In a similar approach the efficiency of TPH precipitation can be estimated. As an average value for TPH concentration in the extract a concentration of 4500 mg L−1 TPH was found (Fig. 2). In 200 ml extract thus a total amount of 900 mg phenolic compounds as gallic acid monohydrate equivalents is present (Fig. 3).
In the precipitation experiments as an average value 0.7 g residue with a TPH content of 45% was obtained. This corresponds to an absolute mass of 315 mg TPH. From the extracted amount of 900 mg TPH a share of 35% could be precipitated as dyestuff containing residue. From 1 L of aqueous extract (TPH 4500 mg L−1) an amount of 1575 mg TPH will precipitate after addition of FeSO4·7H2O solution.
In the literature the concentration of phenolic compounds extracted from Solidago canadiensis in aqueous and alcoholic medium has been analysed by HPLC methods [12]. From the analysis of the phenolic compounds in the aqueous extracts a concentration of 559 mg L−1 rutin, 78 mg L−1 isoquercitrin and 110 mg L−1 quercitrin were reported and a total concentration of phenolics of 747 mg L−1 was found. In this study extraction of the dry plant material was performed 1:10, compared to 1:40 in [23]. Thus an approximately 4 times higher concentration of coloured phenolic compounds in the order of 2900 mg L−1 could be expected in the extract based on reference [23]. In this study approximately a TPH concentration of 4000 mg L−1 TPH was determined, which can be explained with the longer extraction time of 60 min, used in this study.