Removal of heavy metals in sludge via joint EDTA-acid treatment: Effects on seed germination

Abstract

Heavy metals (Cu, Pb, Zn, Ni, Cr, and Cd) were removed from sludge via joint treatment with ethylene diamine tetraacetic acid (EDTA) and three organic acids (citric acid, glutamic acid, or aspartic acid) at optimal EDTA-acid concentration ratios of 1:1, 1:2 and 2:1, respectively. Heavy metal removal rates and post-treatment nutrient retention in sludge was then analyzed. The effects of different proportions sludge and soil mixes on the germination of Chinese white cabbage (Brassica campestris L. ssp. Chinensis Makino) seeds was then studied, and the mechanism by which nutrient content in the soil/sludge mixture affects seed germination was explored. The results indicated that the removal rate of the heavy metals decreased in the order of Zn > Ni > Cd > Pb > Cu > Cr, when EDTA was used in conjunction with citric acid and glutamic acid. In contrast, when EDTA was combined with aspartic acid, the removal rate of the heavy metals decreased in the order of Ni > Zn > Cd > Cu > Pb > Cr. Regarding the effect of heavy metal removal and sludge nutrient retention, EDTA-citric acid and EDTA-aspartic acid treatment had optimum results at a 1:1 ratio, while EDTA-glutamic acid treatment was optimum at a 1:2 ratio. At an optimum sludge to soil ratio of 1:4, the germination and root elongation inhibition rate of Chinese white cabbage seeds could be promoted, and the sludge could meet standard agricultural requirements. SPSS correlation analysis demonstrated that the seed germination index and nutrient content in sludge/soil mixtures were significantly correlated, demonstrating the feasibility of sludge for agricultural purposes.