Assessment of Hydrolyzed Keratin from Waste Sheep Wool for Mitigating Cadmium Toxicity in Lettuce

The effects of hydrolyzed keratin (HK) treatments on the growth, cadmium (Cd) accumulation, nitrogen (N) concentration, and FTIR spectral properties of lettuce grown in Cd-contaminated soil was investigated. Curly lettuce was grown in a naturally lit greenhouse. The treatments were as follows: 1-Control 2-Cd-contaminated [Cd, 30 mg Cd kg−1 kg−1]: 3-Cd+HK1 [30 mg Cd and 1.0 mL HK kg−1] 4-Cd+HK2 [30 mg Cd and 2.0 mL HK kg−1]. Cadmium-contamination improved weights of lettuce plants compared to the control, with a similar trend observed in the Cd+HK treatments. The Cd in control plants was 0.48 mg kg−1, which increased to 14.1 mg kg−1 with Cd application. In contrast, HK treatments reduced Cd accumulation to below 11.5 mg kg−1. Although Cd-contamination reduced plant N concentration, the HK2 treatment mitigated Cd toxicity by enhancing N concentrations compared to Cd treatments. FTIR analysis revealed that the spectral region between 1039 and 1392 cm−1 remained unaffected across treatments. However, marked spectral shifts were observed: the peak at 1645 cm−1 in control plants shifted to 1630 cm−1 with Cd-contamination and broadened with Cd+HK1 treatment, while Cd+HK2 restored it to a pattern like the control. Unique peaks at 1720 cm−1 appeared in the HK treatments, and the peak at 2100 cm−1, absent in Cd treatments, reemerged with HK. Additionally, peaks at 2345 and 3738 cm−1, conspicuously present or absent in control plants, were intensified with Cd+HK treatments. Hydrolyzed keratin reduces Cd accumulation in lettuce, partially alleviates Cd-induced stress, and induces specific structural changes detectable by FTIR.