## 1 Introduction
For instance, [PERSON] et al. [18] tested the sorption capacities of KOH-modified hydrochar from switchgrass for removing copper and cadmium from aqueous solutions: they found a removal of about 100% within 24 h of contact time while the raw hydrochar only removed 16% of copper and 5.6% of cadmium. Similarly, [PERSON] et al. [14] showed that alkali modification improved the sorption ability of hydrochars produced by HTC of sawdust, wheat straw and corn stalk. Moreover, [PERSON] et al. [19] tested a low-cost adsorbent derived from HTC of waste activated sludge after KOH treatment at room temperature for the removal of orthophosphate from the effluent of a municipal WWTP. The study demonstrated that modified hydrochar achieved more than 97% orthophosphate removal. Thus, literature studies demonstrate that the alkali modification process may positively affect hydrochar surface, thus improving its sorption capacity [14]. To date, no study has examined the adsorption capacity of municipal sewage sludge-derived hydrochar [13] compared to alkali modified hydrochar and, in addition, considering different HTC treatment temperatures. Thus, the aim of the present study is to investigate the adsorption capacity of sewage sludge hydrochar (raw or KOH-modified) obtained at different HTC operating conditions for dye removal from aqueous solutions using methylene blue (MB) as the adsorbate. Importantly, these materials present some elements of complexity in comparison with other classes of organic-derived adsorbents: (i) the lower treatment temperatures give rise to products displaying lower specific surface area values than activated carbon-based adsorbents, usually ranging from 800 to above 1100 m\({}^{2}\)-g\({}^{-1}\); (ii) the presence in sewage sludges of alkaline and heavy metals cannot be neglected. Thus, the feasibility of this application must take into account these two aspects: (1) the adsorption mechanism has to involve not only the surface morphology of the samples, but also some chemical reactions have to occur; (2) the presence of metal ions can provide acid-base or oxy-reductive processes with respect to the adsorbate: actually, MB is a positive-charged dye and it also displays reducing behavior in solutions (it can be used as an indicator in redox titrations). With all these features in mind, the aim of this work is reached by performing the following main steps:  
* preparation of three hydrochars from municipal sewage sludge by performing HTC at 190, 220 and 250 \({}^{\circ}\)C with a reaction time of 3 h;
* modification with KOH in order to increase the adsorption capacity of hydrochars;
* comparison of the physicochemical properties of raw and modified hydrocarbons using elemental analysis, thermogravimetry (TGA), nitrogen physisorption analysis, Fourier-transform infrared spectroscopy (FTIR), inductively coupled plasma spectroscopy (ICP) and flow injection mercury system (FIMS);
* comparison of the potential application of raw and modified hydrochars as adsorbents for MB removal by performing batch adsorption studies, namely adsorption isotherms and adsorption kinetics tests. Even if the HCT of sewage sludge has been previously addressed by several authors, as far as we know, there are no studies performing such a detailed evaluation considering sewage sludge as feedstock for the HTC process and the derived hydrochar, possibly KOH-modified, as adsorbent material for MB removal. Therefore, the novelty of this study is on the investigation of the adsorption capacity of the derived hydrochars, the main adsorption mechanisms involved and the assessment of the best production conditions that enhance the adsorption capacity of sewage sludge brochars.