Cinnamic acid (CA), p-coumaric acid (4-hydroxycinnamic acid, 4-HCA), caffeic acid (3,4-vdihydroxycinnamic acid, 3,4-dHCA), and 3,4,5-trihydroxycinnamic acid (3,4,5-tHCA) were studied for their structural, anti-/pro-oxidant properties and biodegradability. The FT-IR, FT-Raman, UV/Vis, ¹H and ¹³C NMR, and quantum chemical calculations in B3LYP/6-311++G** were performed to study the effect on number and position of hydroxyl group in the ring on the molecular structure of molecules. The antioxidant properties of the derivatives were examined using DPPH^● and HO^● radicals scavenging assays, ferric ion reducing antioxidant power (FRAP), cupric reducing antioxidant capacity (CUPRAC), inhibition of linoleic acid oxidation, as well as the biological antioxidant assay with Saccharomyces cerevisiae. Moreover, the pro-oxidant activity of compounds in Trolox oxidation assay was estimated. The effect of the derivatives on environment on the basis of increasing the carbon and nitrogen compounds transformation processes occurring in biological wastewater treatment was studied.

Complexes of chlorogenic acid (5-CQA) with copper(II) and iron(III) were synthesized in a solid state and examined by means of FT-IR, thermogravimetric, and elemental analyses. The molar stoichiometric ratios of metal:ligand for the solid forms of the complexes were established as Cu(II):L = 1:2 and Fe(III):L = 2:3 (L: 5-CQA), with the possible coordination through the carboxylate group and the hydroxyl group from the catechol moiety. In an aqueous solution at pH = 7.4, the composition of the complexes was Cu(II):L = 1:1, and Fe(III):L = 1:1 and 1:2. The Cu(II) and Fe(III) complexes with 5-CQA showed lower antioxidant properties, as estimated by the spectrophotometric methods with DPPH^•, ABTS^•+, and HO^• radicals, than the ligand alone, whereas in the lipid peroxidation inhibition assay, the metal complexes revealed a higher antioxidant activity than 5-CQA. Cu(II) 5-CQA showed the highest pro-oxidant activity in the Trolox oxidation assays compared to the other studied compounds. The lipophilic parameters of the compounds were estimated using the HPLC method. 5-CQA and its complexes with Fe(III) and Cu(II) were not toxic to HaCaT cells in a tested concentration range of 0.15–1000 nM after a 24 h incubation time.

The copper (II) complex of ursolic acid (Cu(II) UA) was synthesized and discussed in terms of its infrared, UV–visible spectra, quantum-chemical calculations at B3LYP/6-31G(d) level and antioxidant capacity. The copper (II) complex was stable in methanolic solution with the molar ratio metal:ligand 1:1. The data obtained by FT-IR confirmed the metal ion coordination through the carboxylate anion. The antioxidant properties of ursolic acid and its complex with Cu were discussed on the basis of energy of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) and values of chemical reactivity parameters. The antiradical properties of ursolic acid and the Cu (II) complex were examined against DPPH^• and HO^• radicals, and the ferric reducing antioxidant power (FRAP) was examined. The Cu(II) complex showed higher antioxidant activity than ursolic acid, i.e., in DPPH^• assay, the EC₅₀ for UA was 47.0 mM, whereas, for Cu(II), UA EC₅₀ = 19.5 mM; the FRAP value for UA was 20.8 µM_Fe2+, and 35.4 µM_Fe2+ for Cu(II) UA (compound concentration 3 mM). Although there was no distinct difference in the antioxidant activity against HO^• between these two chemicals, they were both better HO^• scavengers than DPPH^• and showed different kinetics in the reaction with DPPH^•.

In this study a cobalt(II) complex of quercetin was synthetized in the solid state with the general formula Co(C₁₅H₉O₇)₂∙2H₂O. The FT-IR, elemental analysis, and UV/Vis methods were used to study the composition of the complex in a solid state and in a water solution. The anti-/pro-oxidant activity of quercetin and the Co(II) complex was studied by means of spectrophotometric DPPH (2,2-diphenyl-1-picrylhydrazyl), FRAP (ferric reducing antioxidant activity) and Trolox oxidation assays. The cytotoxicity of quercetin and Co(II)-quercetin complex in HaCat cell lines was then established.

The release of toxic substances in the environment continues to be a problem despite increased efforts to reduce this. The commonly used methods of removing heavy metal ions from water and wastewater have many disadvantages, including a low efficiency and high cost. Heavy metals and dyes are the most problematic pollutants due to their toxicity and stability in the environment. For this reason, in recent years, remediation technologies such as the sorption on materials of natural origin, have been developed. However, these technologies are still rarely used at an industrial level. Recently, scientists have attempted to apply the promising properties of nanotechnology to this field, conducting research on the possibility of using biosorbents in the nanoscale in wastewater treatment. Much attention is currently paid to the preparation of cost-effective, efficient, and environmentally friendly adsorbents, as well as their chemical modifications, to increase the metal removal efficiency from water and wastewater. The waste materials from the agricultural industry are cheap adsorbents that require little treatment. This kind of biosorbent is not able to remove specific metal ions, but by chemical modifications its adsorption capacity and specificity can be increased. The data from the literature are reported for various types of bio-adsorbent materials, e.g., fruit or vegetable pomace and nut shells. One of the most promising raw materials is apple pomace. It was estimated that, in recent years, global apple production reached approximately 75 million tons, equating to 5–7 million tons of apple pomace per year. The management of such waste is a serious challenge. The rational management of exhausting resources requires looking at waste in terms of its use as a reusable raw material. The aim of this paper was to collect information and compare the parameters (pH, dose of adsorbent, and kinetics, etc.) of heavy metal sorption on apple pomace in order to demonstrate the potential of this adsorbent application.