## 2 Results  
[PERSON] and co-authors [1] prepared and investigated the use of polyvinyl alcohol/bacterial cellulose/biochar-nanosilver (PVA/BC/C-Ag) antibacterial composite membrane materials by uniformly dispersing C-Ag particles in a PVA/BC mixed gel. Their findings indicated that the BC was consistently mixed into the PVA gel and that the C-Ag particles were evenly immobilized in the PVA/BC hybrid membrane. The PVA/BC/C-Ag composite membranes displayed exceptional antibacterial activity against _Escherichia coli_. In the treatment of contaminated water, the composite membranes demonstrated sustained antibacterial activity and good reusability. This study highlights the importance of investigating materials that show antibacterial activity, which is a very important topic in water treatment, especially with emphasis when water reuse is aimed [14]. In the group of adsorption studies, [PERSON] and co-authors [2] reported on the synthesis, characterization, and utilization of polyaniline (PANI) and its composites with Fe\({}_{3}\)O\({}_{4}\) for the removal of hexavalent chromium Cr(VI) and divalent nickel Ni(II) ions from water. The authors tested Freudlich, Langmuir, Tempkin, Dubbanin-Ruddishkawich (D-R), and Elovich adsorption isotherms for the description of the process and found that the Freundlich isotherm fits more closely. Various other parameters were investigated, such as the solution pH, initial concentration, contact time, ionic strength, and adsorbent dosage on adsorption behavior were investigated. Uptake of Cr(VI) on all the materials was high in an acidic environment while that of Ni(II) was optimum in a slightly basic environment. Competition by co-existing anions was observed. In particular, removal of HCrO\({}_{4}\)\({}^{-}\) was competed with by HSO\({}_{4}\)\({}^{-}\) in acidic environments while that of Ni(II) was competed with by Na\({}^{+}\) ions when the solution was slightly basic. The maximum amount of Cr(VI) adsorbed by PANI/Fe\({}_{3}\)O\({}_{4}\) composites was 174.09 mg\(\cdot\)g\({}^{-1}\), while that of Ni(II) was 134.88 mg\(\cdot\)g\({}^{-1}\). The kinetics of adsorption was pseudo-second order. The negative sign with the values of Gibb's free energy changes and enthalpy and entropy changes confirmed the spontaneous and exothermic nature of adsorption. A second study tested the removal of hexavalent chromium and reported the results in the present special issue. [PERSON] and co-authors examined the use of a pipe flocculation reactor, followed by sand filtration. Ferrous sulphate was dosed in an inline mode to the Cr(VI) contaminated groundwater and through an oxidation reduction reaction, Cr(VI) was reduced to Cr(III), which was then adsorbed in the formed iron oxides and removed through sand filtration [3], in agreement with previous studies, which demonstrated by using XPS analysis [15]. Experiments were conducted at pH 7.2, using Thessaloniki tap water, which has a typical composition of a groundwater. The water was spiked with Cr(VI) at various concentrations. The required ratio of [Fe]/[Cr] for an efficient Cr(VI) removal, at pH 7.2 was slightly above 10, much above the stoichiometry of the reaction between Fe(II) and Cr(VI). Previous relevant studies accomplished in batch mode [16] have demonstrated that the main reason for this difference is the competition of this reaction by oxygen. Oxygen reacts also with Fe(II) and the kinetics of Fe(II) oxidation by oxygen increase as pH increases above pH 7 [17]. [PERSON] et al. [4] in their study managed to improve the adsorption and separation efficiency of lead-containing wastewaters by graphene oxide (GO), which was used through a simple hydrothermal reaction. The prepared GO was characterized by SEM, TEM, FTIR, and XPS. The adsorption characteristics of the prepared GO were studied and found that the maximum adsorption capacity of Pb\({}^{2+}\) on GO at pH 5 and 25 \({}^{\circ}\)C was 450.9 mg/g.