability[52]; inert electrolytes2020, 12, 7315 will affect the electrostatic point on the surface of the soil colloidal particles16 of 19 since the soil colloid is charged [53].

800 a 700

600 As Cu 500 Zn Pb 400 Cr Cd 300 Hg

200 Adsorption capacity (mg/kg)

100

34567891011 pH

5 b As Cu 4 Zn Pb 3 Cd Cr Hg 2

Adsorption capacity(mg/kg) 1

0 10-5 10-4 10-3 10-2 10-1 -- NaNO concentration 3 Figure 9. Adsorption capacities of heavy metals adsorbed on the soil at different diﬀerent pH values (a) and

NaNO3 concentrationsconcentrations ( b). 4. Conclusions 4. Conclusions Topsoil samples collected from Lixisol of Guizhou Province were selected in this study, and the Topsoil samples collected from Lixisol of Guizhou Province were selected in this study, and the adsorption behavior of heavy metals on the soil samples was explored by indoor simulation. adsorption behavior of heavy metals on the soil samples was explored by indoor simulation. The The contents of Cu, Zn, Cd, Cr, Pb, Hg and As in the soil material were 10.8 mg/kg, 125 mg/kg, contents of Cu, Zn, Cd, Cr, Pb, Hg and As in the soil material were 10.8 mg/kg, 125 mg/kg, 0.489 0.489 mg/kg, 23.5 mg/kg, 22.7 mg/kg, 58.3 mg/kg and 45.4 mg/kg, respectively. The soil pH values increased with the increase of CaCO3 concentration in the soil, and the ﬂuctuation of the soil pH values was weak after the CaCO3 concentration reached 100 g/kg. The adsorption of lime soil increased by approximately 10 mg/kg on average, and the desorption capacity decreased by approximately 300 mg/kg on average. The desorption of all heavy metals in this study did not change with increasing clay content. Pseudo-second-order kinetics were more suitable to describe the adsorption kinetics of heavy metals on the soil sample, as evidenced by the higher R2 value. Furthermore, the Freundlich model can better describe the adsorption process of As on lime soil. The process of As, Cr, Cd and Hg adsorption onto the soil material was spontaneous and entropy-driven. Additionally, the process of Cu and Pb adsorption onto the soil sample was spontaneous and enthalpy-driven. It was found that the slow adsorption processes occurred at pH < 7.00, implying that the adsorption process of heavy metals in the soil was slow under acidic conditions. In contrast, the fast adsorption stage occurred at pH > 7.00, indicating that the adsorption process of heavy metals in the soil was fast under alkaline conditions. Generally, the adsorption and desorption of heavy metals in polluted soil increased and decreased, Sustainability 2020, 12, 7315 17 of 19

respectively, with increasing CaCO3 content. The eﬀect of calcium carbonate on the accumulation of heavy metals in soil was greater than that of clay.

Author Contributions: G.H., Z.Z., X.H. and J.Z. designed the research; G.H. and Z.Z. wrote the paper and performed the experiments; M.C. and X.W. performed the data analysis. All authors have read and agreed to the published version of the manuscript. Funding: This work was ﬁnancially supported by the Guizhou science and technology support plan project (No. [2020] 1Y178, [2019] 2840, [2019] 1217, [2017] 1176); the basic condition platform construction project of Guizhou science and Technology Department [2019] 5701. Conﬂicts of Interest: The authors declare no competing interests.

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