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Degradation of chlorophyll was studied in leaves of spinach grown in soil containing cadmium and tetracycline, based on spectroscopic measurements and biochemical analyses of plant extracts. It was shown that plant exposure to the highest levels of tetracycline and cadmium resulted in 64% and 68%, respectively, reduction in chlorophyll content. The chlorophyll degradation rate constants were determined, and they were found to increase with increasing doses of tetracycline and cadmium. The rate constant of chlorophyll degradation by tetracycline ranged from k = 960 M −1 day −1 to k = 2180 M −1 day −1 , and the rate constant of chlorophyll degradation by cadmium ranged from k = 1130 M −1 day −1 to k = 2580 M −1 day −1 , depending on dose. Plant stress responses to tetracycline are much stronger than to cadmium, as is visible from the activity of guaiacol peroxidase and catalase. However, phytotoxicity of cadmium, measured by the rate of chlorophyll degradation and enzyme activities, is much higher compared to tetracycline. The spectroscopic measurements were taken 10 days after tetracycline and cadmium were added to the reagent grade chlorophyll which was used at the concentration of chlorophyll in plants. Changes in absorption and fluorescence spectra are likely to result from removal of magnesium from the chlorophyll molecule, and thus they indicate the formation of pheophytin. Cadmium, on the other hand, is probably bound into the chlorophyll molecule, substituting its magnesium.

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This paper analysed chlorophyll degradation in spinach leaves from plants affected by cadmium and tetracycline soil contamination, stored for 32 weeks at t= -18 °C. The first-order reaction kinetics were determined for chlorophyll degradation. In plants growing on soil containing tetracycline and cadmium, two ranges of chlorophyll degradation occur: 1 -a slow degradation, during storage for up to 20 weeks, occurring at constant degradation rate, k = 0.00594 ± 0.00029 week -1 for tetracycline and k = 0.00761 ± 0.00026 week -1 for cadmium, and 2 -a fast degradation, occurring later, with constant k = 0.04484 ± 0.00137 week -1 and k = 0.06777 ± 0.00171 week -1 for tetracycline and cadmium, respectively. Those two ranges were not observed for spinach growing without contamination (k = 0.00679  0.00027 week -1 ). Chlorophyll degradation occurred eight times faster for tetracycline and ten times faster for cadmium. After 32 weeks of storage 19.4% and 13.6% of chlorophyll were left in small leaves affected by tetracycline and cadmium, respectively. In spinach leaves of harvesting maturity (50-days-old, large leaves), lower values of rate constants were found for range 2, amounting for dose (90 mg/kg soil) to k = 0.02945  0.00151 week -1 (for tetracycline) and k = 0.03511  0.00124 week -1 (for cadmium) and 50% chlorophyll was left after 32 weeks. Chlorophyll degradation degree depends on soil contamination, leaf size and time of exposure to contaminants.

Iron ions can be used to degrade tetracycline dispersed in nature. Studies of absorption and fluorescence spectra and quantum chemistry calculations showed that iron is more readily released from Fe(III)-citrate than from Fe(III)-EDTA, so Fe(III)-citrate (Fe(III)-Cit) is more suitable for tetracycline (TC) degradation. At 30 °C, a severe degradation of TC by Fe(III)-Cit occurred as early as after 3 days of incubation in the light, and after 5 days in the dark. In contrast, the degradation of TC by Fe(III)-EDTA proceeded very slowly in the dark. By the fifth day of incubation of TC with Fe(III)-Cit in darkness, the concentrations of the former compound dropped by 55% and 75%, at 20 °C and 30 °C, respectively. The decrease in tetracycline concentrations caused by Fe(III)-EDTA in darkness at the same temperatures was only 2% and 6%, respectively. Light increased the degradation rates of TC by Fe(III)-EDTA to 20% and 56% at 20 °C and 30 °C, respectively. The key role of the light in the degradation of tetracycline by Fe(III)-EDTA was thus demonstrated. The TC degradation reaction showed a second-order kinetics. The rate constants of Fe(III)-Cit-induced TC degradation at 20 °C and 30 °C in darkness were k = 4238M−1day−1 and k = 11,330M−1day−1, respectively, while for Fe(III)-EDTA were 55 M−1day−1 and 226 M−1day−1. In light, these constants were k = 15,440M−1day−1 and k = 40,270 M−1day−1 for Fe(III)-Cit and k = 1012 M−1day−1 and 2050 M−1day−1 at 20 °C and 30 °C; respectively. A possible reason for the higher TC degradation rate caused by Fe(III)-Cit can be the result of its lower thermodynamical stability compared with Fe(III)-EDTA, which we confirmed with our quantum chemistry calculations. Two quantum chemistry calculations showed that the iron complex with EDTA is more stable (the free energy of the ensemble is 15.8 kcal/mol lower) than the iron complex with Cit; hence, Fe release from Fe(III)-EDTA is less effective.