Comparison of ashing and pyrolysis treatment on cadmium/zinc hyperaccumulator plant: Effects on bioavailability and metal speciation in solid residues and risk assessment.

Phytoremediation of metal(loid)s contaminated sites is widely used, while there is scarce of investigation on the metal-enriched biomass waste safely disposal which resulted in risks of causing secondary pollution to the soil and water bodies and even to human health. Thus, this study compared the effects of ashing and pyrolysis treatments on cadmium (Cd) and zinc (Zn) hyperaccumulation plant Sedum plumbizincicola. Chemical speciation, the Toxicity Characteristic Leaching Procedure (TCLP), and diethylenetriamine pentaacetic acid (DTPA) extraction were employed to characterize the bioavailability and leachability of Cd and Zn in the solid residues after pyrolysis and ashing. The risk assessment code (RAC) and potential ecological risk index (RI) were subsequently used to evaluate the risk of the solid residues to the environment. The results showed that both ashing and pyrolysis treatments could transform the bioavailable Cd and Zn in S. plumbizincicola into a more stable form, and the higher the temperature the greater the stablility. Pyrolysis converted a maximum of 80.0% of Cd and 70.3% of Zn in S. plumbizincicola to the oxidisable and residual fractions, compared with ashing which achieved only a ∼42% reduction. The pyrolysis process minimised the risk level of Cd and Zn to the environment based on the RAC and RI assessments. The results of the TCLP test, and DTPA extraction confirmed that the leaching rate and the bioavailable portion of Cd and Zn in the biochars produced by pyrolysis were invariably significantly (p < 0.05) lower than the solid residues produced by ashing, and reached the lowest at 650 °C. In other words, pyrolysis was better than ashing for thermal treatment of the metal-enriched hyperaccumulator plant, in view of minimising the bioavailability and leachability of Cd and Zn from the solid residues to the environment. This study provides fundamental data on the choice of treatments for the disposal of metal-enriched plant biomass.

Replacement of mineral fertilizers with anaerobically digested pig slurry in paddy fields: assessment of plant growth and grain quality.

Rice cultivation requires large quantities of irrigation water and mineral fertilizers. This provides an opportunity for the recycling of the plant nutrients in anaerobically digested pig slurry, large amounts of which are generated in Chinese pig farms. Hence, to promote the sustainable development of livestock and poultry breeding and rice production, a micro-plot field experiment was carried out to assess whether or not slurry can replace mineral fertilizers in rice paddy production in terms of plant tillering, grain quality, and yields. The results indicate that the total N content of the slurry can serve as an alternative source of N when compared to the control (450 kg ha-1 commercial compound fertilizer (N/P2O5/K2O = 15:15:15) as basal fertilizer, 300 kg ha-1 urea (N% = 46), and 150 kg ha-1 commercial compound fertilizer as top-dressed fertilizer). No negative effects on plant growth or grain yield were observed, although there may be a potential risk due to an increase in grain Cu concentration. The amylose content and gel consistency of the rice grains were enhanced significantly by the use of slurry as a basal fertilizer, but the grain protein and total amino acid contents decreased. The results suggest that anaerobically digested pig slurry can replace mineral fertilizers in rice production when applied as a basal dressing together with urea and commercial compound fertilizer as top-dressed fertilizers.