**4. Environmental Remediation**

#### *4.1. Adsorption of Contaminants*

Waste biomass has been widely experimented for the realization of activated carbons for air or water remediation [106]. Indeed, as already seen for the preparation of supercapacitors, biomass can be transformed into a highly carbonaceous porous media, with superior properties in term of specific area and surface composition. Contaminants are adsorbed within the carbon matrix both through physical trapping on the internal surface or chemical bonding on the charged surface of the porous material. Moreover, depending on the characteristics needed for the final material, the HTC treatment and activation step can be further tailored by adjusting the operative parameters.

Two studies involving the use of digestate material as precursor for the realization of activated carbon for the adsorption of contaminants were found in the literature [107,108]. The research conducted by Bernardo et al. [108] on the use of hydrochar from digestate of municipal solid waste, showed a promising perspective for its application on the removal of phosphate from waste water. Digestate was hydrothermally carbonized at 250 ◦C for 1 h, in acid (with addition of H2SO4) or native conditions, before being activated at 600 ◦C for 2 h with a KOH-to-hydrochar ratio of 3:1 in mass. Despite the reduction in specific surface area when biomass is subjected to acid treatment, the two sets of chars present the same adsorption capacity of 12 mg of orthophosphate for each gram of carbon. Results indicate that morphological properties do not play a significant role in orthophosphate adsorption. According to the authors, the presence of Al3<sup>+</sup>, Ca2<sup>+</sup>, Fe2+/3<sup>+</sup> and Mg2<sup>+</sup> ions in the acid hydrochar, can boost the formation of mineral complexes with phosphate ions, reducing its content. Similar behaviors were also reported in [109–111], when operating with bio-chars for the removal of phosphate from aqueous streams.

Hydrochars were also successfully used to remove CO2 from gaseous streams by using agro-industrial waste or Coca Cola® [98,107,112]. Bio chars from agro-industrial waste were produced at different temperatures (190–250 ◦C), carbonization times (3 and 6 h) and pH levels (5 and 7), in order to evaluate the optimal reaction conditions [107]. Activated carbons were then realized by mixing hydrocarbons (HCs) with KOH in a mass ratio of 1:4 and then heated up to 600 ◦C for 2 h. Activation considerably boosted surface area and micro-meso porosity of the material. When testing the adsorption capacity, authors found a much greater affinity toward CO2 than CH4, with an adsorption capacity of 8.8 molCO2 kg−1, for the carbon produced at 250 ◦C for 6 h at a pH of 5. Tests on the adsorption of CO2 were also performed with activated carbons realized from the HTC of garlic peel and Coca Cola® [98,112]. Hydrochar from garlic peels [112] were activated under different activation temperatures (600, 700 and 800 ◦C) and KOH-to-char ratios (0:1; 2:1 and 4:1) in order to investigate their effect on porosity development and adsorption capacity. Tests revealed that an increase in activation temperature and KOH amount led to a rise in specific surface area and pore volume. However, CO2 adsorption is mainly driven by micropores availability, resulting in lower adsorption capacity as long as temperature and potassium hydroxide increases. Indeed, too severe activation conditions can induce pore enlargement, as well as pore walls collapse, reducing the possible sites for CO2 immobilization. Superior performances were achieved when using Coca Cola® as precursor in the HTC treatment [98]. The work conducted by Boyjoo et al. showed outstanding capacity when Coca Cola® was hydrothermally carbonized at 200 ◦C for 4 h and further activated either with ZnCl2 or KOH. This latter condition induced the highest increment in the adsorption ability, leading to an adsorption capacity of 5.22 mmol g−<sup>1</sup> at 25 ◦C and 1 Atm, which is one of the highest ever recorded for biomass precursors.

Bernardo et al. produced activated carbon by hydrothermal carbonization of digested sludge and tested their activity toward phosphorous adsorption. They demonstrated that the high porosity together with a high concentration of cations as in Ca, Al, Fe etc., favored phosphates removal from wastewater [108].

Experiments for heavy metals removal were also conducted starting from agro-industrial wastes as reported in [113–115]. In [114], authors tested the removal capacity of Antimony (III) and Cadmium (II) on pyro-char and hydrochar realized from animal manure. The results of the adsorption tests showed higher yields when pyrolytic chars were used, as well as an increase in the removal capacity as long as pH was raised from 3 to 6. The total adsorption potentials for Antimony were 2.24–3.98 and 4.44–16.28 mg g<sup>−</sup>1, respectively, for hydrochar and pyrochar, while higher capacities were reported for the case of Cadmium, achieving a removal of 19.80–27.18 and 33.48–81.32 mg g−1. Aside from Cadmium removal, promising results on the adsorption of heavy metals contaminants such as lead (Pb) [116,117], copper (Cu) [118–120], Zinc (Zn) [55,113,115] as well as pharmaceutical and chemical waste were obtained in different researches [106,121–124]. For copper removal, different studies proved that surface charge, pH and adsorbent dosage were the main parameters affecting the adsorption capacity, while for lead superior efficiency (>99.5%) was achieved with Ni/Fe-doped hydrochar [125]. Additionally, organic compounds like methylene blue, methyl orange and Congo red were removed from contaminated solutions through the adsorption on hydrochar produced from biomass waste [126–132]. Up to 655.7 mg of methylene blue for each gram of hydrochar was absorbed when bamboo sawdust was carbonized into 1 M hydrochloric acid solution, before being further treated with NaOH for 1 h. Acid modification led to 100%–200% increase in the adsorption capacity when the solution was kept in the pH range of 10–12 [127].

The removal of Congo red and 2-naptol was studied by Li et al. in two following works, using bamboo sawdust as starting material [133,134]. Both the two pollutants can have toxic effects on both aquatic life and human organisms and, therefore, need to be removed from the water stream. Moreover, due to their recalcitrant behavior to biological and thermal treatment, the use of adsorbent is more and more studied. In both the studies published by the authors, they found that pore development and the presence of oxygen functional groups on the surface can positively influence the adsorption capacity of the activated carbon. HTC reaction was conducted according to three main procedures: with pure water, with acid medium (HNO3, H2SO4 and H3PO4) or through a two-stage operation with a first hydrothermal carbonization in a 5% weight NaOH solution followed by an acid one in HNO3, H2SO4 or H3PO4. The two-stage process was used to perform a first delignification in order to favor the penetration of the acid inside the material and enhance pore development. The results showed that, among all, the best adsorption performances were achieved in a two-stage process, with H2SO4 or H3PO4 as acid medium and were equal to 90.51 and 72.93 mg g−1, respectively, for Congo red and 2-naphtol.

Adsorption of crystal violet (C25H30IN3) and malachite green was also investigated in biomass-derived hydrochar in [135–137]. For both contaminants, pH represents a key factor for adsorption since it is responsible for the surface charge modification of the hydrochars and the ionic charge modification of the contaminants. It was found that for crystal violet, the optimum pH was 10, while for malachite green it was found to be 7. Food leftovers were also transformed in adsorbent material to remove rare earth ions [138] and toluene [139] from contaminated streams.

Magnetic modified hydrochar, produced from different biomasses through the addition of iron compounds in the reaction media, were also used to remove tetracycline, roxarsone and persistent free radicals from waste water [140–142]. The magnetic properties of the obtained material can reduce the whole purification process time, by shortening the time needed to remove the adsorbent from the solution and saving costs of operation. In Table 3, some of the presented works are reviewed, according to the starting biomass, HTC condition and contaminant removed.


**Table 3.** Biomass type and HTC condition for removal of contaminants from waste waters.

To sum up, the last five years of studies on the possible use of hydrochar material as precursor for the realization of the adsorption of different contaminants, showed interesting potential for the further development of substrates with enhanced capacities that could represent, in the near future, cheaper and more environmentally friendly options with respect to artificially synthetized materials.
