Pyrolyzed biowastes deactivated potentially toxic metals and eliminated antibiotic resistant genes for healthy vegetable production

Abstract

Potentially toxic metals (PTEs) and antibiotic resistance genes (ARGs) present in bio-wastes were the major environmental and health risks for soil use. If pyrolyzing bio-wastes into biochar could minimize such risks had not been elucidated. This study evaluated PTE pools, microbial and ARGs abundances of wheat straw (WS), swine manure (SM) and sewage sludge (SS) before and after pyrolysis, which were again tested for soil amendment at a 2% dosage in a pot experiment with a vegetable crop of pak choi (Brassica campestris L.). Pyrolysis led to PTEs concentration in biochars but reduced greatly their mobility, availability and migration potential, as revealed respectively by leaching, CaCl₂ extraction and risk assessment coding. In SM and SS after pyrolysis, gene abundance was removed by 4-5 orders for bacterial, by 2-3 orders for fungi and by 3-5 orders for total ARGs. With these material amended, PTEs available pool decreased by 25%-85% while all ARGs eliminated to background in the pot soil. Unlike a >50% yield decrease and a >30% quality decline with unpyrolyzed SM and SS, their biochars significantly increased biomass production and overall quality of pak choi grown in the amended soil. Comparatively, amendment of the biochars decreased plant PTEs content by 23-57% and greatly reduced health risk of pak choi, with total target hazard quotient values well below the guideline limit for subsistence diet by adult. Furthermore, biochar soil amendment enabled a synergic improvement on soil fertility, product quality, and biomass production as well as metal stabilization in the soil-plant system. Thus, biowastes pyrolysis and reuse in vegetable production could help build up a closed loop of production-waste-biochar-production, addressing not only circular economy but healthy food and climate nexus also and contributing to achieving the United Nations sustainable development goals.

Keywords: Antibiotic resistant genes; Bio-waste; Biochar; Clean production; Potentially toxic metals; Soil amendment; Vegetable production.

Graphical abstract

Fig. 1

Research flows of the study, focusing on changes in environmental and health risks of the biowastes before and after pyrolysis.

Fig. 2

SEM images of wheat residue (a), swine manure (b) and sewage sludge (c) respectively before (left) and after (right) pyrolysis.

Fig. 3

Metal speciation of Cu (a), Zn (b), Pb (c) and Cd (d) of wheat residue, swine manure and sewage sludge before (left) and after (right) pyrolysis. F1, exchangeable fraction; F2, reducible fraction; F3, OM bound fraction; F4, (mineral) residual fraction. UWS and WSB, USM and SMB, and USS and SSB, wheat residue; swine manure and sewage sludge respectively before and after pyrolysis.

Fig. 4

Abundances of microbial genes and antibiotic resistance genes (ARGs) detected in the biowastes. a, Bacteria; b, Fungi; c, genes related to antibiotic resistance (ARGs and intI1). USM and SMB, and USS and SSB, swine manure and sewage sludge respectively before and after pyrolysis. Different uppercase letters above the bars in a single pair of amendment and different lowercase letters above all the bars, represents a significant difference at p < 0.05 (One-way ANOVA followed by Turkey test) between before and after pyrolysis, and among all the treatments, respectively.

Fig. 5

Abundances of bacterial (a), fungal (b) genes and total antibiotics resistance genes (ARGs and intI1) (c) in pot soil under amendment treatments. CK, control; UWS and WSB, USM and SMB, and USS and SSB, wheat residue, swine manure and sewage sludge respectively before and after pyrolysis. Different uppercase letters above the bars in a single pair of amendment and different lowercase letters above all the bars, represents a significant difference at p < 0.05 (One-way ANOVA followed by Turkey test) between before and after pyrolysis, and among all the treatments, respectively.

Fig. 6

Aboveground biomass yield (a) and overall quality index (b) of harvested pak choi grown 40 days in pot soil under amendment of bio-wastes before (green) and after (gray) pyrolysis, compared to control (blank); CK, control; UWS and WSB, USM and SMB, and USS and SSB, wheat residue, swine manure and sewage sludge respectively before and after pyrolysis. Different uppercase letters above the bars in a single pair of amendment and different lowercase letters above all the bars, represents a significant difference at p < 0.05 (One-way ANOVA) between before and after pyrolysis, and among all the treatments, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 7

Total THQ values of potentially toxic metals by adding up the individual values of Cu (blue), Zn (gray), Pb (yellow) and Cd (black) in the pak choi via diet consumption by adult. CK, control; UWS and WSB, USM and SMB, and USS and SSB, wheat residue, swine manure and sewage sludge respectively before and after pyrolysis. Different uppercase letters above the bars in a single pair of amendment and different lowercase letters above all the bars, represents a significant difference at p < 0.05(One-way ANOVA followed by Turkey test) between before and after pyrolysis, and among all the treatments, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)