Overview
Boostani and colleagues tested whether bentonite combined with pristine or phosphoric-acid-modified biochars could immobilize cadmium in contaminated calcareous soil. The study is mitigation evidence: it uses Cd-spiked soil and amendment treatments, so its values should not be routed into coffee, food, or ingredient occurrence pools.
Key numbers
The source soil was a loam from Darab, Fars province, southern Iran, with pH 7.40, EC 1.40 dS/m, 43.7% calcium carbonate equivalent, 1.50% organic matter, and total cadmium (Cd) below AAS detection before spiking. The experimental soil was spiked to 50 mg Cd/kg soil using cadmium chloride.
The 90-day incubation used three bentonite levels: B0 0% wt., B1 1% wt., and B2 2% wt. Biochar treatments were no biochar control, coffee grounds biochar (G), phosphoric-acid-modified coffee grounds biochar (GH), municipal solid waste biochar (M), and phosphoric-acid-modified municipal solid waste biochar (MH), each at 2% wt. The full factorial design had three replications.
Table 3 reports water-soluble plus exchangeable Cd (WsEx). The unamended C + B0 control had 8.66 mg/kg. Coffee grounds biochar without bentonite (G + B0) lowered WsEx to 4.79 mg/kg, but adding bentonite to G raised WsEx to 5.41 mg/kg at B1 and 7.68 mg/kg at B2. The abstract summarizes those bentonite additions as increasing WsEx by 12.9% and 60.3% compared with G alone.
Table 3 reports Cd mobility factor. The C + B0 control was 50.5%. G + B0 was 37.5%, a 25.7% reduction. M + B2 was 39.9%, a 21% reduction. The text states that bentonite alone lowered mobility factor by 15.6% from B0 to B2, while municipal-solid-waste biochar plus bentonite showed a synergistic immobilization effect.
Table 3 also shows fraction shifts. M + B2 had WsEx 5.95 mg/kg, carbonate-bound Cd 14.0 mg/kg, Fe-Mn-oxide-bound Cd 4.51 mg/kg, residual Cd 25.5 mg/kg, and mobility factor 39.9%. G + B0 had WsEx 4.79 mg/kg, carbonate-bound Cd 14.0 mg/kg, Fe-Mn-oxide-bound Cd 4.26 mg/kg, residual Cd 27.0 mg/kg, and mobility factor 37.5%.
Figure 3 and the desorption text report that M + B2 had the lowest 24-hour EDTA-extracted Cd release at 22.4 mg Cd/kg soil, an 18.7% reduction compared with C + B0. Table 4 reports that M + B2 produced the greatest decrease in the power-function e constant (-29.8%) and the highest increase in f (+40.0%); the initial desorption-rate parameter ef was 1.14 for M + B2 versus 1.29 for C + B0.
Methods (brief)
Soils were air-dried, sieved, spiked with CdCl2 solution to 50 mg Cd/kg, and repeatedly wetted and dried to equilibrate Cd with the soil. Biochars were prepared from coffee grounds and municipal solid waste by slow pyrolysis at 400 C for 2 hours under limited oxygen; modified biochars were treated with 5 N phosphoric acid.
After 90 days of incubation at near field capacity, soil Cd was separated into water-soluble/exchangeable, carbonate-bound, Fe-Mn-oxide-bound, organic-matter-bound, and residual fractions. EDTA desorption used 0.01 M EDTA at pH 7.0 over 5 to 1440 minutes, with Cd measured by atomic absorption spectroscopy.
Related evidence
Update history
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