acid-wash

The contents of ash and REEs in acid-washed oil shale samples are given in Table 6.

From Table 6 it can be seen that the content of REEs is significantly lower in the acid-washed samples than in the original ones.

In Figure 3, the results presented in Table 7, the chondrite-standardized distribution of REEs in the original oil shale samples, the density of each component, and REEs contents in the acid-washed samples are illustratively summarized.

In order to elucidate the role of organic matter in aggregating REEs in oil shale, the ratios of HAW/HOS and LAW/LOS in the acid-washed oil shale samples were calculated.

The content of ash and REEs in acid-washed oil shale samples, [micro]g/g Sample Ash, % REEs, [micro]g/g La Ce Pr Nd Sm Eu Gd Tb HOS 50.28 23.92 40.77 5.55 19.57 3.40 0.76 3.27 0.50 HAW 4.13 8.18 13.77 1.84 6.63 1.15 0.26 1.39 0.23 LOS 72.24 30.86 64.22 6.88 26.61 4.97 1.18 4.61 0.78 LAW 5.43 22.40 35.82 4.60 15.10 2.59 0.52 2.47 0.40 Sample REEs, [micro]g/g Dy Ho Er Tm Yb Lu HOS 2.58 0.51 1.53 0.25 1.44 0.22 HAW 1.43 0.31 0.97 0.16 0.91 0.19 LOS 3.75 0.76 2.15 0.31 1.96 0.30 LAW 2.29 0.51 1.69 0.29 1.90 0.40 Note: HOS--original Huadian oil shale sample; HAW--acid-washed Huadian oil shale sample; LOS--original Luozigou oil shale sample; LAW--acid-washed Luozigou oil shale sample.

These biochars are referred to as acid-washed biochar to distinguish them from the untreated biochars.

The content of acid groups on the acid-washed biochar samples was determined by suspending 0.1 g acid-washed biochar sample in 50 mL of 0.1 M KN[O.sub.3] (n = 3) as the inert electrolyte.

In order to describe the surface chemistry of biochar samples by providing qualitative information about functional groups and at the same time evaluate any change caused by the pretreatment, both untreated and acid-washed biochar were characterised using spectroscopic analysis.

Untreated and acid-washed biochar subsamples (0.5 mg; ground <0.5 mm) were placed onto the sample chamber of an infrared spectrometer (FT/IR-4200; JASCO, Easton, MD, USA).

Concentrations of carboxylic groups, as estimated by potentiometric titration of acid-washed biochar samples, ranged between 20 and 70 mmol [kg.sup.-1] (Table 1); values were always higher for low-temperature biochar samples than the corresponding high-temperature samples, regardless of feedstock, in agreement with the results from the Boehm titration.