The dissolved organic matter from the co-decomposition of Chinese milk vetch and rice straw induces the strengthening of Cd remediation by Fe-modified biochar

Abstract

Fe-modified biochar (FB) and co-using Chinese milk vetch and rice straw (MR) are two effective ways for mitigating the cadmium (Cd) contamination in paddy fields in southern China. Nevertheless, the effects of FB combined with MR on Cd passivation mechanism remain unclear. In the current study, the strengthening effects of FB induced by MR were found and the mechanisms of the extracted dissolved organic matter (DOM) from the co-decomposition of MR on Cd alleviation were investigated through pot experiment and adsorption experiment. Pot experiment demonstrated that co-incorporating FB and MR decreased available Cd by 23.1% and increased iron plaque concentration by 11.8%, resulting in a 34.7% reduction in Cd concentrations in brown rice compared with addition of FB. Furthermore, co-using FB and MR improved available nutrients in the soil. The molecular characteristics of DOM derived from the decomposition of MR (DOM-MR) were analyzed by fluorescence excitation emission matrix spectroscopy-parallel factor analysis (EEM-PARAFAC) and Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS). Results showed that lignin/carboxylic-rich alicyclic molecules and protein/amino sugar were the main compounds, potentially involved in the Cd binding. Adsorption experiments revealed that the addition of DOM-MR improved the functional groups, specific surface area, and negative charges of FB, inducing the strengthening of both physisorption and chemisorption of Cd(II). The maximum adsorption capacity of Fe-modified biochar after adding DOM-MR was 634 mg g⁻¹, 1.30 times that without the addition of DOM-MR. This study suggested that co-incorporating MR, and FB could serve as an innovative practice for simultaneous Cd remediation and soil fertilization in Cd-polluted paddy fields. It also provided valuable insights and basis that DOM-MR could optimize the performances of Fe-modified biochar and enhance its potential for Cd immobilization.

Highlights

•	Chinese milk vetch and rice straw strengthened the Cd remediation by biochar.
•	Soil DOM was the key influencing factor on soil Cd immobilization.
•	DOM optimized the properties of biochar and produced richer functional groups.
•	The combination of DOM and biochar had a higher specific surface area than biochar alone.
•	DOM strengthened the physisorption and chemisorption capacities of Cd(II) on biochar.

Keywords

Cd(II) / Fe-modified biochar / DOM / FT-ICR MS / EEM-PARAFAC / Acidic functional groups

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Funding

The National Key Research and Development Program of China(2021YFD1700200); The earmarked fund for CARS‐Green manure(CARS-22); The Agricultural Science and Technology Innovation Program of Chinese Academy of Agricultural Sciences