Adsorption of EDCs On Reclaimed Water-Irrigated Soils: A Comparative Analysis of A Branched Nonylphenol, Nonylphenol and Bisphenol A


 Nonylphenol (NP) and bisphenol A (BPA) are two typical endocrine disrupter chemicals (EDCs) in the reclaimed water. The adsorptions of NP, a branched NP (NP7) and BPA on reclaimed water-irrigated soils were studied by isothermal experiments, and the different environmental factors on their adsorptions were investigated. The results showed that the adsorptions of NP and NP7 on soils conformed to Linear model, and the adsorption of BPA conformed to Freundlich model. The adsorptions of NP, NP7 and BPA on soils decreased with the increasing temperatures and pHs, while increased with the cation valence. Adsorption equilibrium constant (Kd or Kf) were maximum at pH=3, temperature 25℃ and As (Ⅲ)-soil, respectively; The adsorption capacity of NP, NP7, and BPA to soils increased in accordance with cation valence: trivalent cations > divalent cations > mono- cations. Kd of NP7 on soil was less than that of NP under the different pH and temperatures; while under different cations concentrations, it was inverse. Fourier Transform Infrared Spectrometer (FTIR) analysis showed alkyl chains of NP and BPA seemed to form van der Waals interactions with the cavity of soil. Results of this study will provide further comprehensive fundamental data for human health risk assessment of nonylphenol in the soil.


Introduction
Irrigation with reclaimed water is a key method of alleviating agricultural water shortages (FAO, 2012).
Recycled water can save water resources and promote the circulation of nutrients, but pollutants in reclaimed water and soil have potential to cause harm to humans, especially the endocrine disrupter chemicals (EDCs), which has attracted wide attention recently. EDCs can lead to endocrine disorders in aquatic organisms, affecting reproductive development and immune system (Lee et  (TDI) value for NP of 5 µg. (day. kg) −1 body weight (Danish EPA 2000). USEPA estimated a reference dose for BPA of 50 µg. (day. kg) −1 body weight (USEPA-IRIS 2014). NP is composed of varieties of isomers. The estrogenic activity and the environmental fate were heavily dependent on the isomers structures, such as the side-chain length, the degree of branching, α-substituent type, steric index (Gabriel et  After the plants' debris and residues were removed, the soils were freeze-dried at -20 ℃ and then 0.9 mmsieved. The physicochemical properties of the soil are shown in Table 1.  (pH 3,7, and 11), temperatures (5℃,25℃, and 35℃) and polyvalent metal ions (Na + , Ca 2+ , and As ( )) with a concentration of 0.002M. For the polyvalent metal ions experiment, the background electrolyte was Na + , Ca 2+ and As( ) solution respectively. The experiment was conducted in triplicate.

Dissolved organic matter (DOM)
In order to verify the effect of temperature on the dissolution of DOM, soil and water were mixed at a ratio of 1:150, shaking for 24 h at 5,25, and 35℃, respectively. Then the solution was centrifuged at 3000 rpm.
The supernatant was measured with TOC.

Data analysis
The commonly used isothermal adsorption models are Linear adsorption model, Langmuir model, and Ce: Concentration of pollutants in the liquid phase at equilibrium (mg. L -1 ).
Cs: Concentration of pollutants in the soild phase at equilibrium (mg. L -1 ).

Organic carbon adsorption constants
The soil sorption constants were normalized to the organic carbon sorption constants (K oc ). Since the main adsorbent is organic carbon in the soil, and K oc is a characterization of the sorption capacity of organic carbon. The formula is: (4) K oc : Adsorption coe cient of pollutants on organic carbon (mg. kg − 1 ).

FTIR analysis
Soil before and after adsorpted NP and BPA were characterize by FTIR-650 with a deuterated triglycine sulfate (DTGS) detector. The instrument was under continuous dry air purge to eliminate atmospheric water vapor. Interferograms were averaged for 400-4000 scans at 4 cm -1 resolutions.

Isothermal adsorption
The adsorption of (a) NP, (b) NP7, and (c) BPA on soil is shown in Fig. 1. The adsorption of NP on soil conforms to Linear and Freundlich models with the correlation coe cient of greater than 0.97( Table 2)  The Koc of NP in irrigated soil was 7.56 × 104 L.kg -1 in this study (Table 3)  . The reason for this change was that NP and BPA were deprotonated when the solution pH reached their dissociation constants (pka), which enhanced the electrostatic interactions between the pollutants and soils by converting molecules into ions when the solution pH reached their pka. As the soil surface was negatively charged, the electrostatic repulsion between EDCs and soil increased with the alkaline conditions of solution, resulting a decrease in adsorption capacity (Tong et al., 2019). Many other factors are likely to be important as well as the electrostatic repulsion, including hydrogen bonding, surface complexion and cation exchange. In addition, the partitioning behavior of the ionizable EDCs is highly susceptible to changes the soil pH, therefore may alter the ionic fraction. Empirical relationship has existed between a chemical's lipophilicity and its a nity to adsorption to soil organic matter (Collins et al., 2011;Dodgen, 2014).  It can be seen from Fig. 4 that the DOM in soil at 35 ℃ was 35.59% and 20.69% higher than that at 5 ℃ and 25℃, which is mainly caused by molecular thermal motion. Increasing temperature led to the increase of the organic matter dissolved into the aqueous phase, so the adsorption of NP and BPA on soil reduced. Similarly,  found that under certain disturbances, DOM can diffuse from soil pore water into water columns, since DOM displays high capacity of BPA and NP binding with its natural ligands and adsorption sites. Therefore the decrease of DOM in soil would lead to the decrease of adsorption of EDCs. On the other hand, DOM can be adsorbed on the soil particles again, causing a redistribution of EDCs in aquatic systems Chen et al., 2018). Meanwhile, with increasing temperature, the solubility of NP and BPA in water increased and the hydrophobicity weakened, which was also the reason of the adsorptions of NP and BPA decreased with the increasing temperature.

Effects of different polyvalent metal ions
The K d of NP were 387.8 L.kg − 1 , 575.60 L.kg − 1 and 607.9 L.kg − 1 , while the K d of NP7 were 341.7, 629.2 and 741.7 L.kg − 1 with a concentration of 0.002 M for Na + , Ca 2+ and As( ), respectively (Fig. 5). The adsorption of NP on soil increased with the cation valence. NP7 followed the same trend. K d of NP7 was 8.52% and 18.04% higher than that of NP for Ca 2+ -soil and As ( )-soil system, respectively; while in soil-Na + system, it was inverse. Similarly, K f of BPA in As ( )-soil system, which was 32.25 mg 1 − 1/n .L 1/n/ kg, was greater compared with Ca 2+ -soil system and Na + -soil system ( Table 6).

FTIR spectra analysis
Soils and soils adsorpted with NP and BPA were characterized by FTIR-650 shown in Fig. 6. The infrared spectroscopy (IR) was normalized with respect to the C-O stretching at the band of 1000-1260cm -1 . The area of adsorptions arising from a particular species is directly proportional to the concentration of that species. Thus, in principle, it was possible to determine the concentration of multiple analytes from a single spectrum ( There were no big differences of spectrums for soil and soil after NP adsorption. But differences between soil and soil after BPA adsorption was signi cant. It was obvious that T of the peaks after BPA adsorption were much smaller than soil before adsorption. Bands at 693,777 and 873 cm -1 regions were by phenyl and phenol groups, such as phenyl C-P or C-H bending vibration; Bands at 1428 cm -1 were caused by scissor bending vibration; Bands at about 1620-1640 cm -1 were caused by the stretching vibration of C = O incorporated in amide groups of proteins or C = C stretching vibration in aromatic ring alkenes. Bands at about 3400 and 3600 cm -1 were O-H in phenol, ethanol (etc.), and/or amide and amine N-H. There is no shifts in peak positions, but changes occurred in intensities of band. This result meant that alkyl chains of NP and BPA seemed to form van der Waals interactions with the cavity of soil.

Conclusion
This study, based on laboratory batch experiments, had been successful in studying the thermodynamics of adsorption of NP and NP7 on reclaimed water-irrigated soils. The results indicated that the adsorptions of NP and NP7 on soil were in accordance with the Linear model, and the adsorption of BPA on soil was in accordance with the Freundlich model. The adsorptions of NP, NP7 and BPA on soils decreased with the increasing temperatures and pH, while increased with the cation valence. The adsorption capacity of NP, NP7, and BPA to soils increased in accordance with cation valence: trivalent cations > divalent cations > mono-cations. The adoption of NP7 on soil was quite different from the total NP, which was a further study of the previous study focusing on the total NP. Alkyl chains of NP and BPA seemed to form van der Waals interactions with the cavity of soil. But reclaimed water-irrigated soils was quite complex, and the mechanism of the differences between the total NP and individual isomer need further study. This study can provide indispensable foundations for the human health risks of EDCs in the future.

Declarations Con ict of interest
The authors declare no con ict of interest.