Abstract
The application of organic amendments to contaminated agricultural lands can immobilize metals and improve soil conditions. The chemical structures and long-term stability of commercial humic substances and other composted organic amendments (sheep and horse manure, vermicompost, pine bark, and pruning waste) were analyzed using 13C CPMAS NMR, FT-IR, and DSC to evaluate their use in soil remediation. The interactions of humic substances and manure with Cu (0 and 5000 mg kg−1) at different pH (2.5 and 5.0) were studied through a batch adsorption experiment observing the changes in their molecular structure using spectroscopic techniques. Humic substances exhibited high aromaticity and phenolic and carboxylic group content, with great affinity for Cu complexation. Humic substances and pruning waste were the most stable due to their high recalcitrant organic matter contents, whereas manure was the least stable, given the labile nature of its organic matter content. There were considerable changes in the carboxylic and phenolic groups of humic substances with pH, and also with Cu, albeit in a lesser extent, especially at pH 5.0, suggesting the great sorption capacity of humic substances and the key role of pH and these functional groups in metal complexation. Manure did not exhibit such changes. Commercial humic substances could be useful amendments for the remediation of contaminated agricultural soils due to their high sorption capacities and long-term stability.
Highlights
The contamination of agricultural soils with heavy metals is a widespread threat for human and animal health
The chemical structures and long-term stability of commercial humic substances and other composted organic amendments were analyzed using 13 C CPMAS NMR, Fourier Transform Infrared Spectroscopy (FT-IR), and DSC to evaluate their use in soil remediation
Cu accumulates in the top layers, but its tendency to be adsorbed by soil organic matter, carbonates, and clay minerals may cause it to transfer into deeper layers [4]
Summary
The contamination of agricultural soils with heavy metals is a widespread threat for human and animal health. Metal accumulation reduces plant growth and soil microbial activity, affects soil functioning, and may endanger nearby soils and water ecosystems, as well as increasing their transfer to the food chain [1,2,3]. Despite being an essential micronutrient for all living organisms in natural ecosystems, Cu is a toxic pollutant when its concentrations are high. Mean Cu concentration in non-polluted soils is 20–30 mg kg−1 ; the Agronomy 2019, 9, 762; doi:10.3390/agronomy9110762 www.mdpi.com/journal/agronomy. Agronomy 2019, 9, 762 range of concentrations in agricultural soils which could produce phytotoxicity is 60–150 mg kg−1 [4]. Cost-effective and environmentally-friendly remediation techniques should be applied to reduce the toxicity and dispersion of Cu and other metals, and to limit exposure for humans and other organisms
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