Specific adsorption sites and conditions derived by thermal decomposition of activated carbons and adsorbed carbamazepine.

The adsorption of organic micropollutants onto activated carbon is a favourable solution for the treatment of drinking water and wastewater. However, these adsorption processes are not sufficiently understood to allow for the appropriate prediction of removal processes. In this study, thermogravimetric analysis, alongside evolved gas analysis, is proposed for the characterisation of micropollutants adsorbed on activated carbon. Varying amounts of carbamazepine were adsorbed onto three different activated carbons, which were subsequently dried, and their thermal decomposition mechanisms examined. The discovery of 55 different pyrolysis products allowed differentiations to be made between specific adsorption sites and conditions. However, the same adsorption mechanisms were found for all samples, which were enhanced by inorganic constituents and oxygen containing surface groups. Furthermore, increasing the loadings led to the evolution of more hydrated decomposition products, whilst parts of the carbamazepine molecules were also integrated into the carbon structure. It was also found that the chemical composition, especially the degree of dehydration of the activated carbon, plays an important role in the adsorption of carbamazepine. Hence, it is thought that the adsorption sites may have a higher adsorption energy for specific adsorbates, when the activated carbon can then potentially increase its degree of graphitisation.


TGA decomposition stages
The TGA graphs shown in Figures 1&2a are characterised by two decomposition stages after carbamazepine adsorption. Table S1 provides the temperature ranges and the temperatures of maximum mass loss rate (DTG max ) for each mass loss stage. The temperature ranges were used to determine the mass losses in Table 2 and to calculate the kinetic parameters shown in Table 3. DTG max temperatures show the shifts of the decomposition stages depending on the activated carbon as graphically assessed in Figure 2a.  Table S1. TGA characteristics for the 1 st & 2 nd mass loss stage of the systems with the highest carbamazepine loadings.

Influence of sample mass on DTG profiles
The activated carbon HK 950 has a comparable low density and therefore TGA sample mass was 15 mg in the crucibles. This could alter thermal decomposition of samples with 20 mg and parameters like DTG max temperatures in Table S1. Figure S1 shows the SAE+CBZ-15 system with different sample masses. There is no shift in DTG max temperature of the sample analysed with 15 mg. Therefore, results in Table S1 are not sensitive to the analysed sample masses with slight mass changes. Figure S1. TGA results for the SAE+CBZ-15 system with varying sample masses. No shift in DTG max temperatures can be observed.

Comparison of kinetic parameters
The kinetic parameters determined by TGA ramp-kinetics are provided in Table S2. The results of Pinto et al. (2014) 2 were conducted by heating rates of 2.5, 5, 10 and 15 • C min -1 . We assume that the magnitude of the pre-exponential factor (k) differs, due to experimental conditions including carbamazepine investigated as adsorbate or pure powder.

Hydrolysis of isocyanic acid
Isocyanic acid (HNCO) can be hydrolysed to carbon dioxide and ammonia ( Figure S2). This reaction was already described at 80 • C and only with traces of water vapour. 3 Figure S2. Hydrolysis of isocyanic acid to carbon dioxide and ammonia.
Since we worked with nitrogen as purge gas with 5.0 purity we did not expect noticeable amounts of water. However, we observed the consumption of water in the release rates of TGA-FTIR during HNCO release in SAE+CBZ-15 (yellow line in Figure S3). Simultaneously, the evolution of carbon dioxide and ammonia increased while isocyanic acid decreased.
This was solely found in the TGA-FTIR measurements of the systems with SAE Super, indicating specific catalytic effects by this activated carbon probably by its inorganic inclusions. The amount of water consumed corresponds with the hydrolysis reaction for isocyanic acid. It equals about 0.01% water in the nitrogen purge gas.

Release of hydrogen cyanide
A specific characteristic of the systems with HK 950 is the release of significant amounts of hydrogen cyanide (HCN) after carbamazepine adsorption. This was observed by TGA-FTIR and can help to interpret the release of decomposition products observed by TED-GC/MS. Figure S4 shows the TGA-FTIR measurements with HCN release rates of the systems with high carbamazepine loadings. Unloaded activated carbons do not release any HCN at the investigated temperatures.

SEM comparisons
Scanning electron microscopy (SEM) images were recorded of the HK 950 and the CCP 90D systems exemplary for loaded and unloaded samples ( Figures S59&S60). The samples were treated identically, but with or without carbamazepine in solution (see Materials and Methods). The images do not show differences in the activated carbon particle's morphology. This confirms that adsorption takes place in the pores, on the inner surface of the particles, even for large amounts of adsorbate.

DRIFTS comparisons
Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) have been applied to all three activated carbons unloaded and loaded with carbamazepine. No differences in the spectra could be observed for the Systems with SAE Super and CCP 90D ( Figure S61). The negative bands are spectroscopic effects due to the strong absorption of the activated carbons (about 4 % in KBr). Figure S62a shows the spectra for the HK 950 systems. These spectra are typical for chemical activated, lignocellulose based materials. 4 Furthermore, additional bands occure in the systems loaded with carbamazepine. Figure S62b highlights the wavelength region between 1800 and 650 cm -1 with the subtraction sepctrum of loaded and unloaded HK 950 as well as the spectrum of pure carbamazepine. Distinct bands can be found and attributed to intact carbamazepine molecules on the surface of the activated carbon. However, it is related to the spectral properties of HK 950 that carbamazepine is detectable by DRIFTS.