Enhanced removal of Cd(II) by poly(acrylamide-co-sodium acrylate) water-retaining agent incorporated nano hydrous manganese oxide

Materials and Design 96 (2016) 195–202

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Enhanced removal of Cd(II) by poly(acrylamide-co-sodium acrylate) water-retaining agent incorporated nano hydrous manganese oxide Liang Peng a,⁎, Yi Xu a, Fang Zhou a, Baorui Sun a, Boqing Tie a, Ming Lei a, Jihai Shao a, Jidong Gu a,b a b

Department of Environmental Science & Engineering, Hunan Agricultural University, Changsha 410128, PR China Laboratory of Environmental Microbiology and Toxicology, School of Biological Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, PR China

a r t i c l e

i n f o

Article history: Received 21 December 2015 Received in revised form 5 February 2016 Accepted 6 February 2016 Available online 8 February 2016 Keywords: Water-retaining agent Manganese oxide Adsorption Cadmium

a b s t r a c t In this study, the hydrous manganese oxide (MO) nano-particles impregnated into a poly(acrylamide-co-sodium acrylate) (PP), was synthesized as PPM, and its characterization, adsorption properties and mechanism were investigated. The transmission electron microscopy (TEM) revealed the mostly MO with size of lower than 20 nm were dispersed in polymer. The MO greatly improved the Cd(II) uptake for PP in aqueous solution, due to its high affinity. These adsorptive processes are corresponded to the Sips isotherm model. It was revealed that maximum adsorption capacity (qm) of PP loaded with MO (Mn wt% = 1.05%, PPM-40) and PP for Cd(II) are 507 and 281 mg/g in present of 0.1 M NaCl. However, the qm of PPM-40 and PP are 698 and 771 mg/g without any NaCl. The preferential adsorption of PPM-40 to Cd(II) was showed when inferenced with salt ion. The XPS exhibited the 67% Cd(II) was captured by MO and 33% was by PP, in present of 0.1 M NaCl. The seed germination and leaching experiments demonstrated that the PPM-40 can be taken as a filter for Cd(II) and supplied “clean” water for plant roots in soil. It is a potential in-situ fixation agent for remediation of agricultural soil polluted with Cd(II). © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Recently, soil contamination of heavy metal is getting more and more attention on treating agricultural source pollution. The Ministry of Environmental Protection (MEP) and the Ministry of Land and Resources (MLR) of the People's Republic of China issued a joint report on the current status of soil contamination in China, in 2014 [1]. Of all the soil area, 16.1% exceed the environmental quality standard set by the MEP; for agricultural soils the percentage of exceedance is even greater at 19.4%. Contamination by heavy metals and metalloids accounted for the majority (82.4%) of the soils classified as being contaminated, with organic contaminants accounting for the rest. Among the heavy metals and metalloids, cadmium ranked the first in the percentage of soil samples (7.0%) exceeding the MEP limit (CCd N 0.3 mg/Kg at pH ≤ 7.5; N 0.6 mg/Kg at pH N 7.5). It is revealed that the heavy metal pollution seriously restricts the agricultural development and threatens the safety of agricultural products. For agricultural soils, in-situ fixation is a more suitable risk-mitigation technology than the phytoremediation [2] and washing remediation [3], because of its low-cost and weak adverse effect. That is by applying a

⁎ Corresponding author at: College of Resource and Environment, Hunan Agricultural University, Changsha 410128, PR China. E-mail address: [email protected] (L. Peng).

http://dx.doi.org/10.1016/j.matdes.2016.02.025 0264-1275/© 2016 Elsevier Ltd. All rights reserved.

fixing additive to soil, Cd(II) is transferred to a form less available for plant uptake. Organic (e.g., leaves, bark sawdust, peat, compost) and inorganic (e.g., lime, hydroxyapatite, lignite) amendments can effectively reduce solubility and plant uptake of Cd(II) in contaminated soils and tailings [4–6], by forming insoluble organic metallic compounds. However, it is rarely reported the fixation with nano-adsorbent for agricultural soil [7, 8]. Factually, the adsorption capacity of nano-materials, especially the novel graphene oxides nanosheets [9, 10], for heavy metal ions is much higher than that of bulk materials. However, it is generally believed that high surface energy will induce the nano-material to agglomerate, or interact with other interference matter such as mineral, humic, root exudates, clay and salt ions in soil. Thus, the fixation efficiency of nano-materials for heavy metals in the complex soil environment is far worse than that in pure single phase system. Many methods were proposed to overcome this issue, including that: 1. To fix nano-materials onto the matrix with high surface area such as mesopose material [11] and porous ion-change resin [12]; 2. To prepare nano materials with three-dimensional nanostructure [13, 14]; 3. To stabilize nano materials through coating a layer of organic matter on the surface [15, 16]. However, there are few efficient application of the materials prepared with method 1, because the matrix is very larger than nano-materials and its efficiency is relative low. The preparation is complex for the materials prepared with method 2. The method 3 is only appropriate for aqueous system. In a word, the dispersion and the stability of the nano-materials are improved after using these

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technologies, however, the adsorption efficiency with nano-materials to removal of heavy metals become lower, because of the more of the nano-materials stability, the less of its adsorption capacity. In agricultural soil, the Cd(II) exists in the interstitial water of soil through which the ion can be easily adsorbed by root system. Therefore, a filtrate progress should be introduced around the plant roots, where the interstitial water with Cd(II) would be clean and then supplied for plant. The water-retaining agent meets the requirement because it can uptake the interstitial water and then release the clean water for plant. Furthermore, the high swelling ratio in volume of water-retaining agent can prevent the nano-materials from aggregation in matrix. Furthermore, the water-retaining agent loaded the nano-material is easier in transportation than the mesopore material, because the volume of water-retaining agent becomes very small after dried. The schematics of adsorption Cd(II) in soil using the water-retaining agent incorporated with inorganic nano materials is shown in Fig. 1. The selectivity to capture heavy metal is other important properties for materials, because the concentration of target heavy metal in soil is much lower than that of the interference matters, such as humic matter, sodium ion and calcium ion. However, the nano-materials such as iron (hyro)oxides [17], aluminum (hyro)oxides [18, 19] and manganese (hyro)oxides [20] are easy interaction with interference matters and lost their selectivity for target heavy metal. The macropore polymer (water-retaining agent) is a natural filtration layer around the nanomaterial to prevent some interference and then enhance the selectivity. The Fe2O3 and Al2O3 in Burkholderia cepacia biofilms can capture more Pd(II) [21]. It is indicated that the higher selectivity for Pd(II) after biofilm covering. Hydrous zirconium oxide nano particles were incorporated in polystyrene anion exchanger D201 to get high selectivity of fluoride [22]. Nanosized hydrous manganese dioxide loaded with polymer exhibited higher selectivity of Pd(II) retention from waters in the presence of competing ion, such as Ca(II), Mg(II), and Na(I) with much greater levels than the target toxic metal [23]. The hydrous manganese oxide (MO) nano particles are selected as the incorporated material into poly(acrylamide-co-sodium acrylate) (PP) based on its higher affinity to Cd(II) [24] than that of other usual absorbent such as active carbon [25, 26], iron oxides [27], aluminum oxides [28]. In the present study, a novel adsorbent was fabricated with high efficient water-retaining agent PP incorporated nano particles manganese oxides (MO) as PPM, and its adsorption capacity, anti-interference effect, adsorption mechanism to Cd(II) was investigated. The enhanced adsorption and selectivity for the Cd(II) were analyzed in aqueous system. The filter effect of PPM for interstitial water in soil polluted with

Cd(II) was tested using seed germination experiment. And the in-situ fixation effect of heavy metal cadmium in soil was identified with the leaching experiment. And experiment results would demonstrate the PPM was an efficient in-situ Cd(II) fixation agent in soil and filter material for the toxicity interstitial water. 2. Materials and methods 2.1. Materials and instruments Acrylic acid (N99.0%) was purchased from Tianjing Guangfu Fine Chemical Research Institute. Sodium nitrate (N99.0%), potassium peroxydisulfate (N 99.5%), N,N′-methylene bisacrylamide (N98.0%) and acrylamide (N98.5%) were supplied by Sinopharm Chemical Reagent Co.Ltd. Calcium nitrate tetrahydrate (N 99.0%), manganese(II) chloride tetrahydrate (N 99.0%) and sodium hydrate (N 96.0%) were acquired from Xilong Chemical Co.Ltd. All other reagents utilized were of analytical grade (AR), and all solutions were prepared with deionized water. The pakchoi seeds were purchased from SXGarden, Hefei Province in China. All of these chemicals were used without any further purification. The material was characterized by transmission electron microscopy (H-7500, TEM-1230(HC), JPN). The IR-spectrum was evaluated with a PerkinElmer Spectrum 65 FT-IR spectrometer (USA). Elemental species analysis was performed with an X-ray photoelectron spectrometer (XPS) (K-Alpha 1063, Thermo Fisher, UK). Transmission electron microscopy (TEM) was carried out with an H-7500 (JEM-1230(HC) Japan). The crystal forms of the materials were measured with a Rigaku-TTRIII diffractometer (Japan). The concentration of cadmium solution was measured with flame atomic absorption spectrometer (AAS990, CN) and graphite furnace atomic absorption spectometry(GFAAS, Perkinelmer, USA). 2.2. Preparation of PPM and characterization Acrylamide with 15.00 g and 0.08 g N,N′-methylene bisacrylamide were both dissolved in 75.0 mL acrylic acid with the use of ultrasonic vibration for the sake of full solution in 5 min. Then extra 0–0.08 mol MnCl2 were dissolved with deionized water and injected into the mixture accompanying consistent agitation. The polymer incorporating 0.02, 0.04 and 0.08 mol manganese was presented as PPM-20, PPM-40 and PPM-80, respectively. NaOH with 25.00 g and 0.25 g K2S2O8 were also dissolved in 40.0 mL and 10.0 mL deionized water, respectively. These two solutions were added slowly and sequentially into the

Fig. 1. The schematics of in-situ fixation Cd(II) in soil using the water-retaining agent incorporated with inorganic nano materials. A) The nano composites are mixed with soil. B) nano composites are swelled. The Cd(II) would get access to nano composite, while the interfering substances are unable. C) Nano materials capture Cd(II). D) release clean water for plant and shrink (A-D process cycle. E) The Cd(II) are immobilized with the nano materials in soil.

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mixture previous in the situation of ultrasonic vibration for the formation of homogeneous and stable mutterlauge and cooled at the room temperature. The adjunction of NaOH mentioned above aimed to adjust the degree of neutralization of solution. The mutterlauge was magnetically stirred for 20 min at room temperature for homogeneity and then dumped into the reaction kettle and placed in electric thermostatic drying oven at temperature of 80 degree for 4 h. After fully matured, the material was placed into the alkaline solutions (pH = 9) and magnetically stirred for 24 h to oxidize divalent manganese then wash with purified water to neutral. The obtained materials were dried at 80 degrees for 14 days and then were preserved in desiccator. The morphological, structural, and chemical characterizations of PPM were inspected by TEM, XRD and FT-IR. The BET surface area of material was measured by methylene bule adsorption method and the cation exchange capacity (CEC) by distillation method of ammonium salt replacement [29]. 2.3. Batch adsorption experiments The adsorption capacity of Cd(II) was measured in batch experiments. In the typical experiment, 20 mg adsorbents was taken to 100 mL of Cd(NO3)2 solution (Cd(II) concentration of 100 mg/L) at stirring of 180 rpm for 2 h. The initial pH value (6.0) of the solutions was previously adjusted with diluted HCl (0.01 M). All batch experiments were performed in duplicate and the averaged values are reported. The amount of metal ion adsorbed on the material at adsorption equilibrium, qe (mg/g), was calculated according to Eq. (1): qe ¼ ðC 0 −C e ÞV=W

ð1Þ

where C0 and Ce are the initial and equilibrium metal ion concentrations (mg/L), V is the volume of the metal ion solution used in the adsorption experiment (L), and W is the weight of the absorbent (g), respectively. The adsorption kinetics experiment was conducted with the same conditions and extra 1.0 mL solution was extracted to analyze the Cd(II) concentration on sorption times of 5, 10, 20, 30, 60, 120 and 240 min. The amount of metal ion adsorbed on the composites was calculated according to Eq. (1). The adsorption capacity of different initial metal ion concentration was investigated in variation of the initial concentration of the metal ion at pH 6.0 and equilibrium time determined as above. pH effect experiment and competing cation ion of Cd(II) adsorption experiment were carried out following the same process except for variation of pH value and concentration of NaCl.

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7 days in illumination incubator at the temperature of 25 degree. This same process was cycled as 4 times in 28 days, except the distilled water was 350 mL (70% humidity) in last 3 cycles. Finally, the 500 mL distilled water was added into the breakers, and mixed as possible. Form that the 100 mL mixture was extracted, and centrifuged with 6000 rpm for 20 min. The supernate was filtered with 0.45 μm film and the cadmium concentration was analyzed with GFAAS. 3. Results and discussion 3.1. Characterization of the materials The morphology and surface structure of nano-composite was analyzed by SEM (Fig. S1 and S2). The surface of PP was smoother than that of PPM-40, indicating that the manganese ion incorporated enhanced the pores and rough of nano-composite through interfacial interactions between the templated copolymer matrix and the manganese oxides [30]. The TEM revealed that MO with size ranging from 5 to 40 nm was incorporated in polymer matrix of PPM-40, while no MO nano particles shown in PP. Most size of distributed MO was less than 20 nm (Fig. 2). The MO nano particles were incorporated in the matrix through interaction with\\OH,\\CONH2 and the\\COO−, which is deduced from changed vibration of these function groups after the MO assembled (Fig. S5). Some important parameters of PP and PPM-40 were revealed in Table 1. The surface area of PPM-40 (738.3 m2/g) was higher than that of PP (529.8 m2/g), which was consistent with above SEM result. The incorporated inorganic nano-materials with high surface area such as zirconium oxide and silica oxides increase the surface area of resin and hydrogel were reported by Bingcai Pan [22] and Soumitra Ghorai [31] as well. However, the cation exchange capacity of PPM-40 was lower than that of PP, which was attributed to that the MO with low CEC than that of polymer matrix. Furthermore, the interaction between MO and polymer matrix would decrease some ion group of matrix, such as COO− and OH− of MO. 3.2. The sorption isotherms The Cd(II) sorption isotherms for PP and PPM at solution without and with 0.1 M NaCl were investigated (Fig. 3). The adsorption capacity of PPM-40, PPM-20, PPM-80 and PP was 471, 450, 434 and 348 mg/g in the initial Cd(II) concentration of 100 mg/L, respectively. The adsorption capacity for PPM-40 was about 135% times higher than that of PP

2.4. Seed germination experiment To prove the filter effect of PPM for intersitial water in soil pollutant with Cd, the seed germination experiment was taken. The pakchoi seeds were taken on the filter paper in a breaker containing three kinds of solution: NO.1)100.0 mL pure tap water, NO.2) 100.0 mL tap water with 100 mg/L Cd(II) and mixture with 20 mg PPM-40, NO.3) 100.0 mL tap water with 100 mg/L Cd(II), respectively. The filter paper touched with the PPM-40 and cannot contact the solution immediately in NO.2. The water in NO.1 and 3 were supply the seed germination immediately, however, the water in NO.2 was filtered with PPM-40 and then supply for germination. The breakers were taken into the illumination incubator for one week at the temperature of 25 degrees for seed germination. 2.5. The leaching experiment To identify the in-situ Cd(II) fixation effect in soil, the leaching experiment was carried out. Every 500 g cultivated soil (pH 5.4, Hengyang Hunan province, China) with 126 mg/g total Cd was mixed with 0.50 g PPM-40, 0.50 g PP and without any adsorbent (CK), respectively. Three 1.0 L beakers were loaded with above mixtures. The 500 mL distilled water was added into every breaker at firstly, and then dried for

Fig. 2. Size distribution of the MO nano particles and the inserts are the TEM images of PP and PPM-40.

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Table 1 Salient Property of PP and PPM-40. PP Polymeride structure Surface group BET Surface area (m2/g) CEC (meq/g) Swelling rate Mn content (mass %)

PPM-40

Poly(acrylamide-co-sodium acrylate) R-CONH2, R-COONa 529.8 738.3 44 31 354 460 0 1.05

indicating the MO nano particles greatly enhanced the adsorption capacity of polymer. This result was consistent with the higher surface area of PPM-40 than that of PP. Similar results showed that the inorganic nano-materials would enhance the adsorption of hydrogel to pollutants. The nanosilica enhanced the removal of methylene blue and methyl violet for polyacrylamide grafted xanthan gum [31] because of its high \\OH groups' density. The montmorillonite increased the adsorption of copper and nickel ion for poly(methacrylamide-co-acrylic acid) as for its high surface area [12]. The Cd(II) adsorption capacity of PPM was higher than that of single MO nano materials as well. Our previous study confirmed that the adsorption capacity of monolayer MO was 221 mg/g, when the initial Cd(II) concentration of 100 mg/L [24]. The higher adsorption capacity of PPM was due to the enhanced effect of Donnan preconcentration given by polymer and specific adsorption by MO. Furthermore, the adsorption capacity of PPM-40 was highest than that of others, implied that an appropriate MO content could maximize Cd(II) adsorptive performance. This result may be due to the aggregated effect of MO nano particles when the MO content becomes high. It is notable that the adsorption isotherm curve was “S” shape for Cd(II) solution without salt interference. This similar result was found in the Cd(II) adsorption process with Fe0-polyacrylamide hydrogel [32], and the adsorption process of Pb(II) and Cd(II) with the poly(acrylamide-co-acrylic acid) modified with porous materials [18]. This phenomenon might be attributed to the adsorbate gathering in the external surface or macropores firstly. And the adsorption would be occurring in the internal surface and micropore of bulky polymer after the absorbate occupying on the most adsorptive sites of external surface. However, adsorption isotherm curve for Cd(II) solution with 0.1 M NaCl displayed as anti-symmetric “L” shape. It was indicated that the external surface or macropores was occupied by high concen2+ tration sodium ion (C+ Na/CCd N 110) firstly, and then only internal surface or micropores were avail for Cd(II). Considering the shape of adsorption isotherms curves, the single Langmuir or Freundlich model is not appropriated to analyze the

adsorption isotherm. Therefore, the Sips model, which is a combination of the Langmuir and Freundlich models, is used to fit the experimental data and the model can be expressed as follows: 1=n

qe ¼

qm kce

1=n

1 þ kce

ð2Þ

where qe and qm are the equilibrium and maximum adsorption capacity (mg/g), ce is equilibrium Cd(II) concentration (mg/L), and k is the equilibrium constant (L/mg). The values obtained by fitting the data for PP and PPM-40 with adsorption model are shown (Table 2). The Sips model provided the fine fit for the data of Cd(II) adsorption on PP and PPM, with the R2 higher than 0.90. This indicates that the adsorption process is a combination of monolayer and heterogeneous adsorption. However, the qm of PP-40, PPM-20, PPM-80 and PP was 698, 500, 440 and 771 mg/g, respectively. The qm of PP is higher than that of PPM-40 without the NaCl, indicating that the total adsorption sites of PPM-40 is lower than that of PP. It is attributed to the adsorption site of MO lower than that of PP. However, the qm of PP-40, PPM-20, PPM-80 and PP was 507, 433, 384 and 281 mg/g in present of 0.1 M NaCl, respectively. The qm of PPM-40 is higher than that of PP, indicating that the polymer matrix is without selectivity between Na(I) and Cd(II), however, MO is preference adsorption Cd(II). The qm of PPM-40 for Cd(II) was higher than that of currently other nano-sized metal oxides adsorbents such as α-FeOOH [33], α-Fe2O3 [34], Al2O3 [35], γ-Fe2O3 [36], TiO2 [37], ZnO [38], Al2O3–Fe3O4 composites [39]. This result demonstrated that the novel water-retaining agent loading nano metal oxides was an efficient adsorbent for Cd(II). 3.3. The sorption kinetics The sorption kinetics of Cd(II) on PPM-40 and PP were shown (Fig. 4 A), the sorption equilibrium was reached in ~60 min for PPM-40, which was some shorter than the equilibration time for PP. The faster adsorption may be attributed to the stronger interaction between the MO and the Cd(II). The data for PPM-40 and PP fitted well to the pseudo-firstorder kinetics equation (Eq. (3)): ln ðqe −qt Þ ¼ lnqe −k1 t

ð3Þ

where qe and qt indicate the amount of adsorbed Cd(II) (mg/g) on adsorbent at equilibrium and time t, respectively. k1 (min−1) is the first order rate constant applied in the present studies of Cd(II) adsorption.

Fig. 3. Adsorption isotherms of PPM and PP in Cd(II) solution A) without NaCl B) with 0.1 M NaCl.

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Table 2 Sips isotherm model constants and correlation coefficients for adsorption Cd(II) in PP, PPM-20, PPM-40, PPM-80 in present of 0 and 0.1 M NaCl. 0 M NaCl

PP PPM-20 PPM-40 PPM-80

0.1 M NaCl

qm(mg/g)

k(L/mg)

n

R2

qm(mg/g)

k(L/mg)

n

R2

771.88 500.03 698.21 440.20

0.00382 0.00150 0.07011 1.165E-9

0.6428 0.2608 0.6525 0.1063

0.9958 0.9583 0.9007 0.9919

281.19 433.49 507.98 384.49

0.00192 0.03932 0.06595 0.01393

0.5429 0.8154 0.9415 0.6691

0.9956 0.9991 0.9990 0.9887

The values of k1 and qe for Cd(II) were calculated from the plots (Fig. 4 A), and were listed (Table 3). The very high R2 value indicated the applicability of pseudo-first-order kinetics for the PPM-40 and PP adsorption process. The adsorption kinetics of some systems can be explained by a pseudo-second-order reaction as well. The pseudo-secondorder equation based on adsorption equilibrium capacity may be expressed (Eq. (4)): t 1 t ¼ þ qt t k2 q2e qe

ð4Þ

where k2 is the rate constant for pseudo-second-order adsorption and qe is the equilibrium adsorption capacity (mg/g). The k2 and qe values for different Cd(II) concentrations were experimentally fitted (Fig. 4 B), and the results are shown (Table 3). The correlation calculated correlations are closer to unity for the second-order kinetics model and calculated equilibrium adsorption capacities for Cd(II) agree with the experimental values. 3.4. Effect of solution pH on the Cd(II) removal The effect of pH on Cd(II) removal by PPM-40 was revealed in the Fig. 5. Results indicated that removal efficiency of PPM-40 was highest at about pH 6.0, while decreasing to 4.0 or increasing to 10.0 the removal efficiency would obviously reduced, at pH 2.0 the removal efficiency even approached to zero. Cd(II) removal decreased below pH 6.0 illustrating the strong interference of proton in acid solution for ion exchange effect. However, the decrease of removal above pH 6.0 might be attributed to the forming of Cd2(OH)+ 3 complex when pH ranging from 6.0 to 10.0 (Fig. 5). The size of complex become larger than that of single Cd(II), and then blocked the accession to internal or micropore of hydrogel. The higher pH (pH N 10.0) effect would not be investigated, as the PPM-40 would be dissolved at high pH. The influence of pH on the Cd(II) removal with PP was similar with that of PPM-40. The strong acidic environment usually induces the dissolution of the MO, however, no Mn element was detected when the PPM-40 was shaken at pH 2.0 for 24 h. At the strong acid environmental, the poly acrylic acid was formed at the surface area of PPM-40, because of its Pka = 4.75. And then, the

polymer was shrinked and prevented from the H+ permeating into the interior to react with MO. This process was similar to that the salt ion prevented water-retaining agent uptaking the water molecules. Thus, the MO nanoparticles in polymer are stable at acid environmental. Some reports of stable MO under acid conditions show a biotic effect, e.g., by a fungus Cephalosporium sp. [40] or biofilm in acid rock drainage [41]. In our opinion, the protected effect of biofilm was similar to the polymer of water-retaining agent. 3.5. Effect of salt concentration on Cd(II) removal The Cd(II) removal of PPM-40 was examined under different ionic strengths at pH 6.0 (Fig. 6). The PP was employed as reference to reveal the effect of MO on preventing the salt ion interferences. The PPM-40 exhibited much higher removal efficiency to Cd(II) than PP regardless of the present of background Na(I). It is reasonably because PP takes up the Cd(II) only through a nonspecific ion exchange or columbic interaction [42], and Na(I) with much higher concentration than that of Cd(II) exhibits more preferable uptake by PP. As for PPM-40, except for the nonspecific interaction between PP and Cd(II), the impregnated MO nano particles could exhibit preferable adsorption toward Cd(II) through ligand exchange or metal-ligand interaction (Eq. (5), where the overbar represents the solid phase) [22]. Such specific interaction between MO and Cd(II) has been revealed on the basis of XPS and other spectroscopic techniques [24]. The underlying mechanism for preferable adsorption of Cd(II) by PPM-40 was discussed in detail in the subsequent section: Mn−ðOH Þn þ xCd →Mn−ðOHÞn−2x O2x −Cdx þ 2xHþ 2þ

ð5Þ

3.6. Adsorption mechanism Adsorption of PPM-40 toward Cd(II) can be explained on the basis of its specific structure. PPM-40 has two distinct active sites available for Cd(II) uptake. One is the negative charged \\COO− groups, and the other one is the MO nano particles impregnated inside PP. As discussed earlier, the \\COO− groups of PP only interact with Cd(II) through

Fig. 4. Kinetics for Cd(II) adsorption with PPM-40 and PP, fitted with A) the pseudo-first-order, B) the pseudo-second-order sorption.

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Table 3 Determined parameters and regression coefficients R2, k and qe of pseudo-first-order model and pseudo-second-order model of PPM-40 and PP. Pseudo-first-order

Pseudo-second-order

C0 (mg/L) k1 (min−1) qe (mg/g) R2 PP 100 PPM-40 100

0.06865 0.08632

348.10 466.26

k2 (min−1) qe (mg/g) R2

0.94 0.00146 0.96 0.00125

377.20 504.28

0.95 0.96

nonspecific columbic electro-attraction. Therefore, the\\COO− groups would provide the potential Donnan membrane effect to pre-enrich Cd(II) from water prior to its preferable sequestration by MO. Detailed mechanism of Donnan membrane principle caused by the charged polymeric hosts and its great potential in fabrication of environmental functional materials has been clearly elucidated elsewhere [42, 43]. We probed the Cd(II) adsorption by PPM-40 based on XPS analysis. The binding energy of Cd(II) preloaded onto PP was centered at 405.38 eV (Fig. 7). It represent as columbic electro-attraction of \\COO− group with Cd(II) [44]. However, the bonding energy of Cd(II) adsorption on PPM-40 was different from that of PP. The XPS spectra of Cd (3d5/2) for PPM-40 were fitted with three response peaks, which was 405.38, 404.92 and 404.41 eV. The peak at 405.38 eV was corresponding to the Cd(II) adsorbed with \\COO− group of matrix. The other two peaks were related with MO, where the hydroxyl complexation and ion-change effect were the main interactions. This result was corresponding to that of FT-IR (Fig. S5). The peak at 404.92 eV should be corresponding to the Cd(II) adsorbed with hydroxyl, where The band of hydroxyl was changed and the bond of Cd\\O appeared when the PPM-40 adsorbed the Cd(II). The 404.41 eV was corresponded to ion-exchange effect of MO, in that the higher affinity of complexation than that of ion-exchange. It was noted that enclose area of 405.38 eV was only of 33.3% of whole peak area as revealed in XPS of Cd(II) to PPM-40. It is implied that main adsorption effect (66.7%) depend on interaction between MO and Cd(II) for PPM-40. The higher affinity between metal oxide and heavy metal ion than that of organic matter have been confirmed [24]. Therefore, some specific nano metal oxide incorporated in organic hydrogel is an efficient method on enhancing the heavy metal uptake. Such results were corresponding to the analysis of theoretical calculation (Fig. 8). The adsorption process of PP and PPM for Cd(II) was simulated using Materials Studio software, and calculated the effect energy between adsorbent and Cd(II) in a visual canonical ensemble. The result displayed that the combination energy of PPM and Cd(II) was −160 kcal/mol, larger than −181 kcal/mol corresponding

Fig. 5. Effect of solution pH on removal of Cd(II) with PPM-40, the insert is the fraction of different Cd(II)species as function of pH.

Fig. 6. Removal efficiency of Cd(II) by PP and PPM-40 at different salt concentration.

to PP and Cd(II). From the result, it is revealed that the incorporated MO nanoparticles into water-retaining agent is an efficient method to enhance the adsorption Cd(II). 3.7. Seed germination test One main application of the prepared PPM was to filter the interstitial water containing heavy metal and then provide the “clean” water for the root of plants. To clarify above function, the seed germination experiment was carried out. The pakchoi was selected as the experimental plant type because its germination was severely sensing to the toxicity of the Cd(II) [45]. The germination of pakchoi lasted 7 days and then the average length of sprout was measured. The average length of sprout was 1.5 cm, 2.0 cm and 0.9 cm after cultivated with tap water (NO.1), the mixture of 100 mg/L Cd(II) tap water with PPM-40(NO.2) and the tap water with 100 mg/L Cd(II)(NO.3), respectively. For prove the water-retaining agent can supply the clean water for plant, the seeds were contacted immediately with the PPM-40 in NO.2, instead of the tap water. These results were shown in the Fig. 9. The length of sprout cultivated by 100 mg/L Cd(II) tap water filtered with PPM-40 was longest than others. It is indicated that the PPM-40 is with high removal efficient on the water polluted with Cd(II) and to supply the clean water for the plants. It was noted that the length of sprout of NO.1 was

Fig. 7. The XPS spectra of Cd (3d5/2) for PP and PPM-40 after sorption of Cd(II).

L. Peng et al. / Materials and Design 96 (2016) 195–202

201

Fig. 8. The simulation to Cd(II) adsorption with adsorbent (A) PP and (B) PPM by using MS software.

shorter than that of NO.2. It might be attributed to some toxicants in tap water was filtered with PPM-40. The detailed is unclear and need further investigated. In all, it is a very excellent adsorbent on removal of pollutants from aqueous solution.

of cadmium in interstitial water of agricultural soil and how to disturb the adsorption needed further investigated.

3.8. The leaching experiment

PPM was prepared by copolymerization with AA, AM and MO, and its Cd(II) removal properties were investigated. TEM revealed the mostly MO nano particles with size of lower than 20 nm were dispersed in water-retaining agent. The MO nano particles greatly improved the Cd(II) uptake for PP from aqueous solution with initial Cd(II) concentration lower than 100 mg/L, due to its high affinity. Furthermore, the preferential adsorption to Cd(II) was showed for PPM-40 in present of high concentration NaCl. The seed germination experiment demonstrated that the PPM-40 can be taken as a filtration for heavy metal and supplied “clean” water for plant roots. The leaching experiment proved that the PPM-40 is an efficient in-situ fixation agent for remediation of agricultural soil contaminated by cadmium.

The cadmium toxicity of agricultural soil is mainly attributed to the cadmium transfer into the plant from soil through the interstitial water. PPM-40 is a potential in-situ fixation agent for soil polluted with cadmium. The main effect of fixation is decrease the cadmium content in interstitial water of agricultural soil. The leaching experiment was set to investigate the fixation effect. The result of leaching experiment was shown in Fig. 10. The Cd concentration of leaching solution was 13.8 μg/L, 8.9 μg/L and 3.7 μg/L for CK, processed with PP and PPM-40, respectively. The leaching solution was standed for the interstitial water of soil. Therefore, the PPM-40 was an efficient material to fix the cadmium in soil. The salt ions and organic matters can greatly disturb the cadmium adsorption in adsorbent in this interstitial water. Thus, the cadmium adsorption capacity was decreased greatly for PPM-40 in soil, even its effiency was much higher than that of PP. This disturb was very complex as our known. Therefore, the detail species

4. Conclusions

Acknowledgements For the financial support we are grateful to the Hunan Agricultural University Innovative Experiment Project, National Natural Science Foundation of China (No.41401260), Natural Science Foundation of Hunan Province (No.13JJ04068). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.matdes.2016.02.025.

Fig. 9. The seed germination experiment of Pakchoi with different processed. Above) the length of stem, below) the pictures of the germination.

Fig. 10. The Cd concentration of the leaching solution of agricultural soil with different processed.

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