Cadmium Binding by Fractions of Dissolved Organic Matter and Humic Substances from Municipal Solid Waste Compost

Abstract

The agricultural practice of amending soils with composted municipal solid waste (MSW) adds significant amounts of organic matter and trace metals, including Cd. Under these conditions, soluble organic complexes of Cd formed in the compost may be more significant than previously thought, due to Cd bioavailability and mobility in the soil environment. To study the relative importance of different types of organic ligands in MSW compost for the binding of Cd, six fractions of the dissolved organic matter (DOM) in addition to humic acid (HA) and fulvic acid (FA) were extracted and their complexation of Cd quantified at pH 7 using an ion-selective electrode (ISE). The highest complexing capacities (CC) for Cd were found for the most humified ligands: HA (2386 micromol Cd g(-1) C of ligand), predialyzed FA (2468 micromol Cd g(-1) C), and HoA, a fulvic-type, easily soluble fraction (1042 micromol Cd g(-1) C). The differences in CC for Cd of the various organic ligands were not directly related to total acid-titratable or carboxylic groups, indicating the importance of sterical issues and other functional groups. The strength of association between Cd and the organic ligands was characterized by calculating stability constants for binding at the strongest sites (pK(int)) and modeling the distribution of binding site strengths. The pK(int) values of the DOM fractions ranged between 6.93 (HiN: polysaccharides) and 8.11 (HiB: proteins and aminosugars), compared with 10.05 for HA and 7.98 for FA. Hence, the highly complex and only partially soluble organic molecules from compost such as HA and FA demonstrated the highest capacity to sequester Cd. However, strong Cd binding of organic ligands containing N-functional groups (HiB) in addition to a high CC of soluble, humified ligands like HoA indicated the relevance of these fractions for the organic complexation of Cd in solution.

Status of Selenium and Trace Elements in some Arid Soils Cultivated with Forage Plants: A Case Study from Saudi Arabia

“…Organic matter has an important factor controlling soil trace elements solubility (McBride et al, 1997). Both solid and dissolved OM contains functional groups responsible for forming complexes with trace elements (Kaschl et al, 2001). Insoluble, high molecular weight organic molecules have a high tendency to trace elements.…”

“…On the contrary, low molecular weight molecules with high solubility are responsible for increasing trace elements solubility. The dissolved organic matter represent the most mobile form of OM in the soil and is released by various processes from the solid phase: simple dissolution, adsorption and desorption processes, exchange protonation and dissolution reactions, biological decomposition of OM, and metabolites released by microorganisms, soil fauna and plants (Kaschl et al, 2001). Complexation of trace elements by soluble organic ligands influences the solubilization and sorption/desorption of trace elements.…”

Phytoextraction: a review on enhanced metal availability and plant accumulation

“…Metal solubility and availability are both dependent on soil characteristics and are strongly influenced by pH and the degree of complexation with soluble ligands (Kaschl et al, 2002). Metals exist in soil in various pools: in solution as ionic or organically complexed species; on exchange sites of reactive soil components; complexed with organic matter; occluded in Fe, Al, and Mn oxides and hydroxides; entrapped in primary and secondary minerals (Shuman, 1985;Mann & Ritchie, 1993).…”

Simulação do intemperismo natural de pilhas zinco-carbono e alcalinas

“…3 A capacidade de transferência dos constituintes da fase sólida para o lixiviado (lixiviabilidade) é determinada expondo-se o material a uma solução com características conhecidas e determinando-se o grau de dissolução dos contaminantes. 4 A lixiviabilidade dos componentes depende de diversos fatores: tamanho de partícula (área superficial exposta); temperatura (solubilidade dos solutos na água); tempo de contato (estabelecimento ou não do equilíbrio de lixiviação); 3,4 agitação; relação líquido/sólido; pH (um dos fatores dominantes da mobilidade, solubilidade, complexação e retenção de metais pesados em solos, sedimentos e resíduos); 5,6 condições redox (importante quando os compostos formados na presença de oxigênio têm diferença significativa na solubilidade em relação àqueles formados sob condições redutoras); presença de agentes complexantes, CO 2 e O 2 ; presença de matéria orgânica dissolvida e em suspensão (ela influencia a biodisponibilidade e a mobilidade de metais pesados); 7,8 presença de micro-organismos (intemperismo biológico). [1][2][3]9 A extensão da liberação de constituintes de um resíduo sólido em ensaios de lixiviação é fundamental para definir: 9,10 o potencial de impacto ambiental através do transporte pela água, incluindo solo, lençol freático e contaminação de água de superfície; os riscos ecológicos e para a saúde humana a partir da disposição inadequada no ambiente; a eficácia de processos de tratamento de resíduos para disposição final; projetos e critérios de aceitação para instalação de programas de gerenciamento de resíduos; o controle de qualidade e classificação dos produtos e resíduos; o modo de degradação dos materiais dispostos no meio ambiente.…”