**3. Attenuated Total Reflection – Fourier Transform Infrared (ATR-FTIR) spectroscopy in environmental studies**

Attenuated Total Reflection – Fourier Transform Infrared (ATR-FTIR) Spectroscopy was introduced in 1960s (Harrick, 1967), and now is widely used in many areas.

The principle of this is FTIR technique is that light introduced into a suitable prism at an angle exceeding the critical angle for internal reflection develops an evanescent wave (a special type of electromagnetic radiation) at the reflecting surface. Interaction of this evanescent wave with the sample determines ATR spectrum recording. The main charactesistic of this techniques is the fact that particle samples are deposited on the surface of a horizontal ATR crystal for spectroscopic analysis (Figure 4). Zinc selenide (ZnSe) or Ge crystals are the most commonly used in ATR-FTIR spectroscopy.

**Figure 4.** The principle of ATR-FTIR where n1 and n2 are the refractive indices of the crystal and the sample, respectively.

The main advantages of ATR-FTIR spectroscopy are: can be applied to a large variety of materials such as: powders, liquids, gels, pastes, pellets, slurries, fibers, soft solid materials, surface layers, polymer films, samples after evaporation of a solvent being a versatile and non-destructive technique; is useful for surface characterization, opaque samples; faster sampling being a non-destructive technique; is considered an extremely robust and reliable techique for quantitative studies involving liquids; excellent sample-to-sample reproducibility.

60 Advanced Aspects of Spectroscopy

By joining FTIR spectroscopy with two dimensional correlation analysis (2DCORR) there will be obtained a device with improved performance in the study of complex environmental systems (Noda and Ozaki, 2005). The two dimensional correlation analysis (2DCORR) is a method to visualize the dynamic relationship between the variables in multivariate data set with application of the complex cross-correlation function. With the help of this analysis there will be identified the spectral features which change in phase (i.e. linearly correlated among them) and out of phase (partially or not at all correlated among them) (Mecozzi *et al*., 2009). This technique can be applied to study the evolution of environmental complex systems. Mecozzi and coworkers applied FTIR spectroscopy joined with two dimensional correlation analysis (2DCORR) to identify the aggregation pathways of extractable humic substance from marine sediments, and to compare the molecular modifications determined by the actions of different pollutants on the marine algae *Dunaliella tertiolecta* that is a biomarker of environmental quality (Mecozzi *et al*., 2009). From this study it can be concluded that FTIR spectroscopy joined with 2DCORR analysis can be

**3. Attenuated Total Reflection – Fourier Transform Infrared (ATR-FTIR)** 

Attenuated Total Reflection – Fourier Transform Infrared (ATR-FTIR) Spectroscopy was

The principle of this is FTIR technique is that light introduced into a suitable prism at an angle exceeding the critical angle for internal reflection develops an evanescent wave (a special type of electromagnetic radiation) at the reflecting surface. Interaction of this evanescent wave with the sample determines ATR spectrum recording. The main charactesistic of this techniques is the fact that particle samples are deposited on the surface of a horizontal ATR crystal for spectroscopic analysis (Figure 4). Zinc selenide (ZnSe) or Ge

**Figure 4.** The principle of ATR-FTIR where n1 and n2 are the refractive indices of the crystal and the

introduced in 1960s (Harrick, 1967), and now is widely used in many areas.

an important tool for evaluating toxic effects on the marine life.

crystals are the most commonly used in ATR-FTIR spectroscopy.

**spectroscopy in environmental studies** 

sample, respectively.

All these advantages make that ATR-FTIR spectroscopy to be used for: analysis of processes at surfaces (Freger and Ben-David, 2005), surface modification (Lehocký et al. 2003; Janorkar et al., 2004), surface degradation (Bokria et al., 2002), study of enzymatic degradation of a substrate film attached to a solid surface (Snabe et al. 2002), study of sunscreens on human skin (Rintoul et al., 1998), research of cereal, food and wood systems (D'Amico et al. 2012), detection of microbial metabolic products on carbonate mineral surfaces (Bullen et al., 2008), self-assembled thin films (Gershevitz et al. 2004), grafted polymer layers (Granville et al. 2004), adsorption processes (Sethuraman et Belfort 2005; Al-Hosney et Grassian 2005) of biological (Jiang et al. 2005; Mangoni et al. 2004) and synthetic (Freger et al. 2002) materials.

The followings are some examples of *in situ* ATR-FTIR spectroscopy's application in environmental studies.

In recent years adsorptive removal of heavy metals from aqueous effluents have received much attention because numerous materials such as: clays, zeolites, activated carbon can be used as adsorbents. The adsorption of inorganic ions on metal oxides and hydroxides was resolved using *in situ* ATR-FTIR spectroscopy. In a review Lefèvre describes and discusses *in situ* ATR-FTIR used in order to obtain information on the sorption mechanism of sulfate, carbonate, phosphate, perchlorate on hematite, goetite, alumina, silica, TiO2 (Lefèvre, 2004). This is due to the fact that FTIR technique allows to analyze the sorption/desorption phenomena *in situ* being helpful in determining of the speciation of sorbed inorganic anions or ternary inorganic complexes formed. In addition this technique offers the possibility to distinguish outer-sphere and inner-sphere complexes. In this regard Yoon and coworkers used *in situ* ATR-FTIR spectroscopy and quantum chemical methods to determine the types and structures of the adsorption complexes formed by oxalate at boehmite (γ-AlOOH)/water and corundum (α-Al2O3)/water interfaces (Yoon *et al*., 2004). They found that the adsorption mechanism of a aqueous HOx species involves loss of protons from this species during the ligand-exchange reaction. The results obtained are useful in establishing the transport model of toxic species in natural waters, and remediation of liquid wastes.

Contamination of soils and groundwater by radioactive wastes containing uranium and other actinides is a significant problem. The fate and transport of these kind of pollutants in aquifers, design of cost-effective remediation techniques for radioactive-contaminated soils, and developing of materials proper for encapsulation and disposal of nuclear waste require knowledge of mechanism of radioactive pollutants – sorbent interactions. For radioactive waste depositories one of the most important factors which has to be considered is the long-term safety of them. For this, natural or anthropogenic barriers for sorption of radionuclides around the depositories are placed. Sorption data at the laboratory scale are useful to predict the behaviour of real systems. For this purpose Lefèvre and coworkers used ATR-IR spectroscopy to study the sorption of uranyl ions onto titanium oxide (mixture of rutile and anatase) and hematite. They found that the uranyl sorption on titanium oxide in the pH range 4-7 occurs by formation of one surface complex with uranium atoms bounded by two different chemical environments (Lefèvre *et al*., 2008), and in case of sorption on hematite they concluded that the same surface species is responsible for the uranyl sorption in the pH range 5-8 (Lefèvre *et al*., 2006). Due to the fact that experiments were reversible the authors concluded that reaction of hematite deposit with uranyl ions is the same with the reaction of it in dispersed suspensions (Lefèvre *et al*., 2006). The sorption of U(V) on different forms of titanium dioxide was also studied using ATR-IR spectroscopy by Comarmond and coworkers. They showed the effect of different sources of sorbent and its surface properties on radionuclide sorption (Comarmond *et al*., 2011). On the same subject Müller and coworkers used the high sensitivity of the *in situ*  ATR-FTIR spectroscopy to establish the mechanism of sorption processes of U(VI) onto TiO2 even at concentrations down to the low micromolar range. The Mid-IR spectra of U(VI) aqueous solutions and of U(VI) sorption onto different TiO2 samples is presented in Figure 5.

**Figure 5.** Mid-IR spectra of U(VI) aqueous solutions and of U(VI) sorption onto different TiO2 samples (the values on the IR spectra are in cm-1) (S1-S7 are different titania samples with different content of anatase and rutil, different particle size, and different origins) (from Müller *et al*., 2012 used with permission (originally published in Geochimica et Cosmochimica Acta, http://dx.doi.org/10.1016/j.gca.2011.10.004))

By comparing the spectrum of the aqueous species spectra with the spectra of samples obtained after U(VI) sorption on TiO2 it can be seen that the frequencies of the ν3(UO2) modes presented at 961 cm-1 for the aqueous species are significantly shifted (with 53-44 cm-1) which suggests that uranyl surface complexes are formed at all titania samples.

This study is one complex due to the fact that authors performed researches to establish the influence of: stages of *in situ* sorption experiments (conditioning, sorption, and flushing), the contact time of U(VI) with the mineral, the initial U(VI) concentration, pH values, the origin and manufacturing procedure of TiO2 samples and the absence of atmospheric-derived carbonate on the species formed in sorption processes of U(VI) on TiO2. The results obtained by authors are relevant to the most environmental scenarios (Müller *et al*., 2012).

62 Advanced Aspects of Spectroscopy

the depositories are placed. Sorption data at the laboratory scale are useful to predict the behaviour of real systems. For this purpose Lefèvre and coworkers used ATR-IR spectroscopy to study the sorption of uranyl ions onto titanium oxide (mixture of rutile and anatase) and hematite. They found that the uranyl sorption on titanium oxide in the pH range 4-7 occurs by formation of one surface complex with uranium atoms bounded by two different chemical environments (Lefèvre *et al*., 2008), and in case of sorption on hematite they concluded that the same surface species is responsible for the uranyl sorption in the pH range 5-8 (Lefèvre *et al*., 2006). Due to the fact that experiments were reversible the authors concluded that reaction of hematite deposit with uranyl ions is the same with the reaction of it in dispersed suspensions (Lefèvre *et al*., 2006). The sorption of U(V) on different forms of titanium dioxide was also studied using ATR-IR spectroscopy by Comarmond and coworkers. They showed the effect of different sources of sorbent and its surface properties on radionuclide sorption (Comarmond *et al*., 2011). On the same subject Müller and coworkers used the high sensitivity of the *in situ*  ATR-FTIR spectroscopy to establish the mechanism of sorption processes of U(VI) onto TiO2 even at concentrations down to the low micromolar range. The Mid-IR spectra of U(VI) aqueous

solutions and of U(VI) sorption onto different TiO2 samples is presented in Figure 5.

**Figure 5.** Mid-IR spectra of U(VI) aqueous solutions and of U(VI) sorption onto different TiO2 samples (the values on the IR spectra are in cm-1) (S1-S7 are different titania samples with different content of anatase and rutil, different particle size, and different origins) (from Müller *et al*., 2012 used with

By comparing the spectrum of the aqueous species spectra with the spectra of samples obtained after U(VI) sorption on TiO2 it can be seen that the frequencies of the ν3(UO2) modes presented at 961 cm-1 for the aqueous species are significantly shifted (with 53-44 cm-1) which suggests that uranyl surface complexes are formed at all titania samples.

permission (originally published in Geochimica et Cosmochimica Acta,

http://dx.doi.org/10.1016/j.gca.2011.10.004))

Sorption of Np(V) onto TiO2, SiO2 and ZnO was investigated using ATR-FTIR spectroscopy. The results showed obtaining structurally similar bidentate surface complexes for all sorbents used (Müller *et al*., 2009).

ATR-FTIR spectra confirmed formation of actinyl-carbonato complexes from interaction of actinide with hematite at a specific pH value. This can control the actinide transport in numerous subsurface receptors due to the abundance of carbonate in aquifers (Bargar *et al*., 1999).

The influence of dissolved CO2 on UO22+ sorption process was determined by Foerstendorf and Heim using ATR-FT-IR spectroscopy. They obtained a similar surface complex of the uranyl ion at the ferrihydrite-phase irrespective of the presence of atmospheric CO2. Sorption of actinide ion on mineral phase determines a change of the carbonate ion from a monodentate to a bidentate ligand (Foerstendorf and Heim, 2008).

ATR-FTIR and FT-IR spectroscopy together with other techniques were used to determine the fate and transport of radionuclides in natural environments. The main mechanisms that are responsible for these are: sorption on organic (living matter and humic materials), sorption on inorganic materials (soil media and minerals), precipitation of them under oxic conditions, reduction in presence of microorganisms, and structural incorporation in different mineral host phases (Duff et *al*., 2002).

Citric acid being a naturally-occuring acid commonly found in soils, and also a strong complexant of UO2 is often found as a component of radioactive waste. Advantages such as: its biodegradability and complexing efficiency make from it a good candidate for remediation of uranium contaminated soils (Kantar and Honeyman, 2006). Factors with influence on the uranyl adsorption process to oxide minerals in presence of citric acid were determined by Logue and coworkers. Redden and coworkers have proposed formation of a ternary uranyl-citrate complexes on goethite (Redden *et al*., 2001). Establishing the interactions between UO2, citrate and mineral surfaces on a molecular level represents a key factor for modeling adsorption phenomena affecting transport in soils. For this purpose Pasilis and Pemberton used ATR-FTIR to elucidate the mechanism of UO2 adsorption on aluminium oxide in the presence of citrate. They found that there is an enhanced citrate adsorption to Al2O3 in the presence of uranyl. This result suggests that uranyl may be the central link between two citrate ligands, and the uranyl is associated with the surface through a bridging citrate ligand. One other observation is that uranyl citrate complexes interact with citrate adsorbed to Al2O3 through outer shere interactions (Pasilis and Pemberton, 2008).

In recent years it ATR-FTIR spectroscopy has been used to investigate the atmospheric heteregenous reactions. Thus Al-Hosney and Grassian (2005) used this technique to investigate water adsorption on the surface of CaCO3. They further used T-FTIR in order to investigate the role of surface adsorbed water in adsorption reactions of SO2 and HNO3 (Zhao and Chen, 2010). In other study Schuttlefield and coworkers (2007a) used ATR-FTIR spectroscopy to provide detailed information about water uptake and phase transitions for atmospherically relevant particles. To determine the factors involved in water uptake on the large fraction of dust present in the Earth's atmosphere, Schuttlefield and coworkers (2007b) used a variety of techniques, including ATR-FTIR. They concluded that water uptake on the clay minerals depends on the type and the source of the clay. These results are important because mineral dust aerosol provides a reactive surface in trophosphere being involved in reactions for atmosphere. The role of halogens in the aging process of organic aerosols was determined by Ofner and coworkers (2012) using long-path FTIR spectroscopy (LP-FTIR), attenuated-total reflection FTIR (ATR-FTIR), UV/VIS spectroscopy, and ultrahigh resolution mass spectroscopy (ICR-FT/MS). They concluded that the aerosol-halogen interaction might strongly contribute to the influence of organic aerosols on the climate system (Ofner *et al*., 2012).

Khalizov and coworkers (2010) investigated the heterogeneous reaction of nitrogen dioxide (NO2) on fresh and coated soot surfaces to assess its role in night-time formation of nitrous acid (HONO) in the atmosphere using ATR-FTIR (Khalizov *et al*., 2010).

Segal-Rosenheimer and Dubowski (2007) combined two setups of FTIR for the parallel analysis of both condensed and gas phases of products resulted at the oxidation of cypermethrin (a synthetic pyrethroid being one of the most important insecticides in widescale use both indoors and outdoors) by gaseous ozone (Segal-Rosenheimer and Dubowski, 2007).

ATR-FTIR and T-FTIR methods provide detailed information on the composition of PM (particulate matter) samples. Both techniques can be used for qualitative and quantitative studies of particulate samples. Thus Veres (2005) used both methods to analyse particulate matter collected on Teflon Filters in Columbus – Ohio. He mentioned that ATR spectroscopy has limited applications in quantitative studies since it has a penetration depth of only a few microns, and this method can be replaced by transmission spectroscopy which penetrates into the bulk of substance (Veres, 2005).

Several groups of researchers used ATR-FTIR to particulate matter analysis. Thus Shaka and Saliba (2004) used ATR-FTIR spectroscopy in order to determine the concentration and the chemical composition of particulate matter at a coastal site in Beirut, Lebanon. Kouyoumdjian and Saliba (2006) determined the levels of the coarse (PM10-2.5) and fine (PM2.5) particles in the city of Beirut using ATR-FTIR spectroscopy. They also showed that nitrate, sulfate, carbonate and chloride were the main anionic constituents of the coarse particles, whereas sulfate was mostly predominant in the fine particles in the form of (NH4)2SO4. Ghauch and coworkers (2006) used the same technique for the determination of small amounts of pollutants like the organic fraction of aerosols in the French cities of Grenoble and Clermont-Ferrand.

The applications of ATR-FTIR cover a wide range of subjects such as estimating of soil composition and fate of some soil components.

64 Advanced Aspects of Spectroscopy

2012).

2007).

into the bulk of substance (Veres, 2005).

Grenoble and Clermont-Ferrand.

In recent years it ATR-FTIR spectroscopy has been used to investigate the atmospheric heteregenous reactions. Thus Al-Hosney and Grassian (2005) used this technique to investigate water adsorption on the surface of CaCO3. They further used T-FTIR in order to investigate the role of surface adsorbed water in adsorption reactions of SO2 and HNO3 (Zhao and Chen, 2010). In other study Schuttlefield and coworkers (2007a) used ATR-FTIR spectroscopy to provide detailed information about water uptake and phase transitions for atmospherically relevant particles. To determine the factors involved in water uptake on the large fraction of dust present in the Earth's atmosphere, Schuttlefield and coworkers (2007b) used a variety of techniques, including ATR-FTIR. They concluded that water uptake on the clay minerals depends on the type and the source of the clay. These results are important because mineral dust aerosol provides a reactive surface in trophosphere being involved in reactions for atmosphere. The role of halogens in the aging process of organic aerosols was determined by Ofner and coworkers (2012) using long-path FTIR spectroscopy (LP-FTIR), attenuated-total reflection FTIR (ATR-FTIR), UV/VIS spectroscopy, and ultrahigh resolution mass spectroscopy (ICR-FT/MS). They concluded that the aerosol-halogen interaction might strongly contribute to the influence of organic aerosols on the climate system (Ofner *et al*.,

Khalizov and coworkers (2010) investigated the heterogeneous reaction of nitrogen dioxide (NO2) on fresh and coated soot surfaces to assess its role in night-time formation of nitrous

Segal-Rosenheimer and Dubowski (2007) combined two setups of FTIR for the parallel analysis of both condensed and gas phases of products resulted at the oxidation of cypermethrin (a synthetic pyrethroid being one of the most important insecticides in widescale use both indoors and outdoors) by gaseous ozone (Segal-Rosenheimer and Dubowski,

ATR-FTIR and T-FTIR methods provide detailed information on the composition of PM (particulate matter) samples. Both techniques can be used for qualitative and quantitative studies of particulate samples. Thus Veres (2005) used both methods to analyse particulate matter collected on Teflon Filters in Columbus – Ohio. He mentioned that ATR spectroscopy has limited applications in quantitative studies since it has a penetration depth of only a few microns, and this method can be replaced by transmission spectroscopy which penetrates

Several groups of researchers used ATR-FTIR to particulate matter analysis. Thus Shaka and Saliba (2004) used ATR-FTIR spectroscopy in order to determine the concentration and the chemical composition of particulate matter at a coastal site in Beirut, Lebanon. Kouyoumdjian and Saliba (2006) determined the levels of the coarse (PM10-2.5) and fine (PM2.5) particles in the city of Beirut using ATR-FTIR spectroscopy. They also showed that nitrate, sulfate, carbonate and chloride were the main anionic constituents of the coarse particles, whereas sulfate was mostly predominant in the fine particles in the form of (NH4)2SO4. Ghauch and coworkers (2006) used the same technique for the determination of small amounts of pollutants like the organic fraction of aerosols in the French cities of

acid (HONO) in the atmosphere using ATR-FTIR (Khalizov *et al*., 2010).

Monitoring of nitrate in soil is very important for managing fertilizer application and controlling nitrate leaching. This monitoring help to adjust nitrate level in soils in order to mantain the soil fertility, or to detect soil pollution. Due to the technological limitations, *in situ* or near real-time monitoring of soil nitrate is currently not feasible. In this purpose can be used the following methods: nitrate selective electrodes (Sibley, 2010), ion sensivitive field effect transistor (Birrell and Hummel, 2001), mid-infrared spectroscopy, and more particularly attenuated total reflectance (ATR) with Fourier transform infrared (FTIR) spectroscopy. Thus Raphael Linker submitted a report to the Grand Water Research Institute about simultaneous determination of 15NO3-N and 14NO3-N in aqueous solutions, soil extracts and soil pastes. The results obtained show that a combination of ATR-FTIR analysis with appropriate chemometrics can be successfully used to monitor 15NO3-N and 14NO3-N concentrations in soil during an incubation experiment (Linker, 2010). From the studies performed about measurement of nitrate concentration in soil pastes it can be concluded that ATR-FTIR appears to be a promising tool for direct and close to real-time determination of nitrate concentration in soils, with minimal treatment of the soil samples (Linker *et al*., 2004; Linker *et al*. 2005; Linker *et al*., 2006; Linker *et al*., 2010). The same technique was used by Du and coworkers in order to evaluate net nitrification rate in *Terra Rosa* soil (Du *et al*., 2009). ATR-FTIR spectroscopy was the technique preferred to mass spectrometry due to reduced cost, it is not time consuming, and doesn't require long and laborious preparation procedures. The results obtained have made major contributions for the estimation of the contribution of applied nitrogen and mineralized nitrogen to net nitrification rates ((Du *et al*., 2009).

Soil paste was used by Choe and coworkers in order to improve the contact between sample and ATR crystal in case of using of the ATR-FTIR spectroscopy to determine the level of nitrate in soils. By comparing the nitrate peak intensity of soil pastes and their supernatant, it was shown that the nitrate dissolved in soil solution of the paste mainly responded to the FTIR signal. The results obtained are useful for the monitoring of nutrients in soils (Choe *et al*. 2010).
