DOC In Solution Research Articles

Fly ash-based waste for ex-situ landfill stabilization of per- and polyfluoroalkyl substance (PFAS)-contaminated soil

In response to world-wide soil and groundwater contamination per- and polyfluoroalkyl substances (PFAS), stakeholders require immediate mitigation. Soil deposition in landfill is a common mitigation scheme, but PFAS losses occur via landfill leachate. These leaching losses can be reduced by strategically utilizing other deposited waste materials for ex-situ contaminant stabilization. This screening study tested activated carbon (AC) and eight types of wastes (compost, rubber granulate, bentonite clay, industrial sludge, incineration slag, incineration bottom ash (n=4), incineration fly ash-based air pollution control residue (FA-APC) (n=16)) in amending (adding 4%, 5%, 10% or 25% sorbent) field-contaminated (n=19) and PFAS-fortified (n=11) soils. A subset of FA-based residue types, all originating from grate-fire incineration (G-F-I) plants, achieved extraordinarily high removal of PFAS. The removal was up to 98% (25% addition) of the sum of six dominant PFAS for field-contaminated soil and >99% of the sum of 11 PFAS for fortified soil (10/25% addition) (>99.9% for PFOS). Calculated partitioning coefficient revealed significant trends between sorption strength and perfluorocarbon chain length (0.21-0.47 log units per CF2-moiety), indicating high importance of hydrophobic sorption (R2>0.98). However, with incremental G-F-I FA-APC addition this relationship disappeared, indicating an alternative sorption mechanism. The exceptional PFAS sorption by G-F-I FA-APC was not explained by G-F-I surface area, surface charge, soil mineral- and metal composition, or solution DOC, metal, or ion composition (H+, Ca2+, Mg2+, Al3+ and Ba2+). Although the mechanism remains unknown, this study showed that landfill sites can utilize G-F-I FA-APC for ex-situ stabilization at negative cost, thus preventing PFAS-containing leachate.

Effects of oxygen concentration in solution and uptake rate by roots on cut roses yield, and nutrients and sugars content in leaves

The response of rose flower yield to hypoxia has been discussed in the literature, but there is still a lack of sufficient data on plant performance as affected by dissolved oxygen concentration in solution (DOC) and uptake flux by roots, to support efficient aeration control. The objective was to study Rosa L. response to DOC by characterizing oxygen consumption rates, leaf contents of nutrients and sugars, and cut-flower yield. Five levels of DOC in solution, ranging from 7.5 to 1.7mg/L, were investigated. In winter and early spring DOC ≥2.5mg/L was sufficient to satisfy plant demands for oxygen. In late spring and summer the required DOC was ~5mg/L, corresponding to an O2 influx of 0.55mggfw−1h−1. When DOC in summer was 1.7mg/L the accumulation rate of Mn, Zn, P, Mg, and K in leaves was inhibited relative to DOC ~5mg/L, transpiration declined, starch and polymerized glucose accumulated in leaves, and cut-flower yield decreased. Indications in solution were reduced capacity to decrease pH after the May harvest, which stemmed from reduced H+ efflux from roots, and stronger decline in EC due to decreased transpiration.

Sorption and Fractionation of Dissolved Organic Matter and Associated Phosphorus in Agricultural Soil

Mobility of dissolved organic matter (DOM) strongly affects the export of nitrogen (N) and phosphorus (P) from soils to surface waters. To study the sorption and mobility of dissolved organic C and P (DOC, DOP) in soil, the pH-dependent sorption of DOM to samples from Ap, EB, and Bt horizons from a Danish agricultural Humic Hapludult was investigated and a kinetic model applicable in field-scale models tested. Sorption experiments of 1 to 72 h duration were conducted at two pH levels (pH 5.0 and 7.0) and six initial DOC concentrations (0-4.7 mmol L(-1)). Most sorption/desorption occurred during the first few hours. Dissolved organic carbon and DOP sorption decreased strongly with increased pH and desorption dominated at pH 7, especially for DOC. Due to fractionation during DOM sorption/desorption at DOC concentrations up to 2 mmol L(-1), the solution fraction of DOM was enriched in P indicating preferred leaching of DOP. The kinetics of sorption was expressed as a function of how far the solution DOC or DOP concentrations deviate from "equilibrium." The model was able to simulate the kinetics of DOC and DOP sorption/desorption at all concentrations investigated and at both pH levels making it useful for incorporation in field-scale models for quantifying DOC and DOP dynamics.

Cadmium and copper release kinetics in relation to afforestation of cultivated soil

Afforestation of cultivated soils causes soil acidification and elevated concentrations of dissolved organic matter (DOC) in the soil solution, and hence, aggravate the risk of heavy metal leaching. The kinetics of cadmium and copper release from an unpolluted arable soil applied with forest floor soil solution was investigated in the laboratory, and the release rates correlated to pH and DOC in solution through log–log equations. The soil solution was isolated from Norway spruce ( Picea abies (L.) Karst.) by centrifugation, and the solution passed a cation-exchange column to remove metal cations and to protonate the DOC. Soil samples from an arable Ap horizon were placed in completely mixed flow cells, and influent solutions with 0 to 5 mM DOC were applied. The solution pH was adjusted to achieve effluent pH values in the range 3.6 to 6.9 in the flow cells at steady-state conditions. Cadmium release rates were very low at pH > 5 and increased exponentially as pH decreased to <5. The release rate was correlated to solution pH in a simple model: log(cadmium release rate) = −0.21 pH − 15.28 ( R 2 = 0.48), and no significant effect of DOC was observed. The kinetics of copper release from the soil was more complicated with effects of both pH and DOC. In experiments without DOC, the release rate of copper was slightly lower at high pH than at low pH. In experiments above pH 5, the presence of 5 mM DOC in the solution increased the release rate of copper. However, the copper release was retarded by DOC in the range pH 3.8 to 5.0, which coincided with a maximum retention of DOC in the flow cells. The release rate of copper was correlated to solution pH and concentration of DOC, including an interaction of pH and DOC: log(copper release rate) = 0.86 pH − 1.26 logDOC + 0.24 pH · logDOC − 19.26 ( R 2 = 0.60). If the changes in soil chemical conditions after afforestation influence the cadmium and copper release rates in a similar way as observed in the flow cell experiments, then the release rate of cadmium will increase exponentially at soil solution pH < 4.5. The inhibition of copper release by DOC observed at pH 3.8 to 5.0 indicates that copper is retained in the soil by interactions with adsorbed organic matter.