Kinetic Intersections as a Source of Information on Rate Coefficients

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Modeling Physical and Chemical Nonequilibrium Transport of Herbicide in Soils From Different Tillage Systems.

The physical and chemical nonequilibrium transport of alachlor were studied in a surface Gigger soil from different tillages through tracer studies, and batch and miscible displacement experiments. Batch experiments indicated initially fast reaction followed by slow adsorption. Adsorption and desorption results indicated time dependent hysteretic behavior and was best described by a multireaction model incorporating nonlinear equilibrium reaction, a reversible kinetic mechanism, and a consecutive irreversible mechanism. The model predicted alachlor hysteresis and adsorption-desorption kinetics satisfactorily based on parameters obtained from adsorption experiments. Tracer (Eosin Y and Blue FCF dyes) studies showed non-uniformly stained areas in undisturbed soil cores (6.4 cm i.d, 15 cm length) and indicated more pronounced preferential flow and physical nonequilibrium solute transport in no-till than in conventional tillage. Tritium breakthrough curves (BTCs) indicated earlier breakthrough associated with bimodal peaks in short pulses for no-till. The shape of BTCs were also dependent on flow direction. The superimposed experimental data from short pulses well predicted the data of long pulses. The classical convective-dispersive equation was inadequate and there was no improvement in describing tritium BTCs using physical nonequilibrium models (mobile-immobile and stochastic models) for soils from no-till. Miscible displacement results indicated that alachlor BTCs in soils of no-till were more asymmetrical, with earlier breakthrough and longer tailing than soils from conventional tillage. A multireaction transport model (MRTM) was not satisfactory for alachlor prediction using independently measured parameters from batch experiments. However, MRTM successfully described alachlor BTCs in a calibration mode where physical and chemical nonequilibrium were dominant. Best-fit parameters indicated the dominance of kinetic reactions compared with parameters from batch experiments and may be attributed to soil heterogeneity. Although no-till increased alachlor retention in batch experiments, an overall estimation based on the sum of kinetic and equilibrium retention showed no significant influence on retention by tillage. High pressure liquid chromatography (HPLC) chromatograms, fitted transport parameters, flow interruptions and percent recoveries indicated a significant consecutive irreversible reaction in soils of conventional tillage. Moreover, no-till increased alachlor transport based on breakthrough time compared with conventional tillage

Transport and Retention of Cadmium, Copper, and Lead in Soils: Miscible Displacement Experiments

Heavy metal contamination is a major concern for soil and water quality. To quantify their potential hazard, it is crucial to understand their mobility and retention in soils. The source of the problem is anthropogenic activities such as mining, smelting, usage of sewage sludge and fertilizers. The objective of this study was to quantify Cd, Cu and Pb transport and retention in three soils having different properties, and evaluate their competitive influence on the mobility of individual heavy metal. The second objective was to quantify Cd, Cu and Pb retention kinetics in the different soils. This study also investigates the extent and distribution of retained Cd, Cu and Pb with depth in soil columns. In the study, batch experiments were carried out for different range of concentrations in time. The results indicated that retention of Cd, Cu and Pb in Windsor, Mahan and Webster soil is nonlinear. Lead exhibited highest retention among all three metals. Moreover, all three metals exhibited highest affinity in Webster soil, which has a higher clay content (mostly smectite), organic material and cation exchange capacity. Sorption of all metals was also observed to be kinetic when retention time increased from one day to seven days. Miscible displacement experiments in saturated soil columns were also carried out in two ways. In the first type, consecutive pulses of Cd, Cu and Pb were sequentially introduced to each soil column followed by an extended period of leaching with the background solution (KNO3, 0.005M). In the second type, two consecutive pulses of mixed solution (Cd, Cu and Pb), each followed by leaching with the background solution, were introduced to soil columns. Results indicate that Cd was the most mobile with the highest recovery the effluent solution, whereas Pb was the least mobile with the lowest recovery among all elements and soils. It was also observed that Pb resulted in enhanced mobility of both Cd and Cu . Efforts to describe results from the column experiments based on a multirection and transport model (MRTM) showed varied degrees of success. Although the models accounts for several sorption mechanisms including nonlinear equilibrium, kinetics, and irreversible reactions, the model was not successful in predicting the competitive behavior of heavy metals in the soil columns

Stability of a commercial lipase from Mucor javanicus: kinetic modeling of pH and temperature dependencies

The present communication reports experimental and modelling work pertaining to the independent roles of pH and temperature on deactivation of a crude lipase from Mucor javanicus. Experimental data oflipolytic activities were generated by a classic pH-stat assay on a triolein emulsion following incubation at several pH values for a fixed time, or at several temperatures for various times; postulated models were then fitted by nonlinear fitting to such data. The pH-dependence data were best fit by assumption of three forms of enzyme with increasing states of protonation, with pKa values of 6.2 and 11.3, respectively, where only the intermediate form is stable within the time frame considered. The thermal-dependence data were best fit by assumption of parallel steps of deactivation and rearrangement, with activation energies of 228.8 and 221.7 kJ mol~l, respectively

Transport and adsorption-desorption of heavy metals in different soils

Understanding the reactivity and mobility of heavy metals in soils is indispensable for assessing their potential risk to the environment. In this study, column transport and batch kinetic experiments were performed to assess the sorption-desorption and mobility of Cd, Cu, Pb, and Sn in alkaline and acidic soils. Furthermore, sequential extractions were accomplished to examine their behavior in soils. Also, the competitive reactivity of Sn and Pb in two acidic soils was quantified. Additionally, the effect of introducing Cd and Cu after a Pb pulse in calcareous soil was presented. Modeling of these heavy metals retention and transport was carried out using different models; multireaction and transport model, CXTFIT model, kinetic ion exchange formulation, and second-order two-site model. The results revealed that: 1) the studied heavy metals exhibited strong nonlinear and kinetic retention behavior; 2) Cd was nearly immobile in alkaline soil with 2.8% CaCO3, whereas 20 and 30% of the applied Cd was mobile in the acidic soil and the subsurface layer of the alkaline soil with 1.2% CaCO3, respectively; 3) for a short Cu pulse, the recoveries were \u3c1 and 11% for alkaline and acidic soils, respectively, whereas, for the long Cu pulse, the recoveries ranged from 27 to 85% for the studied soils; 4) tin was highly sorbed in acidic soils where more than 99% of applied Sn was retained in the acidic soils columns; 5) the presence of Sn in solution reduced Pb retention in soils since the Pb recovery in the effluent solution ranged from 37.4 to 96.4%; and 6) the multireaction approach was capable of describing heavy metals retention and transport in soil columns. Moreover, a field study of the spatial distributions and the accumulation of Pb, Cd, Cu, and Ni among soil depth as consequence of irrigation with domestic wastewater were studied. The results of this research showed that Pb, Cu, and Ni had high affinity for retention in the surface soil layer whereas Cd results showed homogeneous distribution within soil depth. The impact of time scale effect on accumulation and spatial distribution of heavy metals indicated the urgent need for remediation and rational management

Retention and transport of mercury and nickel in soils

Nickel (Ni) is one of many trace metals widely distributed in the environment. High concentrations of Ni in soils and aquifers have been observed worldwide, causing several potential human health impacts. Better understanding of Ni transport in soils and aquifers is necessary to assess and remediate insitu environmental contamination. The movement of Ni in soils and aquifers is highly dependent on adsorption-desorption reactions in the solid phase. In this study, kinetic batch, sequential extractions, and miscible displacement experiments were conducted to investigate the effect of several of environmental factors including soil type, reaction time and competing ions, on the fate of Ni in soils. In addition, forward and inverse modeling efforts were made to mathematically predict the reactivity of Ni transport in soils. Based on batch study results, adsorption of Ni was highly nonlinear and strongly kinetic. The comparison of Ni sorption on soil followed the sequences: Windsor \u3c Olivier \u3c Webster, which was related to soil propertities (CEC, clay content, pH and organic matter). Desorption of Ni from all soils were hysteretic in nature which is an indication of lack of equilibrium retention and/or irreversible or slowly reversible processes. A sequential extraction procedure provided evidence that a significant amount of Ni was irreversibly adsorbed on all soils. Moreover, a multi-reaction model (MRM) with equilibrium, kinetic and irreversible sorption successfully described the adsorption kinetics of Ni in Windsor, Olivier and Webster soils and was capable of predicting the desorption of Ni from these soils. Column transport experiments indicated strong Ni retardation followed by slow release or extensive tailing of the breakthrough curves (BTCs). We evaluated several MRM formulations for prediction capability of Ni retention and transport in soils and concluded that nonlinear reversible, along with a consecutive or concurrent irreversible reactions were the dominant mechanisms. The use of batch rate coefficients as model parameters for the predictions of Ni BTCs underestimated the extent of retention and overestimated the extent of Ni mobility for all soils. When utilized in an inverse mode, the MRM model provided good predictions of Ni BTCs and the distribution of Ni with soil depth in soil columns. In natural soil and water environments the competition between Ni and Cadmium (Cd) has the potential of increasing Ni mobility and bioavailability. Our results from batch experiments demonstrated that rates and amounts of Ni adsorption by these soils were significantly reduced by increasing Cd additions. The presence of Cd in soils increased mobility of Ni in columns as well as forced Ni sorption at higher affinity (or specific sorption) sites. The simultaneous presence of Ni and Cd also changed the distribution of Ni and Cd from an accumulation pattern to a leaching pattern in Olivier soil column, which has the potential risk of contamination of ground water

Retention and Transport of Arsenic in Soils

Arsenic transport in soils and aquifers is highly dependent on the adsorption-desorption reactions in the solid phase. Results from our kinetic batch experiments indicated that adsorption of arsenate [As(V)] was highly nonlinear and strongly kinetic. Desorption of As(V) were hysteretic in nature and a significant amount of As(V) was irreversibly adsorbed on all soils. Results from column experiments indicated strong As(V) retardation followed by slow release or extensive tailing of the breakthrough curves (BTCs). Sharp decrease in As(V) concentration during flow interruption verified the extensive non-equilibrium condition which was likely due to the dominance of kinetic retention processes. We evaluated several multireaction model (MRM) formulations for its prediction capability of As(V) retention and transport in soils and concluded that nonlinear reversible along with a consecutive or concurrent irreversible reactions were the dominant mechanisms. The use of batch rate coefficients for the predictions of As(V) BTCs underestimated the extent of retention and overestimated the extent of As(V) mobility for all soils. When utilized in an inverse mode, the MRM model provided good predictions of As(V) BTCs. The competition between arsenate and phosphate (P) has the potential of increasing arsenic mobility and bioavailability in soils. Our kinetic batch studies demonstrated that rates and amounts of As(V) adsorption by soils were significantly reduced by increasing P additions. In a separate experiment, the presence of P in soils increased mobility of As(V) in saturated columns. The use of flow interruptions verified the dominance of time-dependent sorption during As(V) and P transport in soils. We further extended the MRM model to simulate retention and transport of multiple solutes in soils. The formulated multicomponent multireaction model was capable of predicting the competition effect of P on As(V) retention and transport given appropriate kinetic coefficients. The mobilization of colloidal particles and its effect on the transport of arsenite [As(III)] was investigated using miscible displacement experiments. Mobilization of colloidal amorphous material and enhanced transport of As(III) was observed when the input solution was replaced with deionized water. Peaks of colloid generation coincided with the peak concentrations of Fe, indicating mobilization of Fe oxides and facilitated transport of As(III)

Silver Transport and Adsorption-Desorption in Soils: Influence of Zinc

Transport of heavy metals such as Ag is affected by several rate-limiting processes including adsorption and release reactions in soils. In this study, the objective was to qualify adsorption-desorption behavior and transport of silver in the different soils. This study also investigated the influence of the presence of Zn on Ag retention and transport in soils. Kinetic batch adsorption-desorption and column experiments were carried out to investigate the adsorption-desorption and transport of silver in soils having different properties in the presence of Zn. Transport of Ag was carried out using miscible-displacement experiments in water saturated soil columns. For all soils, results indicated that adsorption isotherms for Ag were highly nonlinear with greater affinity for Webster soil. Moreover, the presence of Zn resulted in reduced Ag sorption indicative of competitive behavior. Measured Ag breakthrough results (BTCs) from the column experiments indicated highest Ag mobility in Olivier soil whereas Webster soil exhibited least mobility. This finding is based on the Ag recovered and the retardation of the arrival of Ag in the effluent solution. Furthermore, the presence of Zn resulted in enhanced mobility of Ag. A multireaction and transport model (MRTM) that accounted for nonlinear reversible kinetics and irreversible reactions was capable of describing both Ag and Zn transport in all soil columns