Remediation of Phenanthrene Contaminated Soil by a Solid State

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Remediation of phenanthrene contaminated soil by a solid state photo-Fenton reagent based on mesoporous magnetite/ carboxylate-rich carbon composites and its phytotoxicity evaluation Zhijun Luo, Jing Wang, Youye Song, Xianrong Zheng, Ling-ling Qu, Zhiren Wu, and Xiangyang Wu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02850 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 24, 2018

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ACS Sustainable Chemistry & Engineering

Remediation of phenanthrene contaminated soil by a solid state photo-Fenton

reagent

magnetite/carboxylate-rich

based carbon

on

mesoporous

composites

and

its

phytotoxicity evaluation Zhijun Luo

a,c

*, Jing Wang a, Youye Song a, Xianrong Zheng b, Lingling Qu d, Zhiren

Wu a, Xiangyang Wu a

a School of the Environment and Safety Engineering, Jiangsu University, 301 xuefu road, Zhenjiang 212013, P. R. China; Email: [email protected] b Jiangsu KangRuiJia Environmental Technology Development Co., LTD, Zhenjiang 212013, P. R. China; c School of Chemistry, Biology and Materials Engineering, Suzhou University of Science and technology, Suzhou 215009, P. R. China; d School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013 P. R. China;

Abstract An environment-friendly soil remediation method based on solid state photo-Fenton reagent was developed for remediation of the phenanthrene contaminated soil. The soil remediation process of solid state photo-Fenton reagent is different from the conventional homogeneous and heterogeneous Fenton reagents (or photo-Fenton

reagents).

A

solid

state

photo-Fenton

reagent,

mesoporous

magnetite/carboxylate-rich carbon (MMCRC) composite, has been one step fabricated at a relative low carbonization temperature. In the porous structure of MMCRC, carbon serves as not only the binder to connect Fe3O4 nanoparticles and form the pore structure of MMCRC but also the supporter to solidify the carboxylate groups. 1

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Carboxylate groups solidified into carbon can coordinate with Fe3O4 nanoparticles and form the ferricarboxylate complexes. The excellent visible light absorption ability and photocatalytic activity of MMCRC originate from the ferricarboxylate complexes. Phenanthrene was chosen as a model to investigate the degradation of polycyclic aromatic hydrocarbons (PAHs) by MMCRC. Without using any other chemical agents, MMCRC can be activated by visible light irradiation and release O2•- by itself to degrade phenanthrene in soil at circumneutral pH, which make the soil remediation process extremely simple. The effects of soil remediation method based on MMCRC on contaminated soil were evaluated by using phytotoxicity test of lettuce cultivation. The phytotoxicity of phenanthrene contaminated soil was significantly reduced after treated by MMCRC and no obvious differences could be observed between non-contaminated soil and phenanthrene contaminated soil treated by MMCRC in terms of the lettuce growth indexes. The soil remediation method based on MMCRC is technologically feasible and has huge potential in the domain of remediation of persistent organic pollutant contaminated soil.

Keywords: remediation of soil; polycyclic aromatic hydrocarbons; photo-Fenton; magnetite

Introduction Polycyclic aromatic hydrocarbons (PAHs) are composed of multiple fused aromatic rings which mainly come from the inadequate combustion of organics. PAHs not only are typical persistent organic pollutants (POPs) but also have toxic, mutagenic, and carcinogenic effects on human health.

1

PAHs are hydrophobic and practically

insoluble in water and often present as non-aqueous phase liquids (NAPLs) in soil. So, soil is considered as the final destination of PAHs. 2 PAHs tend to be sequestrated in the soil matrix and the binding force increases with the prolonged aging time, which often make the remediation process burdensome. To realize the sustainable development and utilization of soil, it is imperative to remediate PAHs contaminated 2


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soil and control soil contamination risks. Fenton reagent, the aqueous mixtures of ferrous iron (Fe(II)) and hydrogen peroxide (H2O2), are widely used to treat rapidly recalcitrant organic contaminants of industrial effluent. Fenton reagents can trigger a series of catalytic chain reactions and release continuously hydroxyl radicals (•OH) through the Fe(II)/Fe(III) catalytic cycle. •OH, the most powerful oxidizing species (redox potential E0 of 2.80V), can degrade recalcitrant organic contaminants without selectivity.

3, 4

Though Fenton reagent has

been discovered and developed over one hundred years, wastewater treatment is still its primary mission and the application of soil remediation is still in its toddler stage.5,6 Unlike homogeneous wastewater, the complicated component and structure of soil set more obstacles and difficulties for the Fenton based soil remediation. These obstacles originate from the intrinsic nature of Fe(II) and H2O2. To keep Fe(II) and Fe(III) as water soluble ions rather than the oxyhydroxide precipitation, conventional Fenton reaction must operate at a low pH value (around pH 3).

7, 8

However, the

natural soil pH is often near neutral. In consideration of the higher buffer capacity of soil, the conventional Fenton reaction requires massive use of acids to attain the reaction pH, which increase the cost of remediation. The higher acidic condition causes soil quality to deteriorate. 9, 10 Especially, it also mobilizes the heavy metals in soil and devastates the soil ecosystem, which makes subsequent revegetation very difficult in treated soil.

11

The unstable nature of H2O2 is another thorny problem.

H2O2 is also stabilized at acid pH and stabilizers such as KH2PO4 were often involved into the reaction system to prevent the decomposition of H2O2 once the reaction pH above 4.

9, 12

Compared with H2O2 concentrations (0.03% (w/v)) in wastewater

treatment, higher H2O2 concentrations (4-20% (w/v)) were recommended in the remediation of PAH-contaminated soil because of the instability of H2O2 and the heterogeneity of soil. 9, 10,13 In addition, the packaging, storage, and transportation of H2O2 need special treatment. So, it is costly and impractical to remediate PAH-contaminated soil by using conventional Fenton reagent. In order to overcome the drawbacks derived from the low reaction pH, chelating agents, such as ethylenediaminetetraacetic acid (EDTA), citric acid (CA), oxalic acid 3



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(OA), ethyl lactate et. al., have been used to form the chelated iron with Fe(II) and Fe(III) which can maintain dissolved form even at circumneutral pH.

12, 14-16

These

soluble chelated iron can catalyze H2O2 and release the •OH at circumneutral pH, namely modified Fenton reagents.

17

Unfortunately, these chelating agents may

increase the use amount of H2O2 due to the competition between chelating agents and PAHs for oxidation reaction.

9, 10

Some chelating agents such as EDTA are

nonbiodegradable and persistent in environment and these chelating agents can also mobilize the heavy metal in soil, easily causing secondary pollution.

18

To avoid

adjusting the soil pH, another method is the soil flushing coupling with Fenton processes. However, the soil flushing make the soil remediation complicated because of the surfactants using of soil flushing. In addition, surfactants just like chelating agents also consume the Fenton reagent. 19 The reduction of Fe(III) to Fe(II) is widely considered as the rate-control step of Fe(II)/Fe(III) catalytic cycle of Fenton reagent.

20

Fe(OH)2+ complex is the main

existent form of Fe3+ in aqueous solution. In consequence of ligand-to-metal charge transfer (LMCT) process, the central Fe3+ ion of Fe(OH)2+ complex is reduced to Fe2+ under UV irradiation (Eqn.1).

21

Through the photoreduction of Fe(III) to Fe(II),

Fe(II)/Fe(III) catalytic cycle is accelerated and the oxidation activity is enhanced, referred to as photo-Fenton reagent.

22

The adsorption wavelength of Fe(OH)2+

complex locate in the UV region. Unfortunately, for organic contaminated soil under sun, it is infeasible to remediate organic contaminated soil by using conventional photo-Fenton reagent because atmosphere blocks a large proportion of the sun’s UV and only 3-5% of sun light is UV at ground. ν (LMCT) Fe(OH)2 + h → Fe2 + + • OH

(1)

Unlike Fe(OH)2+ complex, other important iron complexes, ferricarboxylate complexes, which absorption wavelength is within visible light region, exhibit excellent visible-light-driven photocatalytic activities.

21,

23,

24

For example,

MIL-53(Fe), a metal-organic framework (MOF) based on ferricarboxylate complexes, exhibited high photocatalytic activity under visible light irradiation. 4


25

Especially,

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these ferricarboxylate complexes can be fabricated by two environment-friendly raw material, iron species (iron ions or iron oxides) and the soluble low-molecular-weight carboxylic acids (LMWCAs) such as tartaric acids, oxalic, citric, malic, et al. Generally, these photochemical reactions of ferricarboxylate complexes were also classified as photo-Fenton reagents.

26-28

In a broad sense, photo-Fenton reagents can

be classified into two types: homogeneous photo-Fenton reagents (Fe2+ or Fe3+/H2O2 or LMWCAs/light irradiation) and heterogeneous photo-Fenton reagents (iron oxides/H2O2 or LMWCAs /light irradiation). From table 1, it is obvious that both homogeneous photo-Fenton reagents and heterogeneous photo-Fenton reagents are not all solid state because the H2O2 and LMWCAs are liquid phase.

Table 1. Categories of photo-Fenton reagents Categories

component

Homogeneous photo-Fenton

Heterogeneous photo-Fenton

Fe2+ or Fe3+

H2O2 or LMWCAs

(liquid phase)

(liquid phase)

Iron oxide

H2O2 or LMWCAs

(solid phase)

(liquid phase)

For the remediation of soil, the conventional chemical oxidants, including Fenton reagent (Fe2+/H2O2), hydrogen peroxide (H2O2), potassium permanganate (KMnO4), and sodium persulfate (Na2S2O8), are mobile phase (liquid phase). 29-31 Because of the good adsorption capacity of soil and liquid phase fluidity of oxidants, both contaminated part of soil and the healthy part of soil are under attack of oxidants. So, large number of fluid oxidants is wasted on healthy part of soil and the whole soil quality is also destructed. In this study, an all solid state photo-Fenton reagent based on mesoporous magnetite/carboxylate-rich carbon (MMCRC) composite was one-step synthesized without inert gas protection. MMCRC was used as oxidant to degrade phenanthrene in soil under visible light irradiaition. The main objectives of this research are (1) evaluate the technical feasibility of remediation PAHs contaminated 5



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soil by solid state photo-Fenton reagent at circumneutral pH; (2) demonstrate that the excellent visible light absorption and photocatalytic activities of MMCRC originate from the ferricarboxylate complexes; (3) speculate a possible photocatalytic mechanism of Fe(II)/Fe(III) photocatalytic cycles based on ferricarboxylate complexes; (4) assess the phytotoxicity of the phenanthrene contaminated soil treated by MMCRC.

Experimeantal section Preparation of catalysts All chemical reagents are of the analytical grade without further treatment. MMCRC was synthesized in a muffle at a relative low reaction temperature. In a typical synthesis, 2 g of sodium tartrate, 0.5 g of glucose, and 2 g of FeSO4.7H2O were successively dissolved in 20 mL deionized water and stirred for 20 min to obtain a homogeneous mixture solution. This mixture solution was heated and kept at 100 ºC for 10 h in an oven to evaporate majority of water and form a viscous gel. Then, the gel was carbonized at 250 ºC for 3 h in a muffle. After cooled naturally to room temperature, the products were collected, washed with deionized water and ethanol alternately three times, and finally vacuum dried at 90 ºC overnight. The control experiments of synthesis of MMCRC was carried out at different reaction temperature from 200 ºC to 450 ºC and different glucose dosages from 0 g to 3 g at 250 ºC. These obtained magnetite/carboxylate-rich carbon (MCRC) composites were denoted as MCRC-a-b, where a and b stands for the reaction temperature and glucose dosage, respectively. To demonstrate the effect of ferricarboxylate complexes in the degradation of phenanthrene in soil, other carbon based materials including carboxylate-rich carbon (CRC), and Fe-rich carbon were synthesized by the similar synthetic route with MCRC. CRC was synthesized by using glucose (0.5 g) and sodium tartrate (2 g). Fe-rich carbon was synthesized by using glucose (0.5 g) and FeSO4.7H2O (2 g). In addition,

magnetite

nanoparticles

were

synthesized

6


via

the

conventional

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coprecipitation method according to our previous report. 23

Characterization The crystal phases of the samples were recorded with a Bruker D8 diffraction analyzer with high-intensity Cu Kα radiation (λ=1.54 Å). Raman spectra were characterized using a Renishaw Raman system model 2000 spectrometer. Thermogravimetric (TG) analysis was recorded using an Integrated thermal analyzer (STA 449C) under a flow of air with a temperature ramp of 5 ºC/min. Scanning electron microscope (SEM) images were collected on a Hitachi S-4800. Transmission electron microscope (TEM) images were collected on a JEOL 2100 electron microscopes (200 kV). The BET surface area and pore size distribution of the samples was characterized from Brunauer-Emmett-Teller (BET) measurements using an ASAP 2020 surface area analyzer. Fourier transform infrared (FTIR) spectra were identified using a Nicolet Nexus 470. X-ray photonelectron spectra (XPS) were characterized using an ESCALAB 250 spectrometer (Thermo-VG Scientific). The UV-visible diffuse reflection spectra (DRS) were obtained for the dry-pressed disk samples on a UV-Vis spectrophotometer (Shimadzu Corporation, UV-2450) with BaSO4 as the reflectance standard (catalyst: BaSO4=1:1 by weight). The electron spin resonance (ESR) analysis was carried out on an electron paramagnetic resonance spectrometer (A300-10/12, Bruker) and DMPO (5, 5-dimethyl-l-pyrroline N-oxide) was used as the spin trapping reagent. The total carboxylate group contents in MCRC were determined by a simple chemical titration method according to previous reports.

32

0.5 g of MCRC was

dispersed into 50.0 mL deionized water. Then, the pH of mixture should be adjusted to 2.0 to convert -COONa groups of the MCRC into -COOH groups. After stirring continuously for 2 h, the mixture was filtered and the solid was dried at 80 ºC for 24 h. 0.1 g of the dried solid was dispersed into 100 mL of NaHCO3 aqueous solution (0.01 mol L-1) and stirred for 2 h in N2 to isolate from air. After stirring, the suspension was filtered and then three aliquots (25.0 mL) of filtrate were titrated with HCl standard aqueous solution (0.01 mol L-1). The content of carboxylate group was calculated 7



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according to Eqn. (2) and the results were given by average.

C COOH =

C NaHCO3 × VNaHCO3 − C HCl × VHCl

m

(2)

Where C NaHCO3 and C HCl (mmol mL-1) are the concentration of NaHCO3 and HCl solution, respectively. VNaHCO3 and VHCl (mL) are the volume of NaHCO3 and HCl solution, respectively. In addition, m (g) is the weight of MCRC.

Soil samples and soil spiked with phenanthrene Surface soil (0-20 cm) was collected from a garden located in Zhenjiang, China. These collected soils were air dried and grinded. Then, the soil was passed through a 0.25 mm-mesh sieve and fully mixed to obtain the soil sample. The sieved soil samples were stored in dark before use. The physical and chemical properties of soils were shown in Table S1. The collected uncontaminated soil was spiked with phenanthrene dissolved in methanol solution. The methanol was evaporated from soil for 24 h at room temperature in a fume hood. Subsequently, the spiked soils were aged for one month in a sealed bottle and kept at 4 ºC in a refrigerator to eliminate possible biodegradation by microbes. The phenanthrene concentration in soil was determined by gas chromatograph (GC) and the phenanthrene concentration in soil is about 200 mg/kg.

Degradation of phenanthrene by solid state photo-Fenton reagent and Fenton reagent. The photodegradation reactions of phenanthrene were carried out in a chamber equipped with a 250 W Xenon lamp with a 420 nm cutoff filter and the illumination could reach 8×104 Lux. The distance between Xenon lamp and samples was 15 cm. Temperature in the chamber was controlled below 30 ºC by a fan. For all photodegradation reactions, the mixture of 15 g of spiked soil and 0.3 g solid state photo-Fenton reagent was evenly distributed on the bottom of a glass dish with 8


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diameter of 15 cm and the glass dish was placed under the Xenon lamp. During the photodegradation process, soil was wetted and the moisture content of soil was kept around 60 %~80 %. At interval of 15 min, the loss of water was supplied by a spray according to the weighing the glass dish to calculate the moisture content. At appropriate time intervals, 0.5 g soil samples were withdrawn from the glass dish to detect the residual phenanthrene in soil samples after visible light irradiation. All degradation experiments were conducted in three replicates. For the purpose to compare the phytotoxicity of conventional Fenton reagent and solid state photo-Fenton reagent, conventional Fenton reagent was also used to remediate the phenanthrene contaminated soil and the remediation process as follow: 5 g of spiked soil and 10 mL of deionized water were added into a beaker and form the slurry. The pH of slurry was adjusted to about 3 by using 1 N H2SO4 aqueous solution. Then, 140 mmol of H2O2 (30%) and 3.4 mmol of Fe2+ were added into the slurry under agitation. After 8 h of Fenton reaction, 0.5 g of soil sample was collected to detect the residual phenanthrene in soil samples.

Phenanthrene extraction from soils and analytical methods The residual phenanthrene and their products were extracted by using dichloromethane (DCM) as extraction solution. 10 mL of DCM was added into 0.5 g of soil sample and form a suspension. The suspension was extracted in an ultrasonic bath for half an hour, and then centrifuged at 8000 rpm for 5 min to separate the supernatant from the solid. Then, the supernatant was filtered through a 0.22 µm membrane in a syringe filter. The concentration of phenanthrene was determined by using a gas chromatograph (GC-2010, Shimadzu), equipped with a flame ionisation detector (FID) and a Shimadzu Rtx-5 capillary column (30 m×0.32mm×0.5 µm). High purity nitrogen was used as the carrier gas. The injection of sample was operated in splitless mode and the injection volume was 1 µL. The injector temperature was set as 280 ºC. The initial oven temperature was set at 80 ºC for 2 min, then increased to 200 ºC with 20 ºC/min of heating rate and held for 5 min, and then increased to 280 ºC with the 20 9



ºC/min and held for 5 min. The detector temperature was 280 ºC. The retention time for phenanthrene was 16.1 min under above conditions.

Phytotoxicity tests Phytotoxicity tests were conducted on the lettuce cultivation in different soils. These four different soils include non-contaminated soil as control, phenanthrene contaminated soil treated by solid state photo-Fenton reagent based on MMCRC, phenanthrene contaminated soil (200 mg/kg), and phenanthrene contaminated soil treated by Fenton reagent. Lettuce cultivation in each soil was performed in four replicates. Seeds of lettuce were first surface-sterilized in 5% (v/v) NaClO for 10 min and then washed with deionized water. 15 g of soil were placed in a plastic pots (side length: 4 cm, height: 3.5 cm). 5 seeds were placed in the soil per pot and the moisture content of soil was kept at 60 % in the whole cultivation process. Then, these six plastic pots were placed into a growth chamber with a fluctuating day/night time and temperatures (16 h/8 h and 22 °C/16 °C). After five days, the seed germination percentage was recorded. Four pots of lettuce seedings growing well were elected and two of five healthy seedlings in each pot were left in soil for subsequent growth experiment. Ten days later, lettuce were harvested and rinsed with deionized water. The root length, leaf length, and fresh weight of lettuce were measured.

Statistical analysis Univariate descriptive statistics analysis was made for all growth indexes involving seeds germination percentage, root length, leaf length, root fresh weight and shoot fresh weight. Data from ecotoxicity evaluation was analyzed using SPSS software, version 22.0 for statistical analysis. A one-way analysis of variance (ANOVA) test was used to test for differences between each treatment type for soil. A comparison of mean rank of all pairs of groups was applied using Student Newman Keuls (SNK) test. A two-tailed p value < 0.05 was considered to indicate statistical significance. 10


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Results and discussion Characterization of mesoporous magnetite/carboxylate-rich carbon (MMCRC)

Fig. 1. (a) XRD pattern and (b) Raman spectra of MMCRC.

The typical XRD pattern of as-synthesized MMCRC, which was synthesized at 250 ºC for 3 h and the glucose dosage was 0.5 g, are shown in Fig. 1a. Six main peaks are well in agreement with those of the Fe3O4 (JCPDS card no.: 11-0614) and correspond to the Bragg reflections from the (220), (311), (400), (422), (511), and (440) planes of Fe3O4, respectively. There is no representative diffraction peaks of crystalline carbon observed. To further distinguish the carbon materials and iron oxides, MMCRC was characterized by using Raman spectrum and shown in Fig. 1b. There are two characteristic peaks of Fe3O4 at 215 and 280 cm-1, which can be assigned to the A1g(1) and Eg(1) vibration modes of Fe3O4, respectively.

33

Two weak peaks can also be

observed at 1361 and 1580 cm-1, corresponding to the typical D and G bands of carbon, respectively. The carbon of MMCRC is amorphous carbon and has a disordered structure due to the lower intensity of D and G bands. It is inevitable that the relative low carbonization temperature (250 ºC) produce carbon with poor crystallinity.

11



Fig. 2. (a and b) SEM images, (c and d) TEM images, (e) HRTEM images, and (f) Thermogravimetric (TG) curve of MMCRC.

The morphologies of MMCRC were investigated by SEM and TEM. From the SEM image of MMCRC (Fig. 2a), MMCRC is nearly elliptical shape without uniform sizes and possess a very rough surface. The surface SEM image of MMCRC (Fig. 2b) shows that MMCRC is composed of large number of particles. The TEM image (Fig. 2c) also demonstrates that MMCRC is constructed by large number of nanoparticles. These nanoparticles aggregate and connect together to produce abundant mesopore. As shown in Fig. 2d, the diameter of these mesopores is about 4 nm. High-resolution 12


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TEM (HRTEM) image taken from the edge of MMCRC reveal the connect mode between these nanoparticles. Fig. 2e shows that several nanoparticles are connected by the carbon coating and the gap between these nanoparticles is mesopore. From the inset of Fig. 2e, clear lattice fringes can be observed on nanoparticle coated by carbon and the interplanar distance is about 0.296 nm which should be attributed to the (220) lattice spacing of Fe3O4. It can be easily deduced that large number of Fe3O4 nanoparticles are connected by carbon and construct abundant mesopores of MMCRC. So carbon content is a very important factor in the structure of MMCRC. The carbon content of MMCRC can be estimated through the weight loss of MMCRC via the thermogravimetric analysis (TGA). Fig. 2f is the TG analysis curve of MMCRC. Obviously, there are two stage weight losses from 25 ºC to 800 ºC. The first weight loss takes place between 25 ºC and 205 ºC, which should attribute to the moisture evaporation of the MMCRC. The subsequent weight change at 205 ºC~750 ºC should attributed to the decomposition of carbon coating and the transformation of Fe3O4 to Fe2O3 in air. According to the result of TGA, the carbon content of MMCRC can be calculated as about 10.00 wt%.

Fig. 3. (a) N2 adsorption and desorption isotherm and (b) BJH pore distribution of MMCRC.

The specific surface area and pore size distribution of MMCRC were investigated by N2 adsorption-desorption isotherms. According to the N2 adsorption-desorption isotherms (Fig. 3a) and pore size distribution curve (Fig. 3b), the BET surface area of 13



MMCRC is calculated as 58.61 m2/g and MMCRC shows a narrow pore size distribution and the average pore diameter is about 3.8 nm, which consist with the result of TEM (Fig. 2d and Fig.2e). The large surface area and porous structure of MMCRC will be propitious to the adsorption and subsequent photodegradation of phenanthrene in soil.

Fig. 4. (a) FT-IR spectra, (b) survey scan XPS, (c) Fe 2p XPS, and (d) C 1s XPS of MMCRC.

The original idea of this work is to solidify carboxylate groups into amorphous carbon and coordinate with Fe3O4, and finally form the ferricarboxylate complexes. So, the solidification of carboxylate groups into carbon is a very important precondition to obtain abundant ferricarboxylate complexes. Fig. 4a shows the FT-IR spectra of MMCRC, CRC, sodium tartrate, and Fe3O4 nanoparticles, respectively. MMCRC has a similar band with Fe3O4 nanoparticles around 580 cm-1, which should ascribed to the Fe-O stretch of Fe3O4.

34

Sodium tartrate present two bands of the

14


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carboxylate group including 1618 cm-1 and 1372 cm-1, which should be ascribed to the antisymmetric stretching mode (νas), and symmetric stretching mode (νs) of carboxylate group, respectively.

35

When the tartrate is solidified into carbon (CRC),

the νs band shift to higher wavenumber, while νas band shift to lower wavenumber. Compared with CRC, the νas band of MMCRC shift to a higher wavenumber, on the contrary, νs band shift to a lower wavenumber. These IR results demonstrate that the carboxylate group of tartrate can coordinate with Fe3O4 nanoparticles and form the ferricarboxylate complexes, FeII(C4H4O6)n(2-2n) and FeIII(C4H4O6)n(3-2n). To determine more information about chemical state and surface properties of MMCRC, X-ray photoelectron spectroscopy (XPS) were carried out and the full scan XPS spectra of MMCRC demonstrated there were four elements of carbon, oxygen, sodium, and iron elements existing on the surface of MMCRC (Fig. 4b). Especially, Na1s peak accompanied by strong Auger peak at ~497 eV. Na Auger peak can be observed because of the combustion of sodium tartrate under carbon. For the Fe 2p XPS spectra (Fig. 4c), two peaks at 711.2 and 724.5 eV are the characteristic peaks of Fe 2p3/2 and Fe 2p1/2 for Fe3O4, which demonstrate the formation of Fe3O4 phase in MMCRC.

36

From the deconvoluted C 1s XPS spectra (Fig. 4d), three component

peaks at 283.1, 283.9, 286.5 eV are identified and these peaks should be ascribed to the C-C, C-O, and C=O bonds, respectively. The strong intensity of C=O bond at 286.5 eV implies MMCRC possesses abundant carboxylate groups.

37

From the

results of IR and XPS, it can be concluded that MMCRC is rich in ferricarboxylate complexes, FeII(C4H4O6)n(2-2n) and FeIII(C4H4O6)n(3-2n).

The main reaction parameters on the synthesis of MMCRC To determine the main reaction parameters on the synthesis of MMCRC, the control experiments were carried out and demonstrated that two reaction parameters, reaction temperature and glucose dosage, play crucial roles during the formation process of MMCRC. The synthesis of MMCRC was carried out under air atmosphere. So, the reaction temperature is a very important parameter to obtain MMCRC. Fig. 5a exhibits the XRD patterns of magnetite/carboxylate-rich carbon (MCRC) composites 15



Page 16 of 36

synthesized at different reaction temperature (250 ºC, 300 ºC, 400 ºC, and 450 ºC) and the glucose dosage is 0.5 g. The carbonization reaction of carbon source including the glucose and sodium tartrate must be kept in a suitable temperature range. If the reaction temperature below 200 ºC, the carbon source could not be carbonized and the product also had no any magnetism which implied the Fe3O4 could not be obtained. On the other side, some impurity may emerge if the reaction temperature was too high. Besides Fe3O4, a small amount of Fe2O3 could be also observed in MCRC-450-0.5 when the reaction temperature exceeded 450 ºC (Fig. 5a). In order to remain rich organic functional groups in carbon as much as possible as well as produce Fe3O4, the optimal reaction temperature was chosen as 250 ºC.

Fig. 5 XRD patterns of MCRC synthesized by different reaction temperatures (a) and different glucose dosages at 250 ºC (b). Table 2. The carbon content, BET surface areas, pore volume, carboxylate group content, and surface iron content (wt %) of MCRC prepared by different glucose dosages. samples

carbon content

SBET

carboxylate group

Fe

(wt %)

(m2/g)

content (mmol/g)

(wt %)

MCRC-250-0

4.27

55.96

0.58

43.64

MCRC-250-0.5

10.00

58.61

3.79

25.77

MCRC-250-1

35.87

44.61

3.93

25.58

MCRC-250-3

66.71

12.69

3.96

13.43

16


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Fig. 6. SEM images of MCRC synthesized by different glucose dosages: (a and b) 0 g, (c and d) 1 g, (e and f) 3 g.

Besides the reaction temperature, the glucose dosage is another important parameter to obtain MMCRC. Fig. 5b shows the XRD patterns of MCRC synthesized at different glucose dosages and the reaction temperature is 250 ºC. When the glucose dosage increase from 0 to 1 g, Fe3O4 is the identical product for all samples (MCRC-250-0, MCRC-250-0.5, and MCRC-250-1) regardless of the glucose dosage. The intensity of diffraction peaks gradually decrease with the increase of glucose dosage. When the glucose dosage reaches 3 g, there is not any diffraction peaks 17



Page 18 of 36

observed on MCRC-250-3 though MCRC-250-3 has magnetism. The properties of MCRC prepared with different glucose dosages, including carbon content, BET surface areas, pore volume, carboxylate group content, and the Fe wt % on the surface, were present in Table 2. Carbon carbonized from the glucose mainly exerts two important roles during the formation process of MMCRC. First of all, carbon served as the binder to connect the Fe3O4 nanoparticles and form mesoporous structures. According to the result of TEM of MMCRC (MCRC-250-0.5), the formation of pore structure depends on the connection of carbon between Fe3O4 nanoparticles (Fig. 2e). So, glucose served as carbon source and the glucose dosage must keep in an appropriate range. If the glucose did not involve into the reaction (MCRC-250-0), sodium tartrate is the only carbon source and the carbon content of MCRC-250-0 is only about 4.27 wt%. It is obvious that the product is composed of many nanoparticles and these nanoparticles are unstructured (Fig. 6a and b). Not getting enough carbon can not fabricate the mesoporous structures. The BET surface area of MCRC-250-0 is about 55.96 m2/g. When the glucose dosage increases to 1 g, MCRC-250-1 is the aggregation of micrometer scale of particles (Fig. 6c and d). The carbon content of MCRC-250-1 increase to 35.87 wt% and the BET surface area decrease to 44.61 m2/g. Once the carbon content exceeds the demand of connecting Fe3O4 nanoparticles, the redundant carbon will block the pore and induce the decrease of pore volume. Compared with MCRC-250-0.5, the carbon content of MCRC-250-3 increase from 10 wt% to 66.71 wt% and the BET surface area decrease from 58.61 m2/g to 12.69 m2/g and the pore volume decrease from 0.06 cm3/g to 0.003 cm3/g. As shown in Fig. 6e and Fig. 6f, MCRC-250-3 has a very thick carbon coating. The reason why MCRC-250-3 possesses magnetism but can not found diffraction peaks of Fe3O4 is probably due to the thick carbon coating on Fe3O4 (Fig. 5b and Fig. 6f). Another important role played by carbon derived from the glucose is served as a supporter to solidify the carboxylate groups. The carboxylate group content of MCRC is an important factor for obtaining more ferricarboxylate complexes. The carboxylate group content of MCRC was determined by chemical titration method. 18


32

As for

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MCRC-250-0, sodium tartrate acts as not only carbon source but also the source of carboxylate groups. MCRC-250-0 contains 4.27 wt % carbon and the carboxylate group content is only 0.58 mmol/g (Table 2). When 0.5 g of glucose served as carbon source and involved into the reaction, MCRC-250-0.5 contains 10.00 wt % carbon and the carboxylate group content significant increases to 3.79 mmol/g, which is higher than that (0.422 mmol/g) of carbon nanotubes treated with hot H2SO4/HNO3. 38 The amorphous carbon of MCRC may have greater capacity for the organic functional groups than the carbon nanotubes, the typical crystalline carbon. However, rising tendency of carboxylate group content in MCRC become slowly once the glucose dosage above 1 g. The carboxylate group content of MCRC-250-1 is 3.93 mmol/g and that of MCRC-250-3 is 3.96 mmol/g. As mentioned above results and discussions, MMCRC (MCRC-250-0.5), which possesses the highest BET surface area among MCRC, only can be obtained in the presence of 0.5 g of glucose at 250 ºC. In the structure of MMCRC, carbon serves as not only the binder to connect Fe3O4 nanoparticles and form the pore structure of MMCRC but also the supporter to solidify the carboxylate groups. The unique mesoporous structure of MMCRC, which make the carboxylate-rich carbon and Fe3O4 nanoparticles have a compact structure and own the strong interaction, are beneficial to the formation of ferricarboxylate complexes.

Photodegradation of phenanthrene at natural soil pH (near neutral)

19



Page 20 of 36

Fig. 7. (a) the photodegradation of phenanthrene by MCRC-250-0.5 (MMCRC) with and without visible light; (b) the photodegradation of phenanthrene by Fe3O4 nanoparticles, Fe rich carbon, CRC, MCRC-250-0, MCRC-250-0.5, MCRC-250-1, and MCRC-250-3; (c) DRS spectra of sodium tratrate, Fe3O4 nanoparticles, Fe rich carbon, CRC, and MCRC-250-0.5; (d) DRS spectra of MCRC-250-0, MCRC-250-0.5, MCRC-250-1, and MCRC-250-3.

The visible-light-driven photodegradation activities of MMCRC were evaluated by photodegradation of phenanthrene in soil. Fig. 7a displays the photodegradation ratio of phenanthrene with an initial concentration of 200 mg/kg under visible light irradiation (λ>420 nm). After 10 h of visible light irradiation, more than 93 % of phenanthrene was removed. In the dark, the degradation of phenanthrene was negligible which demonstrate the role of light. To distinguish the main component of MMCRC which is responsible for the excellent photocatalytic activity, Fe3O4 nanoparticles (without carboxylate groups and carbon), Fe rich carbon (without carboxylate groups), and CRC (without iron) were also used as photocatalyst, respectively, to degrade the phenanthrene in soil. Because of the absence of carboxylate groups or iron, the ferricarboxylate complexes can not be fabricated in Fe3O4 nanoparticles, Fe rich carbon, and CRC. As shown in Fig. 7b, it is obvious that MCRC containing rich ferricarboxylate complexes has enhanced visible

light

photodegradation

activity

over

those

photocatalysts

without

ferricarboxylate complexes including Fe3O4 nanoparticles, Fe rich carbon, and CRC. To clarify the electronic interaction between carboxylate-rich carbon and Fe3O4 20


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nanoparticles and distinguish the effect of ferricarboxylate complexes, the DRS of sodium tartrate, Fe3O4 nanoparticles, Fe-rich carbon, CRC, and MMCRC were investigated and shown in Fig. 7c. Sodium tartrate has a characterize peak at about 190 nm and can not absorb visible light. Compared with Fe3O4 nanoparticles, Fe-rich carbon and CRC, MMCRC displays significant UV and visible light absorption expect the region above 562 nm. These control experiments demonstrate that ferricarboxylate complexes are the key component which is the origin of excellent visible light absorption abilities and photocatalytic activities of MMCRC. The photocatalytic activities of MCRC originate from the Fe(II)/Fe(III) photoredox cycle of ferricarboxylate complexes. So, it is reasonable that the more iron and carboxylate groups there are on the surface of MCRC, the higher enhanced photocatalytic activities MCRC possesses. As shown in Fig. 7b, the photodegradation rate

of

phenanthrene

follows

the

order:

MCRC-250-0.5>

MCRC-250-0>

MCRC-250-1> MCRC-250-3. MCRC-250-0.5 possesses the highest photocatalytic activity because MCRC-250-0.5 has simultaneously abundant iron (25.77 wt %) and carboxylate groups (3.79 mmol/g) which means the existing rich ferricarboxylate complexes on the surface. Due to the lower carboxylate group content (0.58 mmol/g), the photocatalytic activity of MCRC-250-0 is lower than that of MCRC-250-0.5 though the iron content (43.64 wt%) of MCRC-250-0 is higher than that (25.77 wt%) of MCRC-250-0.5. It is interesting that MCRC-250-0 and MCRC-250-0.5 have similar visible light adsorption abilities (Fig. 7d). Maybe, the reason is there is no enough ferricarboxylate complexes to provide consecutively for the photocatalytic process due to the lack of carboxylate groups, though ferricarboxylate complexes on the surface of MCRC-250-0 is enough for the visible light adsorption. The lower Fe wt % makes MCRC-250-1 and MCRC-250-3 have the lower photocatalytic activities than that of MCRC-250-0.5 and MCRC-250-0 though the MCRC-250-1 and MCRC-250-3 have relative higher carboxylate group content, 3.93 mmol/g and 3.96 mmol/g. According to these results mentioned above, it can conclude that carbon, the supporter to solidify the carboxylate groups, play a crucial role in the formation of ferricarboxylate complexes. 21



Fig. 8. Effect of soil thickness on the degradation of phenanthrene in soil.

As is well known, it is difficult for light to penetrate the soil. So, the photodegradation of phenanthrene in soil was limited by the soil thickness. Fig. 8 is the photodegradation ratios at different soil thickness in the presence of MMCRC. The soil thickness was controlled by a glass ring with a piston (Fig. S1). The soil thickness can be adjusted via the movement of piston. The outer ring was covered by silver paper to prevent the light through the glass ring. As shown in Fig. 8, the photodegradation ratio decrease steeply with the increasing of soil thickness. When the soil thickness was 2 mm, the photodegradation ratio of phenanthrene was 91.1 %. However, the photodegradation ratio of phenanthrene decreased to 42.4 % while soil thickness increased to 10 mm. Obviously, phenanthrene in soil could be efficiently degraded if soil particles were crushed adequately and turned over at regular intervals.

Reaction mechanism

Fig. 9. ESR spectra of radical adducts trapped by DMPO from MMCRC in the dark and under visible light irradiation, respectively 22


Page 22 of 36

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Until now, photochemical reaction mechanism of ferricarboxylate complexes is still controversial due to its complicated reaction process. To clarify the photochemical reaction mechanism of MMCRC, the ESR technique was used to confirm the main active species. As shown in Fig. 9, there is no signal detected from the MMCRC in dark. However, there are six characteristic peaks observed under visible light irradiation and these ESR peaks is similar with the previous report about the DMPO-O2•- adduct. 39, 40 In addition, there is not clear DMPO-•OH spin adduct signal. These ESR spectra results demonstrate MMCRC can be activated by visible light irradiation and release O2•-. According to experiment results mentioned above, a possible photocatalytic mechanism

of

MMCRC

are

speculated.

Ferricarboxylate

complexes,

FeII(C4H4O6)n(2-2n) and FeIII(C4H4O6)n(3-2n), are the key component and O2•- is the main active species of MMCRC. The intermediate active species O2•- are formed through the following processes: (i) FeIII(C4H4O6)n(3-2n) are activated into a charge-transfer state FeII(C4H4O6)n(3-2n)* (designated by an asterisk) through the ligand-to-metal charge transfer process (LMCT) under visible light irradiation (Eq.(3)).

21,26

(ii) The

photolysis process of FeII(C4H4O6)n(3-2n)* not only release the Fe(II) but also initiate the decarboxylation reaction to yield CO2•- (Eq.(4)). can produce superoxide (O2•-) (Eq.(5))

14, 41

41

The reaction of CO2•- with O2

and the O2•- was detected by ESR spectra

(Fig. 9). (iii) The acid-base pair HO2•-/O2•- (pKa=4.8) will trigger a dismutation reaction and generate H2O2, (Eq.(6)).

26,27

Then, H2O2 formed from HO2•-/O2•- reacts

with Fe(II) yielding •OH (Eq.(7)). 14 However, in this soil remediation process, the pH value of soil is 6.31. So, there is not enough H+ to provide for the dismutation reaction of HO2•-/O2•- and generate H2O2 and •OH, which means that Eq.(6) and Eq.(7) are suppressed at near neutral soil condition. This is probably the main reason why the DMPO-O2•- adduct signal was detected but the DMPO-•OH adduct signal was not clear. The dominant active species of MMCRC is O2•- rather than •OH and O2•- can degrade the phenanthrene adsorbed on clay (Eq.(8)). In previous report, O2•- is also the dominant active species of Fe(III)-smectite under visible light irradiation.

14

It is

easy that Fe(II) is oxidized to Fe(III) by O2 and other oxidants at circumneutral pH. 23



Page 24 of 36

Then, Fe(III) coordinate with tartaric acid and form the FeIII(C4H4O6)n(3-2n) complexes and realize the Fe(II)/Fe(III) photocatalytic cycles.

Fe III (C 4 H 4 O 6 ) (3n− 2 n) + hν (λ > 420 nm ) LMCT → Fe II (C 4 H 4 O 6 ) (3n− 2 n) * •

Fe II (C 4 H 4 O 6 ) (3n− 2 n) * → Fe(II) + (C 4 H 4 O 6 ) (n2−2 n) + CO •2−

(3) (4)

CO •2− + O 2 → CO 2 + O •2−

(5)

2 HO •2 /O •2− + H + / 2 H + → H 2O 2 + O 2

(6)

Fe(II) + H 2 O 2 → Fe(III) + • OH + OH −

(7)

O •2− + phenanthrene → degradation products

(8)

Phytotoxicity evaluation

Fig. 10. The comparison of lettuce growth in different soils, including non-contaminated soil (control), phenanthrene contaminated soil treated by MMCRC (treatment), phenanthrene contaminated soil (PHE), and phenanthrene contaminated soil treated by Fenton reagent (a) before and (b) after harvest.

In the field of remediation of PAHs-contaminated soil, most of literatures have focused on the development of new remediation methods and the removal of organic pollutant from soil. Another important work, the ecological risk assessment after soil remediation has been ignored. To preliminary assess the ecological risk of soil remediation method based on solid state photo-Fenton reagent, phytotoxcitiy evalution were conducted on the lettuce cultivation experiments. Fig. 10 exhibits the comparison of lettuce growth in different soil including non-contaminated soil, 24


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phenanthrene contaminated soil treated by MMCRC, phenanthrene contaminated soil, and phenanthrene contaminated soil treated by Fenton reagent, respectively. It is obvious that phenanthrene in soil exerted higher toxicity and inhibited the growth of lettuce. After treatment by MMCRC, the phytotoxicity of phenanthrene contaminated soil was reduced and there was not clear difference for lettuce growth between non-contaminated soil and soil treated by MMCRC. Especially, the germination of lettuce seeds was completely hindered in the soil treated by Fenton reagent. The toxicity derived from Fenton treatment is higher than that of phenanthrene in contaminated soil and previous report also demonstrate the similar results. 42

Fig.11. The growth indexes of lettuce cultivated in non-contaminated soil (control), phenanthrene contaminated soil treated by MMCRC (treatment), and phenanthrene contaminated soil (PHE). (a) seed germination percentage; (b) root length; (c) leaf length; (d) fresh weight of lettuce.

To investigate the effect of treatment based on MMCRC on lettuce growth, seed 25



germination percentage, root length, leaf length, and fresh weight of lettuce were considered as the growth indexes to evaluate the phytotoxcitiy. Fig. 11a shows the germination percentage of lettuce seed in non-contaminated soil (93.3 %), phenanthrene contaminated soil treated by MMCRC (90.0 %), and phenanthrene contaminated soil (86.7 %). Phenanthrene in contaminated soil had no marked negative effect on the seeds germination percentage and similar finding were also reported.

42, 43

The reason why phenanthrene has no toxic effect on the seed

germination process should probably be attributed to the low bioavailability. 43 Significant differences were observed in the roots length, leaf length, and fresh weight of lettuce. The roots length, leaf length, and fresh weight of lettuce cultivated in different soils are shown in Fig. 11b-Fig. 11d. The growth of lettuce in phenanthrene contaminated soil was obviously depressed (roots length: 1.35 cm, leaf length: 1.6 cm, fresh weight: 0.0319 g) compared to that (roots length: 3.03 cm, leaf length: 2.5 cm, fresh weight: 0.0411 g) of non-contaminated soil. After treated by MMCRC, toxicity of phenanthrene in contaminated soil was markedly reduced and lettuce showed good growth (roots length: 2.81 cm, leaf length: 2.4 cm, fresh weight: 0.0405 g). No significant differences were observed between the non-contaminated soil and phenanthrene contaminated soil treated by MMCRC. According to the results of phytotoxicity test, it can be demonstrated that the soil remediation method based on the MMCRC can efficiently remove phenanthrene from contaminated soil and markedly reduce the phytotoxicity.

The comparison of Fenton reagent and solid state photo-Fenton reagent in the remediation of phenanthrene contaminated soil.

26


Page 26 of 36

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Fig. 12. (a) conceptual model of PAHs-organo-clay complex; (b) the soil remediation process by using conventional Fenton (or photo-Fenton) reagents; (c) the soil remediation process by using solid state photo-Fenton reagent based on MMCRC

Soil is a mixture of minerals, organic matter, gases, and liquids, and the mineral components of soil are sand, silt and clay. Previous reports demonstrate that the majority of soil organic matter (SOM) tend to be absorbed by the clay and form the organo-clay complex due to the high surface areas of clay. Once PAH enter into soil, it will be selectively adsorbed on the surface of organo-clay complex and form the PAHs-organo-clay complex.

2

Fig. 12a is the conceptual model of PAHs-organo-clay

complex. Obviously, solid state photo-Fenton reagent based on MMCRC differs from the conventional homogeneous and heterogeneous Fenton (or photo-Fenton) reagents in the remediation of PAHs contaminated soil and these differences were depicted in Fig. 12b and Fig. 12c. As shown in Table 1 and Fig. 12b, conventional Fenton (or photo-Fenton) reagents are not all solid state because the H2O2 and LMWCAs are liquid phase. Due to the 27



Page 28 of 36

fluidity of liquid, H2O2 or LMWCAs is adsorbed on everywhere of soil during the remediation process. Only a small proportion of H2O2 or LMWCAs is absorbed on the PAH-organo-clay complex of soil and degrades PAHs. Large number of H2O2 or LMWCAs not only is wasted but also causes the quality of uncontaminated part of soil to deteriorate due to the oxidation of H2O2 and the heavy metal mobilized by LMWCAs.

11,18

Conventional Fenton reaction should be kept at low pH (pH 2~3)

which must consume massive use of acids. The higher acidic condition of conventional Fenton reaction induces the deterioration of soil quality. To extend application range from acidic condition to neutral condition, chelating agents and H2O2 stabilizers were used to stabilize iron ions and H2O2 at neutral condition. In addition, to exert optimal oxidation efficiency, H2O2 (or LMWCAs), iron ions (or iron oxides), chelating agents, and H2O2 stabilizers must uniform mixing as far as possible and the ratio of these chemical agents must be kept in an optimal range.

9,10

The

massive use of different chemical agents is imperative for conventional Fenton reactions, which make the soil remediation process complicated, expensive and hazardous. Unlike conventional Fenton reagent, solid state photo-Fenton reagent based on MMCRC can independently release active radicals O2•- without using any other chemical agents to degrade PAHs. The remediation process can be activated by visible light irradiation after mix of MMCRC and PAHs contaminated soil, as shown in Fig. 12c. The whole remediation process does not need adjust the pH of soil and other chemical agents. Especially, after treated by MMCRC, the pH of soil increased from 6.31 to 7.16 which was still kept at circumneutral pH (Table S1). Conductivity of soil increased from 65.41 µS/cm to 74.10 µS/cm and organic matter content of soil increased from 4.44 % to 5.13 %. So, the soil remediation method based on solid state photo-Fenton reagent did not change significantly the basic properties of soil. Conventional Fenton reagent affects every soil particles which attack not only the contaminated part of soil but also the healthy part of soil due to its fluidity. So, large number of fluid Fenton reagent is wasted on healthy part of soil and the quality of healthy part of soil is also destructed by Fenton reagent at the same time. Compared 28


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with conventional Fenton reagent, solid state photo-Fenton reagent based on MMCRC can minimize the damage to soil because it mainly affects the nearby soil particle which is demonstrated by the phytotoxicity test.

Conclusion In summary, a solid state photo-Fenton reagent based on MMCRC has been synthesized at a relative low carbonization temperature. Two reaction parameters, carbonization temperature and glucose dosage, play crucial roles during the formation process of MMCRC. The relative low carbonization temperature (250 ºC) can not only ensure the carbonization of carbon source including the glucose and sodium tartrate but also remain rich carboxylate groups in carbon as much as possible. Porous structure of MMCRC is fabricated by the large number of Fe3O4 nanoparticles connected by carbon. In addition, abundant carboxylate groups are solidified into carbon and coordinate with Fe3O4 nanoparticles to form the ferricarboxylate complexes. So, the glucose dosage must be control in an appropriate range. Experimental results demonstrate that ferricarboxylate complexes are the key component of MMCRC which is the origin of excellent visible light absorption abilities and photocatalytic activities. In the application of remediation of phenanthrene contaminated soil, solid state photo-Fenton reagent based on MMCRC has several advantages over conventional Fenton reagents and these advantages are summarized as follows: (1) Without adjusting the soil pH, the soil remediation process based on MMCRC can be operated at circumneutral pH. (2) Without using H2O2, MMCRC can be activated by visible light irradiation and release O2•- to degrade phenanthrene in soil. (3) Without using any other reagents, MMCRC is the only reagent used to remediate the phenanthrene contaminated soil, which enormously simplifies the packaging, storage, transportation, and operation process. (4) The soil remediation method based on MMCRC can significantly reduce the 29



phytotoxicity of phenanthrene contaminated soil which is propitious to the subsequent revegetation.

Acknowledgments We thank Dr. Xiazhang Li in Changzhou University for his help with TEM characterization. This work was financially supported by Zhenjiang’s Key Project of Research Plan (Social Development) (SH2017044), Highly Qualified Professional Initial Funding of Jiangsu University (10JDG120), and Jiangsu Collaborative Innovation Center of Technology and Material of Water Treatment.

ASSOCIATED CONTENT Supporting Information Basic properties of non-contamination soil and phenanthrene contaminated soil treated by MMCRC and photos of a glass ring with a piston.

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35



Graphical Abstract:

Without adjusting the soil pH and using H2O2, MMCRC can remediate the phenanthrene contaminated soil under visible light irradiation.

36


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Remediation of Phenanthrene Contaminated Soil by a Solid State

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