Assisted Phytoremediation between Biochar and Crotalaria pumila to Phytostabilize Heavy Metals in Mine Tailings

Abstract

The increasing demand for mineral resources has generated mine tailings with heavy metals (HM) that negatively impact human and ecosystem health. Therefore, it is necessary to implement strategies that promote the immobilization or elimination of HM, like phytoremediation. However, the toxic effect of metals may affect plant establishment, growth, and fitness, reducing phytoremediation efficiency. Therefore, adding organic amendments to mine tailings, such as biochar, can favor the establishment of plants, reducing the bioavailability of HM and its subsequent incorporation into the food chain. Here, we evaluated HM bioaccumulation, biomass, morphological characters, chlorophyll content, and genotoxic damage in the herbaceous Crotalaria pumila to assess its potential for phytostabilization of HM in mine tailings. The study was carried out for 100 days on plants developed under greenhouse conditions under two treatments (tailing substrate and 75% tailing/25% coconut fiber biochar substrate); every 25 days, 12 plants were selected per treatment. C. pumila registered the following bioaccumulation patterns: Pb > Zn > Cu > Cd in root and in leaf tissues. Furthermore, the results showed that individuals that grew on mine tailing substrate bioaccumulated many times more metals (Zn: 2.1, Cu: 1.8, Cd: 5.0, Pb: 3.0) and showed higher genetic damage levels (1.5 times higher) compared to individuals grown on mine tailing substrate with biochar. In contrast, individuals grown on mine tailing substrate with biochar documented higher chlorophyll a and b content (1.1 times more, for both), as well as higher biomass (1.5 times more). Therefore, adding coconut fiber biochar to mine tailing has a positive effect on the establishment and development of C. pumila individuals with the potential to phytoextract and phytostabilize HM from polluted soils. Our results suggest that the binomial hyperaccumulator plant in combination with this particular biochar is an excellent system to phytostabilize soils contaminated with HM.

Keywords: Crotalaria pumila; biochar; heavy metals; hyperaccumulator; mine tailing; phytostabilization.

Figure 1

Heavy metal concentration (average ± standard error), two-way ANOVA to determine the effect of treatment, time, and interaction (treatment × time) in root of C. pumila growing under greenhouse conditions. Regression analysis between exposure time and heavy metal concentration in root. The asterisks denote significant differences between treatments by exposure time with p < 0.05 (Tukey). ANOVA test: *** = p < 0.001, * = p < 0.05.

Figure 2

Heavy metal concentration (average ± standard error), two-way ANOVA to determine the effect of treatment, time, and interaction (treatment × time) in leaves of C. pumila growing under greenhouse conditions. Regression analysis between exposure time and heavy metal concentration in root. The asterisks denote significant differences between treatments by exposure time with p < 0.05 (Tukey). ANOVA test: *** = p < 0.001, * = p < 0.05. n.s. = not significant differences.

Figure 3

Average (±standard error) biomass of C. pumila roots and leaves growing in greenhouse conditions on mine tailing substrate and mine-tailing/substrate. Two-way ANOVA to evaluate the effect of time (100 days) and treatment on root and leaves biomass characters, and simple regression analysis to evaluate the relationship between exposure time to the substrate and biomass characters. The asterisks denote significant differences between treatments by exposure time with p < 0.05 (Tukey). ANOVA test: *** = p < 0.001, * = p < 0.05.

Figure 4

Average ± standard error of chlorophyll a and b from leaves from C. pumila growing in greenhouse conditions on mine tailing substrate and mine-tailing/substrate. Two-way ANOVA to evaluate the effect of time (100 days) and treatment on chlorophyll content from leaves, and simple regressions analysis to evaluate the relationship between exposure time to the substrate and chlorophyll content. The asterisks denote significant differences between treatments by exposure time with p < 0.05 (Tukey). ANOVA test: *** = p < 0.001, * = p < 0.05.

Figure 5

Average ± standard deviation of genetic damage (single strand breaks) in foliar tissue from C. pumila growing in greenhouse conditions on mine tailing substrate and mine tailing/substrate. Two-way ANOVA to evaluate the effect of time (100 days) and treatment on genetic damage, and simple regression analysis to evaluate the relationship between exposure time to the substrate and genetic damage. The asterisks denote significant differences between treatments by exposure time with p < 0.05 (Tukey). ANOVA test: *** = p < 0.001, ** = p < 0.01, * = p < 0.05.