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The removal of the herbicides diuron, isoxaben, oryzalin, and clopyralid by shredded cedar mulch was studied in laboratory batch and column experiments. The distribution coefficients ([K.sub.d]) for the herbicides diuron, isoxaben, and oryzalin measured in batch experiments ranged from 129 to 187 L/kg for raw mulch and from 153 to 341 L/kg for ground mulch. [K.sub.d] could not be accurately determined for clopyralid because of its weak sorption. A reactive transport model that described adsorption-desorption using a linear two-rate (site) reversible submodel provided a good fit for the experimental breakthrough data for isoxaben from column studies; adjusting the exchange rate coefficients using molecular diffusion coefficient ratios allowed breakthrough curves for oryzalin and diuron to be predicted effectively using no adjustable parameters. Model sensitivity analysis indicates that over 80% herbicide removal efficiency can be obtained for detention times greater than 8 min and storm durations of less than 100 min for the ground mulch particles. Widespread use of mulch as landscape material and the short detention times required for reasonable removal efficiency suggests that a mulch treatment system may be an efficient best management practice: these experimental and model results provide a basis for future pilot testing. CE Database subject headings: Herbicides; Adsorption; Coefficients: Model analysis; Highways; Storm water management; Abatement and removal.

A physically based, one-dimensional, mathematical model is developed to simulate transport of herbicides applied to roadsides into adjacent surface water. Herbicide transport by overland flow, infiltrating water, and subsurface flow is considered. The model accounts for advection, dispersion, equilibrium linear sorption in the soil, and first-order decay of the herbicides. Chemical transport between soil and overland flow is described using a rate-limited mass exchange between the two compartments that is proportional to the difference between the chemical concentration in overland flow and the pore water with a proportionality constant that is a function of overland flow characteristics and rain intensity. For simulating overland and subsurface flow, the kinematic wave equation is solved using a Crank-Nicolson finite difference scheme. Richards' equation is solved using a Galerkin finite element method to model infiltration, and the transport equation is solved using a finite difference method. The model is calibrated and verified using extensive field data on the runoff of five herbicides with a range of physical-chemical properties at two field sites. Good agreement is obtained between predicted and measured event-mean concentrations in runoff and the herbicide concentration remaining in the application-zone soil. CE Database subject headings: Overland flow; Vadose zone; Contamination, Storm runoff; Mathematical models; Numerical models; Herbicides.

Perchlorate (CI[O.sup.-.sub.4]) can be reduced by iron surfaces, suggesting that permeable reactive barriers may represent a useful groundwater remediation strategy. However, chloride produced by the reaction inhibits further perchlorate removal. Adsorption of chloride on iron filings was investigated as a potential mechanism of chloride interference. The effect of chloride on the removal of nitrate, another oxyanion reactive at iron surfaces, was also investigated to draw more general conclusions about anion competition when target compounds adsorb electrostatically. A triple layer adsorption model was used to describe chloride sorption isotherms on the iron filings using magnetite as the model surface and defining a single type of surface hydroxyl sorption site. The model considered electrostatic attraction, specific sorption, and the effect of adsorbed [Fe.sup.2+] on chloride sorption. Experimental and modeling results indicate that chloride competition is probably not of concern for nitrate reduction in permeable reactive barriers. However, perchlorate reduction is significantly inhibited by chloride in both buffered and unbuffered solutions, possibly because the reactive sorbed [Fe.sup.2+] sites may be preferentially occupied by chloride. DOI: 10.1061/(ASCE)0733-9372(2005)131:6(924) CE Database subject headings: Ground-water pollution; Ion adsorption; Remedial action; Mathematical models; Chlorides; Barriers; Nitrates.

Controlled precipitation of struvite from postdigestion, sludge lagoon supernatant was studied using a pilot scale fluidized bed reactor (FBR). The process effectively and rapidly removed struvite constituents from the supernatant and produced effluent significantly undersaturated for struvite. Recently published information on struvite thermodynamic and kinetic properties was incorporated into the FBR system design and selection of components. Struvite crystals were selected as seed media, which enabled precipitation of nearly pure crystals and allowed nucleation, with its associated lag time, to be bypassed. Mixing energy in the FBR was demonstrated to be sufficient to overcome transport limitations to struvite growth, allowing growth kinetics to be governed by the surface integration rate of constituent ions into the growing crystal lattice structure. Struvite removal was optimized by incorporating pH elevation into the FBR system. Process fluid was circulated in the pH adjustment tank/FBR system, which was configured as a continuous feed stirred-tank reactor. Struvite removal efficiency exceeded 80% in continuous feed experiments where the hydraulic detention time exceeded lh.

The kinetics of struvite (MgN[H.sub.4]P[O.sub.4] [center dot] 6[H.sub.2]O) formation was studied to determine the cause of preferential accumulation at certain locations within the anaerobic digestion and postdigestion processes in wastewater treatment. Mechanisms controlling nucleation were also studied in laboratory induction time experiments. A field study of struvite accumulation on postdigestion process appurtenances was conducted to evaluate mechanisms affecting crystal growth rates. Struvite nucleation was found to be reaction-controlled, with nucleation strongly dependent on the struvite supersaturation level. Increased mixing intensity at constant supersaturation had a minor effect on induction times. The crystal growth rate was found to be transport-limited, with the growth rate a function of mixing energy input. Evaluation of the effect of pH change resulting from C[O.sub.2] evolution in high turbulence areas showed that it has a minor influence on the crystal growth rate. Preferential struvite accumulation was found to be primarily controlled by growth kinetics, which was strongly dependent on mixing energy. Preferential accumulation of struvite on selected construction materials was also studied, and experimental evidence suggested that surface roughness exerted the greatest control over preferential struvite accumulation on various materials under similar mixing and supersaturation conditions.

The removal of the herbicides diuron, isoxaben, oryzalin, and clopyralid by shredded cedar mulch was studied in laboratory batch and column experiments. The distribution coefficients (Kd) for the herbicides diuron, isoxaben, and oryzalin measured in batch experiments ranged from 129 to 187 L/kg for raw mulch and from 153 to 341 L/kg for ground mulch. Kd could not be accurately determined for clopyralid because of its weak sorption. A reactive transport model that described adsorption-desorption using a linear two-rate (site) reversible submodel provided a good fit for the experimental breakthrough data for isoxaben from column studies; adjusting the exchange rate coefficients using molecular diffusion coefficient ratios allowed breakthrough curves for oryzalin and diuron to be predicted effectively using no adjustable parameters. Model sensitivity analysis indicates that over 80% herbicide removal efficiency can be obtained for detention times greater than 8 min and storm durations of less than 100 min for the ground mulch particles. Widespread use of mulch as landscape material and the short detention times required for reasonable removal efficiency suggests that a mulch treatment system may be an efficient best management practice; these experimental and model results provide a basis for future pilot testing. [ABSTRACT FROM AUTHOR]