Lewis A. Gaston

Enhancing phosphate adsorption by Mg/Al layered double hydroxide functionalized biochar with different Mg/Al ratios

Mg/Al ratio plays a significant role for anion adsorption by Mg/Al-layered double hydroxides (Mg/Al-LDHs) modified biochar. In this study, Mg/Al-LDHs biochar with different Mg/Al ratios (2, 3, 4) were prepared by co-precipitation for phosphate removal from aqueous solution. Factors on phosphate adsorption including Mg/Al ratio, pH, and the presence of other inorganic anions were investigated through batch experiments. Increasing Mg/Al ratio in the Mg/Al-LDHs biochar composites generally enhanced phosphate adsorption with Langmuir adsorption maximum calculated at 81.83mg phosphorous (P) per gram of 4:1Mg/Al-LDHs biochar at pH3.0. The adsorption process was best described by the pseudo-second-order kinetic model. Solution pH had greater effects on the phosphate adsorption by Mg/Al LDHs biochar composites with lower Mg/Al ratios. The presence of other inorganic anions decreased the phosphate adsorption efficiency in the order of F(-) > SO4(2-) > NO2(-) >Cl(-). Phosphate adsorption mechanism involves ion exchange, electrostatic attraction and surface inner-sphere complex formation. Overall, Mg/Al-LDHs biochar composites offer a potential alternative of carbon-based adsorbent for phosphate removal from aqueous solution.

Tillage and cover crop effects on cyanazine adsorption and desorption kinetics

Accumulation of partially decomposed plant residues under no-tillage (NT) and cover crop management systems can affect herbicide fate in the soil. This study evaluated adsorption and desorption of cyanazine {2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl]amino] -2-methylpropanenitrile} in soils and herbicide-killed Italian ryegrass (Lolium multiflorum Lam.) residues collected from a long-term conventional tillage (CT) and NT cotton field. The four cotton production systems included were CT and NT, each with and without ryegrass as a cover crop. Adsorption was determined by reacting 0.5 g of soil or ryegrass residue with 8 mL of 14 C-cyanazine solution (five concentrations: 0.13 to 15.68 μmol L -1 ) for 48 h. The Freundlich K f values were higher in NT than in CT soils and higher in soils from ryegrass cover crop than in soils from no cover crop. The K f was higher in ryegrass residue (13.33) than in soils (1.77 to 2.94). The N values for soils (>0.90) and ryegrass residue (>0.95) indicated nearly linear adsorption. Time-course adsorption data analyzed by an equilibrium/kinetic model indicated that adsorption was rapid initially (within 1 h), followed by a slow increase in CT and NT soils from ryegrass plots. In contrast, adsorption achieved equilibrium within 48 h of reaction time in ryegrass residue. Cyanazine adsorption increased with increased decomposition of plant residues. The K f for ryegrass residues sampled at 5 weeks after cotton planting was 17% higher than the residues sampled at 3 weeks before planting. The CaCl 2 -desorbable cyanazine in two consecutive 24-h cycles ranged from 77 to 88% in soils and from 46 to 47% ofthat adsorbed in ryegrass residues. Two additional 24-h desorptions with methanol removed most of the remaining cyanazine. Under field conditions, the plant residues on the soil surface in NT and cover crop systems can apparently intercept and temporarily retain cyanazine.

Metribuzin mobility and degradation in undisturbed soil columns

Metribuzin [4-amino-6-(1–1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] is a widely used soil-applied herbicide, and its dissipation was assessed using undisturbed soil columns to simulate field conditions. Metribuzin (technical grade and 14C-labeled dissolved in 0.1 M CaCl2) was applied to the soil (Dundee loam) surface at a concentration equivalent to 0.68 kg metribuzin ha−1. Water was added to the surface at 1, 8, 15, 22, 29, and 36 days after treatment. Chloride (Cl−) and 14C were measured in the effluent. Twenty-four hours after each leaching, three cores were randomly selected, sectioned at 0–3.3-, 3.3–6.6-, and 6.6–10-cm depth increments, and analyzed. Metribuzin mineralization was monitored weekly by trapping 14CO2-C in 0.5 M NaOH. The pattern of Cl− appearance in the effluent indicated possible preferential flow. Metribuzin degradation in the 10-cm profile followed first-order kinetics, with parent metribuzin being the dominant extractable species until the last 14 days when a composite of unidentified polar metabolites was prevalent. Metribuzin was the primary single component measured in the effluent until 37 days after application. However, from 9 days after herbicide application through the remainder of the experiment, the fraction of total applied 14C present as metabolites (summation of known metabolites diketometribuzin [DK], deaminated metribuzin [DA], and deaminated diketometribuzin [DADR] and unidentified polar metabolites) exceeded metribuzin in the effluent. As an average over time, the 3.3–6.6-cm section contained the most methanol unextractable 14C

Conservation cotton production in the southern United States: herbicide dissipation in soil and cover crops

Abstract Soil and surface residues from cotton field studies in Stoneville, MS (1994 through 1996) and Florence, SC (1995 through 1996) were sampled to evaluate effects of cover crop and tillage on herbicide dissipation. Mississippi treatments included tillage (conventional [CT]; none [NT]) and cover crop (ryegrass; none [NC]). South Carolina treatments included tillage (CT; reduced tillage [RT]) and cover crop (rye; NC). Fluometuron was applied preemergence (PRE) in both Mississippi and South Carolina, and norflurazon was applied PRE in Mississippi. Soils were sampled various times during the growing season (depths: 0 to 2 cm, 2 to 10 cm). Cover crop residues were sampled from RT or NT cover crop areas. Soil and cover crop sample extracts were analyzed for herbicides. Soil organic carbon tended to increase with tillage reduction and presence of cover crop and was positively correlated with herbicide sorption, especially in the surface. Across locations, herbicide half-lives ranged from 7 to 15 d in the soil surface. Tillage had mixed effects on herbicide persistence in surface soil, with higher herbicide concentrations in CT at early samplings, but differences were insignificant later on. The most consistent effects were observed in RT/NT with cover crops, where cover crop residues intercepted applied herbicide, impeding subsequent movement into soil. Herbicide dissipation in cover crop residues was often more rapid than in soil, with half-lives from 3 to 11 d. Herbicide retention in cover crop residues and rapid dissipation were attributed to strong herbicide affinity to cover crop residues (e.g., fluometuron Kd = 7.1 [in rye]; Kd = 1.65 [in Mississippi Dundee soil CT, NC]) and herbicide co-metabolism as cover crop residues decomposed. A fluometuron metabolite, desmethyl-fluometuron, was observed in most soil and cover crop samples after 1 wk. Only minimal herbicide or metabolite moved into the subsurface, and little treatment effect could be ascribed to herbicide or metabolite movement below 2 cm. Nomenclature: Desmethyl fluometuron; fluometuron; glyphosate; norflurazon; paraquat; pendimethalin; cotton, Gossypium hirsutum L. ‘Stoneville 506’, ‘Delta & Pine Land DES 119’; Italian ryegrass, Lolium multiforum Lam.; rye, Secale cereale L.

PREDICTING ALACHLOR MOBILITY USING BATCH SORPTION KINETIC DATA

The ability to accurately predict the mobility of a soil-applied herbicide is important to product registration and recommendations for environmentally safe use. Commonly, mobility is affected by time-dependent sorption processes. Parameters for such nonequilibrium sorption may be conveniently determined from batch kinetic studies; however, applicability of these parameters for subsequent use in transport modeling must be verified. The objective of this study, therefore, was to determine whether batch kinetic data were appropriate for predicting nonequilibrium sorption during alachlor [2-chIoro-N-(2,6-diethylphenyl)-N-(methoxymethyl)-acetamide] transport in Dundee silt loam soil. Batch (solution and extraction) data previously generated in this laboratory were used to determine sorption parameters for two- or three-site equilibrium/kinetic sorption models. Predictions of alachlor mobility generated from the corresponding transport models were then compared with miscible displacement data. Use of sorption parameters determined from batch solution data alone resulted in only qualitative predictions of alachlor movement. However, more accurate predictions were obtained if sorption parameters were based on both soil extraction and soil solution data.

Degradation kinetics assessment for the fungicide bas 505 in intact soil cores versus batch soils

Field degradation rates of pesticides are often different than determined in the laboratory using homogeneous soil. This project developed an intact soil core method for determining aerobic degradation rate that is intended to address such discrepancies. The fungicide BAS 505 (phenyl-U-14C-labeled) [N-methyl-(E)-2-methoxyamino-2-(2-((2,5-dimethylphenoxy)methyl)phenyl)acetamide] was applied to surface 0-7.5 cm of Ruston (fine loamy, siliceous, thermic Typic Paleudults) soil cores (triplicate and duplicate series) and homogeneous (batch) soil in biometer flasks (triplicate). Recovery was measured 12 times over the 360-day incubation. Mineralization rate in cores was initially slower than in batch soil but increased to give more 14CO2 lost by Day 360 (11% and 8% of applied, respectively). Unextractable 14C was 21% in cores, similar to that for batch soil (18%). Recovery of BAS 505 in combined MeOH and MeOH-water extracts (HPLC-LSC analysis) after 360 days averaged 36% from cores and 57% from batch soil. Degradation rate in both systems decreased over time and could be described by Nth order kinetics but not first order. Recoveries of BAS 505 by 360 days were lower in cores, indicating faster degradation than in batch soil after long-term incubation. Lack of nutrient inputs may account for decreasing degradation rates; however, decreasing microbial activity with time was not shown by the highly variable biomass C data. Faster degradation in cores may have been due to higher microbial populations/nutrient levels in the surface soil.

Facilitative capture of As(V), Pb(II) and methylene blue from aqueous solutions with MgO hybrid sponge-like carbonaceous composite derived from sugarcane leafy trash

Enhancing the contaminant adsorption capacity is a key factor affecting utilization of carbon-based adsorbents in wastewater treatment and encouraging development of biomass thermo-disposal. In this study, a novel MgO hybrid sponge-like carbonaceous composite (HSC) derived from sugarcane leafy trash was prepared through an integrated adsorption-pyrolysis method. The resulted HSC composite was characterized and employed as adsorbent for the removal of negatively charged arsenate (As(V)), positively charged Pb(II), and the organic pollutant methylene blue (MB) from aqueous solutions in batch experiments. The effects of solution pH, contact time, initial concentration, temperature, and ionic strength on As(V), Pb(II) and MB adsorption were investigated. HSC was composed of nano-size MgO flakes and nanotube-like carbon sponge. Hybridization significantly improved As(V), Pb(II) and methylene blue (MB) adsorption when compared with the material without hybridization. The maximum As(V), Pb(II) and MB adsorption capacities obtained from Langmuir model were 157 mg/g, 103 mg/g and 297 mg/g, respectively. As(V) adsorption onto HSC was best fit by the pseudo-second-order model, and Pb(II) and MB with the intraparticle diffusion model. Increased temperature and ionic strength decreased Pb(II) and MB adsorption onto HSC more than As(V). Further FT-IR, XRD and XPS analysis demonstrated that the removal of As(V) by HSC was mainly dominated by surface deposition of MgHAsO4 and Mg(H2AsO4)2 crystals on the HSC composite, while carbon π-π* transition and carbon π-electron played key roles in Pb(II) and MB adsorption. The interaction of Pb(II) with carbon matrix carboxylate was also evident. Overall, MgO hybridization improves the preparation of the nanotube-like carbon sponge composite and provides a potential agricultual residue-based adsorbent for As(V), Pb(II) and MB removal.

Losses of surface runoff, total solids, and nitrogen during bermudagrass establishment on levee embankments.

Nutrient and sediment runoff from newly constructed levee embankments pose a threat to water quality during soft armor vegetation establishment. Research was initiated in 2008 and 2009 to evaluate the effect of bermudagrass ( L.) coverage and N source on nutrient and sediment runoff from levee embankments during establishment. Bermudagrass plots were seeded at 195.3 kg pure live seed ha and fertilized at 50 kg N ha using a water-soluble N source, urea or NH-NO, or slow-release N source, S-coated urea (SCU) or urea formaldehyde (UF), with controls unfertilized. Vegetative cover percentage, time until the onset of runoff, runoff volume, and total solids (TS), NO-N, and NH-N concentrations were measured from simulated and natural rainfall events for 70 d in 2008 and 56 d in 2009. Bermudagrass at 90% grass cover delayed the onset of runoff an additional 441 to 538 s and reduced runoff volumes 74 to 84% of that exhibited at 10% grass cover. Nitrogen fertilizers did not accelerate bermudagrass growth sufficiently, however, to reduce TS loading compared with unfertilized bermudagrass in either year of the study. The application of urea and SCU resulted in cumulative N losses of 2.45 and 3.13 kg ha compared with 1.59 kg ha from the unfertilized bermudagrass in 2008, and 1.73 kg ha from NH-NO vs. 0.24 kg ha from controls in 2009. Only UF increased bermudagrass establishment without increasing cumulative N losses compared with unfertilized bermudagrass. Therefore, the benefit of greater erosion and runoff resistance expected from N-accelerated vegetative growth did not occur but had the unintended consequence of higher N losses when water-soluble N and SCU fertilizers were applied.

Dimoxystrobin Sorption and Degradation in Sandy Loam Soil: Impact of Different Landscape Positions

Abstract Pesticide sorption and degradation may be affected by soil spatial variability ranging from field to pore scale. The study objectives were to (i) determine whether soil sampling by landscape position (shoulder, side slope, and foot slope) accounts for the range in soil properties that may affect pesticide sorption and degradation, (ii) measure the sorption and degradation of the fungicide dimoxystrobin ((DMS) (E)-2-(methoxyimino)-N-methyl-2-[&agr;-(2,5-xylyloxy)-o-tolyl]acetamide) at these positions, and (iii) determine the effect of disrupting natural soil pore geometry (homogenizing) on DMS degradation. A 50 × 90–m section of grassed hillside on Ruston soil (fine-loamy, siliceous, thermic Typic Paleudult) was sampled on a 10-m grid to 7.5-cm depth, and enzymatic activity, organic C, pH, and texture were measured. Shoulder position soil had more clay and lower pH than soil downslope. Sorption and degradation of DMS in surface 7.5 cm homogenized or intact core samples from these landscape positions were determined. Soil treated with 14C-labeled DMS at 0.28 kg ha−1 was incubated at 23°C up to 120 days, then extracted with methanol, and analyzed for DMS (high performance liquid chromatography) and non–DMS 14C. Sorption was nearly linear and greatest on shoulder soil, with a distribution coefficient of 5.4 L kg−1 compared with 3.8 L kg−1. Degradation of DMS was slow (∼65% recovery at 120 days) and unaffected by either homogenizing soil or landscape position. However, unextractable and mineralized 14C was greater in homogenized than that in intact soil and generally increased downslope, perhaps reflecting more favorable conditions for biodegradation or decreasing sorption. Degradation of DMS followed nonlinear kinetics.

Tillage Effects on Soil Properties and Spatial Variability in Two Mississippi Delta Watersheds

This study evaluated changes in soil properties several years after implementation of conservation measures. Two approximately 50-ha fields within two Mississippi Delta oxbow lake watersheds (Deep Hollow and Beasley) were laid out in 60-m grids. Soil from a tilled cotton field in Deep Hollow watershed was sampled at each node in 1996 and again in 2000 after 4 years of reduced tillage cotton (Gossypium hirsutum) and winter wheat (Triticum aestivum L.) cover crops; soil also was collected from conventional tillage cotton in Beasley Lake watershed in 1996 and again in 2006 from the same grid nodes after 4 years of reduced tillage cotton and 5 years of reduced tillage soybeans. Organic matter levels in the soil surface were higher in both watersheds after conservation tillage was implemented, likely caused by increased plant residue accumulation and limited soil mixing. Higher soil P levels in both watersheds under conservation management were attributed to less distribution in soil because of reduced tillage. Lower NO3-N in Deep Hollow in 2000 suggested N immobilization in the soil surface. Nitrogen was not analyzed for Beasley soil samples. Soil pH values were also higher in the later samplings for both watersheds, but because lime was applied in the interim, it is difficult to ascribe an effect from tillage. Potassium, calcium, and magnesium were higher in 2000 after the 4-year reduced tillage practice in the Deep Hollow watershed. However, potassium, calcium, and magnesium were lower in 2006 than in 1996 at the Beasley Lake watershed because no fertilizer was applied to the field since 2001. Regardless of tillage system, relatively high P levels in soils from both watersheds are indicative of high native soil P levels in the Mississippi Delta soils. Similar to other studies, water-soluble P was positively correlated with Mehlich III P. Spatial relationships of P examined using kriging showed that P data were spatially dependent, and their spatial dependence was impacted by tillage practices.