Kinetic and isotherms studies of phosphorus adsorption onto natural

Environ Monit Assess (2015) 187: 381 DOI 10.1007/s10661-015-4621-4

Kinetic and isotherms studies of phosphorus adsorption onto natural riparian wetland sediments: linear and non-linear methods Liang Zhang & Chao Du & Yun Du & Meng Xu & Shijian Chen & Hongbin Liu

Received: 12 January 2015 / Accepted: 19 May 2015 / Published online: 28 May 2015 # Springer International Publishing Switzerland 2015

Abstract Riparian wetlands provide critical functions for the improvement of surface water quality and storage of nutrients. Correspondingly, investigation of the adsorption characteristic and capacity of nutrients onto its sediments is benefit for utilizing and protecting the ecosystem services provided by riparian areas. The Langmuir and Freundlich isotherms and pseudosecond-order kinetic model were applied by using both linear least-squares and trial-and-error non-linear regression methods based on the batch experiments data. The results indicated that the transformations of non-linear isotherms to linear forms would affect the determination process significantly, but the non-linear regression method could prevent such errors. Non-linear Langmuir and Freundlich isotherms both fitted well with the phosphorus adsorption process (r2 >0.94). Moreover, the influences of temperature and ionic strength on the adsorption of phosphorus onto natural riparian wetland sediments were also studied. Higher temperatures were suitable for phosphorus uptake from aqueous solution using the present riparian wetland sediments. The adsorption capacity increased with the L. Zhang : C. Du : Y. Du : M. Xu : S. Chen Key Laboratory of Environment and Disaster Monitoring and Evaluation, Institute of Geodesy and Geophysics, Chinese Academy of Sciences, Wuhan 430077, China H. Liu (*) Key laboratory of nonpoint source pollution control, Ministry of Agriculture, Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Beijing 100081, China e-mail: [email protected]

enhancement of ionic strength in agreement with the formation of inner-sphere complexes. The quick adsorption of phosphorus by the sediments mainly occurred within 10 min. The adsorption kinetic was well-fitted by pseudo-second-order kinetic model (r2 >0.99). The scanning electron microscopy (SEM) and Fourier transformation infrared (FT-IR) spectra analyses before and after phosphorus adsorption revealed the main adsorption mechanisms in the present system. Keywords Honghu Lake wetland . Non-linear regression . Soil . Isotherm parameters . Phosphate

Introduction Riparian wetlands are essential components of the landscape, providing critical functions for the improvement of surface water quality and storage of nutrients (Theriot et al. 2013). Phosphorus, one of the most important elements in the ecosystems, is often considered to be the major limiting nutrient in freshwater systems (USEPA 2000; Zhang et al. 2010a). Phosphorus occurs predominantly as phosphate in natural waters and wastewaters. Phosphates could be classified as ortho-phosphate, condensed (pyro-, meta-, and poly-) phosphates, and organically bound phosphate (USEPA 2000). The phosphorus removal and retention mechanisms and processes within wetlands are highly complex and include microbial, biological, physical, and chemical processes that may occur sequentially or simultaneously (Hammer 1989; USEPA 2000; Vymazal 2007). Among those

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processes, phosphorus adsorption from aqueous solution by sediments and precipitation reactions play an important role (USEPA 2000; Vymazal 2007). Sediments, collected from estuary, marine, wetland, canal, river, and lake, in Florida, USA, were used to evaluate phosphorus adsorption behavior; the result indicated that high capacities to retain phosphorus by sediments from estuary and wetland could play a critical role in buffering some chemical and ecological changes and benefit aquatic eco-environments (Wang and Li 2010). Investigation of the adsorption characteristic and capacity of pollutants onto natural sediments is benefit for utilizing the ecosystem services provided by riparian areas. Adsorption equilibrium and kinetics studies are always conducted to assess the adsorption characteristic of phosphorus onto various sediments in lakes (Du et al. 2011), reservoirs (Wang et al. 2012), rivers (Wang et al. 2009), constructed wetlands (Yoo et al. 2006), and agricultural drainages (Sugiyama and Hama 2013). Adsorption isotherms and kinetic models are commonly used to describe adsorption behaviors of pollutants from liquid solutions. The adsorption of uranium ions from the aqueous solution by using activated carbon was investigated, and a comparison of best-fitting was performed using the coefficient of correlation; Langmuir isotherm was found to well represent the measured sorption data, and kinetics data were best described by pseudosecond-order model (Belgacem et al. 2014). Tomar et al. (Tomar et al. 2014) studied the adsorption behavior of fluoride from aqueous media using Citrus limonum (lemon) leaf, and the result revealed that both the Langmuir and Freundlich isotherm models fitted with the fluoride sorption process. Langmuir and Freundlich isotherms equation were also used to fit the adsorption isotherms for As(V) onto an activated red mud adsorbent, and the adsorption kinetics fit well with the pseudo-second-order equation (Zhang and An 2013). However, those studies were usually based on linear regression method in obtaining the best-fitting coefficient of correlation. But some research presented that using this linear regression method for comparing the best-fitting models might be inappropriate (Ho 2004; Ho and Ofomaja 2006a; Zhang et al. 2010b). Therefore, in order to preferably use the ecosystem services provided by natural riparian wetland, there is a need for detailed and comparative research for the

Environ Monit Assess (2015) 187: 381

adsorption characteristic and capacity of phosphorus onto sediments. The sediments collected from a natural riparian wetland were used as absorbents in this study. Batch adsorption experiments were conducted and isotherm, and kinetic models were applied by using both linear least-squares and trial-and-error non-linear regression method to investigate the adsorption characteristic and capacity of phosphorus onto sediments and reveal the influences and adsorption mechanisms of the present system.

Materials and methods Sampling site description Sediments (site position, 113° 25′ E and N 29° 48′ N) were collected from the riparian area of Honghu Lake wetland, Hubei, China. Honghu Lake wetland, which is located at the north bank of the middle reaches of the Yangtze River and near the Three Gorges Dam Project, is the largest natural wetland in Jianghan Plain in central China. It has an area of approximately 344 km2, with an average water depth of 1.34 m and a maximum depth of 2.3 m (Li et al. 2009). This area had a subtropical monsoon climate. The average air temperature and annual precipitation during 2001– 2010, obtained from the China Meteorological Data Sharing Service System, were 17.8 °C and 1367.6 mm, respectively. Sediments analysis The samples were prescreened to remove larger gravel and wood chips, finely ground and sieved to 0.99) indicated that the adsorption of phosphorus onto the present riparian wetland sediments followed the pseudo-second-order kinetic expression (Table 4). SEM and FT-IR analyses To better understand the present system between the phosphorus and sediments, the SEM and FT-IR analyses were performed to study the characteristics of sediments before and after the kinetic experiment (with initial 10 mg/L phosphorus, 0.02 mol/L Cl−, 298 K temperature, and at a constant agitation speed of 170 rpm for 4 h). The SEM images of sediments were shown in Fig. 4. The sediments after reaction presented relatively rough surface (Fig. 4b). It showed that there were much more small particulate materials attached to the large particle surface after reaction. It also could be found that the spacing between the minerals was small. Most of the gap may be occupied by the newly generated small precipitates (Xue et al. 2009). It can be found that the sediments have more crushing surface structure which Table 4 Pseudo-second-order kinetic parameters by linear and non-linear methods

Methods

Plot

Linear

t qt

Non-linear

vs:t qtvs.t

may also result from the appearance of new precipitation. Thus, it could be concluded that the formation of precipitation in chemical adsorption might be one of the main adsorption mechanisms of phosphorus adsorption onto natural riparian wetland sediments. In FT-IR spectra (Fig. 5), the adsorption bands near the 3696 cm−1 region result from the OH stretching vibrations in the kaolin surface, and the razor edge near 3621 cm−1 belongs to the OH stretching vibrations of the alumina octahedral sheet which contained in crystal lattice of kaolinite (Du et al. 2008). The appearance of the characteristic absorption peaks at band of 3429 cm−1 confirms the presence of montmorillonite in this sediments sample. The strong inflection at 1635 cm−1 belongs to water bending vibrations. The bands in the 1425 cm−1 region are associated with the CaCO3 contained in soil. The most intense bands near 1032 cm−1 are attributed to Si–O–Si stretching vibrations of the tetrahedral sheets. But the double-peak characteristic bands at 776–779 and 797–800 cm−1 are assigned to the symmetric stretching vibration of O-SiO in soil, which represent the crystalline quartz and amorphous silica, respectively (Madejova et al. 2009). Besides, the Si–O–Mg bending vibrations bands appear in the 522–528 cm−1 region. By comparing the spectra qe (mg/g)

k (mg/g min)

r2

x2

0.187

3.58

0.9995

0.0049

0.183

5.35

0.9908

0.0018

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(a)

(b)

Fig. 4 Scanning electronic microscope (SEM) of adsorbents a before and b after adsorption experiment

before and after adsorption experiment, it can be found that the absorption peaks at 1032 cm−1 which belongs to SiO2 reduced. It indicated the participation of SiO2 in the absorption of phosphorus in aqueous solution. But considering the stability of SiO2, it was unlikely to directly involve in the reaction. So it could be concluded that some new formed other precipitation covered on the surface of SiO2. As the absorption peaks at 1425 cm−1 which belongs to CaCO3, it reduced at the same time. It can be concluded that the formation of new Ca3(PO4)2 precipitation caused by the precipitation reaction leads to the present result.

Conclusion In this study, batch experiments were conducted to assess the adsorption characteristics of phosphorus onto natural riparian wetland sediments, linear least-squares, and trial-and-error non-linear regression methods were both used to fit the isotherms and kinetic models. Moreover, the influences of temperature and ionic strength on the adsorption system were also investigated. The trial-and-error non-linear regression method showed advantageous to obtain the parameters for the

110

Fig. 5 Fourier transformation infrared (FT-IR) spectra of adsorbents a before and b after adsorption experiment

(b)

100

Transmittance

90

3696 779 796

1635 1425

(a)

80 3621

70

3429

60

526

50 40 30 4000

1032 3500

3000

2500

2000

1500 -1

Wave number (cm )

1000

500

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adsorption system of phosphorus onto natural riparian wetland sediments. Non-linear Langmuir and Freundlich isotherms both fitted well with the experiment data (r2 >0.94). The adsorption of phosphorus onto the present riparian wetland sediments followed the pseudo-second-order kinetic expression (r2 >0.99). The capacity of the sediments for phosphorus adsorption increased with the increase in temperature and ionic strength. The formation of precipitation in chemical adsorption might be the main reason causing the phosphorus uptake from aqueous solution. Acknowledgments The authors thank the support by the National Natural Science Foundation of China (41001333 and 41471433), the National Key Technology R&D Program of China (2012BAC06B03), the Central Public-interest Scientific Institution Basal Research Fund (2014–37), and Executive Office of Three Gorges Project Construction Committee State Council of the People’s Republic of China (JJ2013012JC011707).

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