Peter Nkedi-Kizza

Catechol and humic acid sorption onto a range of laboratory-produced black carbons (biochars).

Although the major influence of black carbon (BC) on soil and sediment organic contaminant sorption is widely accepted, an understanding of the mechanisms and natural variation in pyrogenic carbon interaction with natural organic matter (NOM) is lacking. The sorption of a phenolic NOM monomer (catechol) and humic acids (HA) onto BC was examined using biochars made from oak, pine, and grass at 250, 400, and 650 degrees C. Catechol sorption equilibrium occurred after 14 d and was described by a diffusion kinetic model, while HA required only 1 d and followed pseudo-second-order kinetics. Catechol sorption capacity increased with increasing biochar combustion temperature, from pine < oak < grass and from coarse < fine particle size. At lower catechol concentrations, sorption affinity (Freundlich constant, K(f)) was directly related to micropore surface area (measured via CO(2) sorptometry) indicating the predominance of specific adsorption. In contrast, HA exhibited an order of magnitude less sorption (0.1% versus 1%, by weight) due to its exclusion from micropores. Greater sorption of both catechol and HA occurred on biochars with nanopores, i.e. biochars made at higher temperatures. These findings suggest that addition of BC to soil, via natural fires or biochar amendments, will sequester abundant native OM through sorption.

Sorption nonequilibrium during solute transport

Abstract The effects of pore-water velocity, solute hydrophobicity, and sorbent organic-carbon content on sorption nonequilibrium during solute transport were evaluated. Nonequilibrium transport was observed to increase with pore-water velocity, solute hydrophobicity, and sorbent organic-carbon content. Nonequilibrium transport of neutral organic compounds was not detected with low organic-carbon (TOC = 0.33 g kg−1) aquifer material, but was detected on higher organic sorbents from the unsaturated zone (TOC = 2.6 g kg−1) and the soil surface (TOC = 6.9 g kg−1). For solute-sorbent combinations yielding retardation factors > 2, nonequilibrium during transport was observed. After experimentally accounting for slow solute diffusion in the aqueous phase and isotherm nonlinearity as potential contributors to nonequilibrium solute transport, sorption nonequilibrium was attributed to slow solute diffusion within the organic-carbon matrix.

MONOLAYER TO BILAYER TRANSITIONAL ARRANGEMENTS OF HEXADECYLTRIMETHYLAMMONIUM CATIONS ON Na-MONTMORILLONITE

A low-charge Na-montmorillonite (SWy-2) was exchanged with hexadecyltrimethyl-ammonium (HDTMA) at levels equal to 20, 40, 60, 70, 80, 90, 100, 150 and 200% of the cation exchange capacity (819 mmol(+)/kg) to determine the nature of adsorption and the ionic composition of the clay interlayers. In contrast with earlier work with smaller aliphatic cations, which suggested random interstratification of interlayers occupied by either organic or metallic cations, there was no evidence of cation segregation into homogeneous interlayers. Instead, X-ray analysis indicated that the organic cations assumed two dominant configurations which were roughly equivalent in prevalence at ∼70% coverage of the CEC. Below 70% exchange the organocations existed predominantly in heterogeneous monolayers with Na+, attaining basal spacings of between 1.41 and 1.44 nm which were sensitive to changes in relative humidity. Relative humidity effects indicated that Na+ and HDTMA occupied functionally discrete domains within the interlayer as shown by the free interaction of water and a neutral organic solute, naphthalene, with Na+ and HDTMA, respectively. At greater levels of HDTMA exchange (up to 100% of the CEC), the organocations assumed a predominantly bilayer configuration. Transition to a fully-developed bilayer indicated by a 1.77 nm d-spacing at 100% coverage was gradual, suggesting some interstratification of the monolayers and bilayer configurations between 70 and 100% exchange. Sorption of naphthalene to the organoclays within this range of coverage was well correlated with clay organic carbon content, consistent with relatively unimpeded interlayer access of neutral organic molecules.

THE INFLUENCE OF MASS TRANSFER ON SOLUTE TRANSPORT IN COLUMN EXPERIMENTS WITH AN AGGREGATED SOIL

Abstract The spreading of concentration fronts in dynamic column experiments conducted with a porous, aggregated soil is analyzed by means of a previously documented transport model (DFPSDM) that accounts for longitudinal dispersion, external mass transfer in the boundary layer surrounding the aggregate particles, and diffusion in the intra-aggregate pores. The data are drawn from a previous report on the transport of tritiated water, chloride, and calcium ion in a column filled with Ione soil having an average aggregate particle diameter of 0.34 cm, at pore water velocities from 3 to 143 cm/h. The parameters for dispersion, external mass transfer, and internal diffusion were predicted for the experimental conditions by means of generalized correlations, independent of the column data. The predicted degree of solute front-spreading agreed well with the experimental observations. Consistent with the aggregate porosity of 45%, the tortuosity factor for internal pore diffusion was approximately equal to 2. Quantitative criteria for the spreading influence of the three mechanisms are evaluated with respect to the column data. Hydrodynamic dispersion is thought to have governed the front shape in the experiments at low velocity, and internal pore diffusion is believed to have dominated at high velocity; the external mass transfer resistance played a minor role under all conditions. A transport model such as DFPSDM is useful for interpreting column data with regard to the mechanisms controlling concentration front dynamics, but care must be exercised to avoid confounding the effects of the relevant processes.

Solvophobic approach for predicting sorption of hydrophobic organic chemicals on synthetic sorbents and soils

Abstract The application of a solvophobic approach for predicting the sorption of hydrophobic organic compounds (HOC) was evaluated with data collected using synthetic sorbents and soils. The experimental data consisted of batch equilibrium sorption coefficients ( K D ), as well as soil-TLC and reversed-phase liquid chromatographic (RPLC) retention factors (κ′). All data were collected using aqueous solutions and binary or ternary solvent mixtures of water, methanol, acetone, and acetonitrile. As predicted by the theory, the chromatographic retention factors and sorption coefficients for HOC decreased log-linearly with increasing fraction of organic cosolvent in binary solvents. Model parameters estimated from the binary solvent data could be used to predict sorption (or retention) from ternary solvents. Reasonable agreement was found between model parameters reported in the literature and those estimated using the data from batch sorption, soil-TLC, and RPLC studies.

Sorption of atrazine and ametryn by carbonatic and non-carbonatic soils of varied origin.

Sorption of two s-triazines, atrazine and ametryn, by carbonatic soils, Histosols, Spodosols and Oxisols was examined. Linear isotherms were observed and sorption coefficients (K(d)) of both compounds were significantly lower (α = 0.05) onto carbonatic soils compared to non-carbonatic soils. Furthermore, among carbonatic soil types, the marl-carbonatic soils had the lowest sorption affinities. K(d) and organic carbon content were highly correlated, suggesting predominant influence of organic carbon in the sorption of the s-triazine, except in Oxisols and Spodosols where variations suggest other factors. Upon removal of organic matter (OM) using sodium hypochlorite and hydrogen peroxide, the K(d) values were reduced by ~90%, indicating minimal contribution of mineral surfaces. Thus OM compositional differences likely explain the large variation in s-triazine sorption within and between soil orders. This study highlights the need to consider OM composition in addition to quantity when determining pesticide applications rates, particularly for carbonatic soils.

Water movement through an aggregated, gravelly oxisol from cameroon

Abstract Increasing population pressures have caused increased utilization of the gravelly (stone line) soils common to the hilly landscapes of equatorial Africa. This study was conducted to determine if macropore water flow and immobile-water regions should be considered in describing solute movement while developing nutrient-management strategies. Break-through curves (BTCs) from miscible displacement experiments with tritiated water were measured from 70-cm long by 9.6 cm internal diameter, water-saturated, undisturbed soil columns. Simulations produced by theoretical transport models were fitted to the BTCs to determine the magnitude of dispersivity and immobile-water regions. Gravel separates composed of kaolinite, gibbsite, goethite and manganese oxides had porosities ranging from 0.13 to 0.32 m 3 /m 3 , with a composite sample porosity of 0.20 m 3 /m 3 . Adsorption coefficients of tritium ranged from 0.031 to 0.052 ml/g for the three horizons in the soil columns. Columns containing gravel (30% by volume and 62% by weight) gave asymmetrical BTCs. A convective-dispersioe (CD) transport model was unable to simulate observed BTCs accurately. The mobile/immobile (MIM) water model provided close agreement to BTCs obtained at flow rates ranging from 2.71 to 111 cm/d. The water-saturated soil columns had about 50% of all water in immobile regions. Soil water dispersivity was 3.3 cm 2− n d n −1 (with empirical constant n =1.3) from a curvilinear plot of the dispersion coefficient and the mobile pore-water velocity. Parameters estimated from one column were applied to the BTCs of a similar soil column. The MIM model showed close agreement between the measured and the independently estimated BTCs. These soil characteristics can contribute to the rapid deep transport of a limited quantity of solute and to the storage and/or slow diffusive mass transport of the remaining solute from within immobile regions.

Effect of Tomato Packinghouse Wastewater Properties on Phosphorus and Cation Leaching in a Spodosol

Land application of wastewater is a common practice. However, coarse-textured soils and shallow groundwater in Florida present favorable conditions for leaching of wastewater-applied constituents. Our objective in this study was to determine phosphorus (P) and associated cations (Ca, Mg, K, Na) leaching in a Spodosol irrigated with tomato packinghouse wastewater. We packed 12 polyvinyl chloride soil columns (30 cm internal diameter × 50 cm length) with two soil horizons (Ap and A/E) and conducted 30 sequential leaching events by irrigating with wastewater at low (0.84 cm d), medium (1.68 cm d), and high (2.51 cm d) rates. The control treatment received deionized water at 1.68 cm d Leachate pH was lower (6.4-6.5) and electrical conductivity (EC) was higher in the wastewater-treated columns (0.85-1.78 dS m) than in the control treatment (pH 6.9; EC, 0.12 dS m) due to the low pH (6.2) and high EC (2.16 dS m) of applied wastewater. Mean leachate P concentrations were greatest in the control treatment (0.70 mg L), followed by the high (0.60 mg L) and low and medium wastewater-treated columns (0.28-0.33 mg L). Leachate concentrations of Na, Ca, Mg, and K were significantly ( < 0.05) greater in wastewater-treated columns than in the control. Concentrations of P, Na, and K in leachate remained lower than the concentrations in the applied wastewater, indicating their retention in the soil profile. In contrast, leachate Ca and Mg concentrations were greater than in applied wastewater during several leaching events, suggesting that additional Ca and Mg were leached from the soil. Our results suggest that tomato packinghouse wastewater can be beneficially land-applied at 1.68 cm d in Floridas Spodosols without significant P and cation leaching.

Characterization of Adsorption and Degradation of Diuron in Carbonatic and Noncarbonatic Soils

The adsorption and degradation of the pesticide diuron in carbonatic and noncarbonatic soils were investigated to better understand the fate and transport of diuron in the environment. Batch adsorption experiments yielded isotherms that were well-described by the linear model. Adsorption coefficients normalized to soil organic carbon content (K(oc)) were lowest for carbonatic soils, averaging 259 +/- 48 (95% CI), 558 +/- 109, 973 +/- 156, and 2090 +/- 1054 for carbonatic soils, Histosols, Oxisols, and Spodosols, respectively. In addition, marl-carbonatic soils had much lower K(oc) values (197 +/- 27) than nonmarl-carbonatic soils. Diuron degradation data fit a first-order reaction kinetics model, yielding half-lives in soils ranging from 40 to 267 days. There was no significant difference between the average diuron degradation rate coefficients of each of the soil groups studied. Given the low adsorption capacity of carbonatic soils, it may be advisable to lower herbicide application rates in agricultural regions with carbonatic soils such as southern Florida to protect aquatic ecosystems and water quality.

Extrinsic spatial variability of selected macronutrients in a sandy soil

Abstract In addition to native or intrinsic variability, the spatial variability of a soil property at a managed site includes an extrinsic component which is due to management practices. The objective of this study was to estimate changes in the extrinsic spatial variability of Ca, Mg, K, and P in the surface soil which resulted from tillage and fertilizer application. Such changes may increase or decrease total variability. The upper 15 cm of soil from a small experimental field (0.31 ha) on Millhopper fine sand (loamy, siliceous, hyperthermic, Grossarenic Paleudult) was sampled with a large bore soil probe. Soil samples were taken before and after plowing and application of mixed fertilizers. Average contents of these elements for subareas of the field were estimated using block kriging techniques. Comparisons of block-kriged estimates for Ca, Mg, K, and P before and after tillage and fertilizer application were used to assess the extrinsic spatial variability of these elements. Results of this study indicated that both tillage and fertilizer application altered the spatial distribution of these macronutrients. Attempted uniform site preparation for planting resulted, therefore, in nonuniform changes in extractable Ca, Mg, K, and P. Those elements which were applied at high rates exhibited large increases in variability and marked changes in distribution patterns. Knowledge of the pre-plant spatial variability and distribution of nutrient elements may aid in the planning and execution of field experiments or serve as a basis for soil specific crop management practices.