# Cadmium and zinc sorption and desorption in soil: the impact of humic-fulvic acids, Bacillus sp., insect frass, and soil aging

## TL;DR

This study explores how soil amendments like insect frass and biostimulants affect the retention and release of cadmium and zinc in soil.

## Contribution

The paper introduces a novel approach to studying the chemical behavior of cadmium and zinc in soil using innovative amendments.

## Key findings

- Insect frass and Bacillus sp. significantly increased cadmium and zinc sorption over time.
- Frass amendments enhanced cadmium desorption but not over time, suggesting reduced metal mobility.
- Soil aging with amendments improved metal retention, promoting soil health and environmental safety.

## Abstract

The sorption and desorption of Cd and Zn were investigated in a soil treated and incubated over a period of 30 days with various innovative amendments (the biostimulants humic and fulvic acids (HFA), Bacillus sp. (BAC), as well as insect frass (FR), and their combination (FR + HFA and FR + BAC)). This is a novel attempt to study the chemical behavior of the two toxic metals induced by the tested materials—a work that is in high want in the literature. The batch sorption test of Cd and Zn was conducted in the beginning and at the end of the incubation period in competitive systems. We found a significant increase in Cd sorption over time at FR and BAC, as recorded in the distribution coefficient (Kd-50), with values increasing from 143.70 to 184.98 L kg−1 and from 144.11 to 196.70 L kg−1, respectively. Zn sorption enhanced at FR, with Kd-50 values rising from 940.09 to 1310.64 L kg−1. Added frass caused an increase in Cd and Zn sorption relative to the unamended control, but only in day 30, probably due to the added ion exchange and the presence of available organic matter-bound retention sites that contributed to the soil matrix after soil aging over time. Desorption was enhanced for Cd at FR and FR + BAC relative to the control (5.74% and 6.26% vs. 2.08%, respectively), but the fact that no further increase was observed over time is rather promising. We conclude that frass, alone or co-amended with the tested biostimulants, can positively affect metal retention, reducing metal mobility and bioavailability, and thus preserving soil health and environmental safety.

The online version contains supplementary material available at 10.1007/s11356-025-36699-4.