2.2.1. Solubility Experiments

Solubility experiments were performed to evaluate the potential of phosphate release from the HTL solid phase if applied directly as a fertilizer or as a phosphate-containing source for subsequent extraction.

To identify the phosphate form that was presented in the HTL solid phase, the study applied semi-sequential fractionation extraction following the standards, measurements, and testing (SMT) harmonized protocol [39,40]. This procedure included 3 separate extraction proceedings to obtain 5 phosphorus fractions: Inorganic (IP) and organic (OP) phosphorus fraction; apatite inorganic (AP: Ca-bound) and nonapatite inorganic (NAIP: Associated to Al, Fe oxides, and hydroxides) phosphorus fraction; and, finally, total phosphorus (TP) as an overall characteristic. In addition, independent solubility behaviors of phosphate and other nutrients from the HTL solid phase in a range of acid-basic environments were studied. 10 mL of a leaching agen<sup>t</sup> was added to 1 g of the dry HTL solid phase. The resulting slurry was shaken overnight at room temperature. After phase separation by centrifugation or filtration with 13 μm filter paper, the equilibrium pH of leachate was measured with a pH electrode HACH HQ40d. Two experimental setups were conducted. The 1st examined various leaching agents (1M HCl, 1M H2SO4, 1M NaOH, 1M citric acid). In the 2nd, sulfuric acid was used as a leaching agent, and their concentrations were varied (0.01M, 0.1M, 0.5M, 1M, 2M). The extraction capacity for phosphorus and other elements was calculated from the amount of the element that was extracted in leachate divided by the sample mass:

$$\text{Extraction capacity} = \frac{\text{c}\_{\text{leachate}} \times \text{V}\_{\text{leaching agent}}}{\text{m}\_{\text{HTL}-\text{X}-\text{SP}}} \tag{1}$$

where cleachate is the concentration of the element in leachate [mg·L−1], and Vleaching agen<sup>t</sup> is the volume of the leaching agen<sup>t</sup> [L].

To estimate the percentage of crop-available phosphate in the HTL solid phase, the study applied the calcium-acetate-lactate (CAL) extraction method (pH 4) [41] that was established in Germany for soil testing. Extraction was performed according to VDLUFA standard methods [42].

The elemental concentration in all extracts and leachates was measured by ICP-OES (Agilent 715). Analyses by SMT fractionation, all leaching tests, and the CAL extractions were conducted twice.
