Dissolved organic matter dynamic in the Amazon basin: Sorption by mineral surfaces

Abstract

In aquatic systems, soprtion of organic matter (OM) on environmental surfaces or its preference to remain dissolved is highly important for determining its potential transport and/or susceptibility to degradation. In the Amazon and other major rivers of the world, transported OM is either adsorbed to fine minerals or remains dissolved. The fate of autochthonous OM in Amazon floodplains and allochthonous OM from the river can be addressed by characterizing the nature of OM interactions with the sediment material. This goal was pursued in this study using Rio Negro and Amazon River samples as well as those collected from floodplain lakes during phytoplanktonic blooms, small black rivers, soil solution issued from podzolic areas in the Rio Negro watershed and “terra firme” stream flowing into the floodplain. The amount of carbon surface loading expressed as the adsorbed organic carbon per unit of surface area of the substrate was obtained by batch experiments with mineral phases representative of soils, suspended matter found in the Amazon basin and extracted OM fractions (the hydrophobic, HPO and the transphilic, TPH fractions of OM). The bulk dissolved organic matter samples were characterized by carbon and nitrogen isotopic measurements as well as elementary composition, specific UV absorbance (SUVA), molecular weight (Mw) and FTIR spectroscopy. To our knowledge, these are the first C and N isotopic composition data reported for extracted OM fractions in the Amazon basin. The Rio Negro basin water samples had high [DOC] correlated with conductivity. The SUVA values ranged from 4.0 to 7.3 m −1l mg −1C and were within the range of measurements reported for the Amazon River. The δ 13C values of all HPO fractions varied from − 27.7 to − 30.2‰. The δ 13C values for the TPH fractions were systematically 1‰ higher than their respective HPO fraction. An increasing trend between the weight average Mw of the DOC as function of the C/N ratio (the higher C/N the higher Mw) is reported. This trend is inferred to result from both mixing between 2 end-members and OM fractionation. Carbon surface loadings calculated for HPO and TPH fractions ranged between 23 to 309 μgC m −2 and 38 to 145 μgC m -2, respectively. Carbon surface loadings with goethite and the Rio Negro organic matter fractions cluster around the average value of 24.5 ± 4 μgC m -2. With the help of the pH dependant dissociation measurements and NICA-Donnan model parameters, it was possible to estimate the amount of deprotonated sites on the OM interacting with the sediment material. The amount of carbon surface loading was correlated with the calculated residual negative charge of the OM fractions (i.e. HPO or TPH). These analyses showed that higher residual negative charges were associated with lower carbon surface loadings on the treated sediment. The decrease of the negative charge was suggested to reduce the net electrostatic repulsion between the OM fractions and the clay surfaces and promote ligand exchange reactions governing sorption to clays. The preferential uptake of the high molecular weight fraction demonstrates that the chemical nature of the organic matter remaining in solution differs from that of the adsorbed fraction.