Abstract
Humic acids in aqueous solution demonstrate inverse temperature-solubility relationships when solution conditions are manipulated to reduce coulombic repulsion among the humic polyanions. These effects were followed by dynamic light scattering (DLS) measurements of the resulting aggregates, as well as the addition of a polarity sensitive fluorescent probe (pyrene). The humic solutions could be primed for temperature induced clouding by carefully lowering the pH to a point where hydration effects became dominant. The exact value of the cloud point (CP) was a function of both pH and humate concentration. The CPs mostly lay in the range 50–90°C, but DLS showed that temperature induced aggregation proceeded from approximately 30°C onward. Similar effects could be achieved by adding multivalent cations at concentrations below those which cause spontaneous precipitation. The declouding of clouded humate solutions could be affected by lowering the temperature combined with mechanical agitation to disentangle the humic polymers.
Highlights
Humic substances (HS) are the decay products of the total biota in the environment
As noted in the Introduction, the occurrence of temperature induced clouding (TIC) in humic solutions is predicated on a reduction of the ionic character of the solute, allowing it to aggregate when the temperature is raised
At 20∘C, a 40 mg/L Latahco silt-loam humic acid (LSLHA) solution did not cloud under these conditions, but when the temperature was increased, visible clouding occurred
Summary
Humic substances (HS) are the decay products of the total biota in the environment. They are grouped into three operationally defined categories: fulvic acid (FA), soluble at all pHs; humic acid (HA), soluble at pH > 2; and humin, insoluble at any pH. In view of these broad size ranges, a high degree of polydispersity exists in aqueous solutions of humic materials [1] They are mildly acidic, primarily due to an abundance of carboxylic groups [2]. (iii) The ionic strength is raised sufficiently to result in a salting-out effect Under these circumstances, the negative charges on the humic polyanions are shielded by the ionic content of the solution and coulombic repulsion among the humates is reduced. The negative charges on the humic polyanions are shielded by the ionic content of the solution and coulombic repulsion among the humates is reduced This leads to the formation of aggregates. It has been observed previously [3] that aqueous HA solutions, not unlike those of nonionic surfactants, can undergo temperature induced clouding (TIC). The bulk of the investigation was done with Latahco silt-loam humic acid (LSLHA), which has been shown to be an effective surfactant [5]
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