Mesotrione reacts with hydrazides or amines to form irreversible enamine linkages, that re potentially still active. We therefore conjugated mesotrione to the nucleophilic PEG derivatives before forming the final gel. Mesotrione was incubated with PEG-CDH, PEG-ADH, or PEG-NH2 at a ratio of 1:4 between mesotrione and PEG end groups. We observed nearly quantitative reaction efficiency between mesotrione and each electrophile . The remaining PEG end groups were purposefully left unreacted to allow for further cross-linking with PEG-BA to form a hydrogel. PEG-BA was added at a ratio of 1:1 between -BA end groups and the remaining -CDH, -ADH- or -NH2 end groups. In acidic conditions and 50 °C, gels formed almost immediately with PEG-CDH and PEG-ADH through facile hydrazone click chemistry. For gels to form with PEGNH2, the reaction pH was slowly increased until gelation occurred. After screening a number of polymer concentrations , we determined 15 wt.% PEG was ideal for gelation rates and properties. Gelation was confirmed by inverting the gelation vial or, for the gel made with PEG-CDH, by rheology . The swelling ratio was calculated as 10.7 g/g by dividing the difference between the swollen weight and dry weight by the dry weight.Owing to their relatively high water solubilities, mesotrione and other cyclic b-triketone herbicides are susceptible to accumulation and leaching in soil and water compartments.The half-life of mesotrione ranges from two day to 18 days in soil with pH 4.2 to 8.3, respectively, due to degradation and dissipation.We hypothesized that by conjugating and encapsulating mesotrione within our PEG networks,large plastic pots mesotrione would release more slowly through soil compared to the free herbicide.

We observed that free mesotrione leaches at a significantly faster rate than mesotrione conjugated to PEG-CDH gel: 25.8 % ± 5.9 % and 87.4 %± 1.9 % residual mesotrione was left, respectively, in the soil after 13.2 mL of water was eluted through the column. Mesotrione is therefore retained in the soil for longer when encapsulated in the hydrogel. To further understand how mesotrione is released from the gel, we conducted aqueous release studies in various pH buffers and with the other PEG conjugates.We tested the release of mesotrione from the three PEG hydrogels in aqueous conditions, at pH 5.5 and 6.8 at 22 °C . The experiments were set up by adding hydrogels into a dialysis bag , submerging in the relevant buffer, and monitoring the external solution for release mesotrione via HPLC analysis. For all three hydrogels, only about 20 % of mesotrione released from within the gel with little to no dependence on pH or PEG conjugation. This agrees with the soil release studies and suggests that mesotrione not eluted from the soil column is likely still attached to the polymer. Since we observed quantitative conjugation and, based on the small molecule model studies discussed in Chapter 4, it is likely that the imine linkages are rearranging to irreversible enamine linkages, thus preventing mesotrione release. These conjugates may still be active as the mode of action of mesotrione only requires two of the three carbonyl groups in mesotrione to participate in b-keto-enol tautomerization.However, distortion of mesotrione planarity has demonstrated negative impacts on its affinity for and activity against HPPD, and further tests would need to be conducted to understand the conjugations’ effects. We therefore tested the efficacy of the hydrogels to see how its herbicidal activity compares to free mesotrione.Hydrogels are more appealing for pre-emergent weed treatments as they can be dried and applied directly on soil in powdered form. We tested the efficacy of mesotrione conjugated and encapsulated within the gel versus free mesotrione against a common weed, Chenopodium album . In maize, mesotrione is effective against pre-emergent Chenopodium album,recommended dosage is 150 g active ingredient per hectare.

In a small trial, we applied 150 g mesotrione ha-1 in the form of free mesotrione or in the PEG-CDH gel as a preemergent treatment. We observed that both free mesotrione and the hydrogel decrease the total number of weeds and their health compared to a control without herbicide . bTriketones inhibit the HPPD enzyme in weeds which ultimately decreases photosynthetic activity, causing bleaching and death of treated plants.18 This was directly observable in the pots treated with herbicide as there were overall fewer germinated Chenopodium album seeds. Moreover, the seeds which did germinate in herbicidal treatments were pale pink, indicating their photosynthetic capabilities were low compared to the green weeds grown from the control. Since we only observed 20 % of mesotrione release from the gels in the soil release experiments, the weed control from these gels may have been because only a fraction of the applied 150 g mesotrione ha-1 is necessary to control Chenopodium album or because the hydrolytically stable conjugate is also active against the weeds. The herbicidal activity could be tested at lower concentrations to see how much mesotrione is necessary to prevent weed growth in these experimental conditions. Alternatively, the inhibition of HPPD by the conjugate could be evaluated through an enzyme activity assay. We wanted to focus on creating a system that could effectively release free mesotrione as this would prove most efficacious for practical use. We therefore explored other possible degradable linkages to form mesotrione-polymeric conjugates.Bio-char is a biomass-derived char material intended for soil application. Bio-char soil application is regarded as a low risk strategy for sequestering carbon and reducing greenhouse gases emission .Bio-char addition to soil can have many agricultural benefits, such as improving soil quality, soil structure, and nutrient availability for plants and microbial populations. In addition, bio-char can increase soil water holding capacity, especially in sandy soils.The high specific surface area and cation exchange capacity of bio-char compared to soil can be effective in reducing leaching of nutrients such as nitrate, ammonium,and phosphorus. Bio-char is a not fully carbonized product produced by pyrolysis of biomass in a low oxygen environment.

Pyrolysis processing factors such as temperature, residence time and oxygen content affect bio-char characteristics, such as surface area, functional groups and bio-char stability.The effects of bio-char soil amendment on, for instance, soil water holding capacity,nutrient retention,herbicide sorption, and reducing heavy metal bio-availability vary with bio-char characteristics. The variation in effects of different bio-char products presents an opportunity to select or even create a bio-char that best matches the needs of a particular agricultural and environmental application. Phenylurea herbicides are widely utilized for herbaceous and perennial weed control of non-crop areas and for pre-emergent treatment of fruit crops.Globally, they are detected in surface water, ground water, soil and sediment in areas wherever there is extensive use.Herbicides leach through soil into groundwater during rainfall and irrigation and may persist in soil and water for a long period of time.Cost-effective and environmentally friendly technologies, such as bio-char amendment, are needed to reduce these losses and impacts of residues in soils. Our objective was to investigate the potential impact of bio-char on phenylurea herbicide environmental behavior and fate. We chose 5 bio-chars, with representative surface characteristics and produced from different common feed stocks, and monuron, diuron and linuron sorption isotherms on bio-chars were measured. The three phenylurea herbicides were selected based on their wide application in agriculture and also their representative lipophilicity among phenylurea herbicides. Sorption of monuron, diuron and linuron on all five bio-chars and soil was determined via batch isotherms. The batch sorption experiments were conducted using 8 mL glass vials with polytetrafluoroethylene -lined screw caps. The initial aqueous phase diuron, plastic pots for plants linuron and monuron concentrations were 0, 0.5, 1, 5, 25 and 50 mg L−1 with. 0.12 g sorbent in 5.2 mL of background solution . Prior to addition of herbicides the vials with sorbent and background solution were reacted on an end-over-end shaker for 48 h to saturate bio-char. Following this pre-equilibration step, the pH was adjusted to approximately 7.0 using 0.05 mol L−1 HCl to minimize pH effects on sorption results. Different amount of herbicides and background solution were added into vials to reach the desired concentrations in a total volume of 6.0 mL. Based on our preliminary study, samples were spiked with the pesticides and rotated in the dark on an end-over-end shaker at 22 ± 1 °C for 48 h. The supernatants were filtered through 0.45 µm Millipore membrane filter and the filtrate collected into 2 mL amber LC vials. External standards were similarly filtered to correct for solute loss due to filtration. Control samples with no bio-char containing sorbate and with bio-char without sorbate were concurrently established with the sorption samples. All experiments were conducted in triplicates. Concentrations of monuron, diuron and linuron in in the aqueous phase were analyzed using an Agilent 1200 Series HPLC System with a DAD and an Agilent ZORBAX Eclipse Plus C18 column, 4.6 mm × 250 mm, 5 μm at a flow rate of 0.4 mL min−1 and an injection volume of 5 μL. The ultraviolet DAD was set at 254 nm for diuron, linuron and monuron determination. Isocratic elution was performed with 0.1% formic acid in water and 0.1% formic acid in methanol. Under these conditions, the elution times were approximately 2.2 min, 3.7 min, and 4.9 min for monuron, diuron, and linuron respectively. Compounds were identified by comparing their retention time values with those of standards. Data was collected and processed using Agilent Chemstation software. The limit of detection for monuron was 1.16 mg L−1, while the limit of detection for diuron was 0.091 mg L−1 for diuron and 0.066 mg L−1 for linuron. The bio-chars used were pyrolyzed from a variety of feed stocks at temperatures ranging 510 to 900 °C and the measured physical and chemical characteristics of these bio-chars varied accordingly . Pyrolysis temperature is the primary factor for differences in the observed physical and chemical characteristics of bio-char. Of these physical characteristics, surface area showed the greatest variability, ranging from 2 to 227.1 m2 g−1.

Generally, a high pyrolysis temperature resulted in a higher surface area. The increase in bio-char surface area with increasing pyrolysis temperature has been previously reported.However, it should be noted that the surface area of bio-chars depends not only on pyrolysis temperature, but also on characteristics of source materials, heating rate and reaction time. Increasing pyrolysis temperatures also can result in greater CO2 release,generally resulting in higher ash contents . Pyrolysis temperature also has a distinct impact on chemical properties of the bio-chars, with aromaticity increasing with pyrolysis temperature as indicated by the decreasing H/C ratios. The atomic ratio of H/C is an index for aromaticity and polarity and this result is in agreement with other studies also showing the ratio of aromatic carbon increases with an increase in pyrolysis temperature.The data describing herbicide sorption to the five bio-chars fit well to a Freundlich model . The one exception was WF bio-char, which fit better to the Langmuir model. The WS bio-char has the largest capacity of the bio-chars for herbicide adsorption and the equilibrium concentrations for all sorption experiments were below detection limit. According to the Freundlich model, nonlinear sorption isotherms were observed in all cases. The KF values suggest that sorption of herbicides in bio-chars decreased in intensity according to the following sequence: linuron > diuron > monuron and the sorption of monuron, diuron and linuron in bio-chars decreased in intensity according to the following sequence: WS > WF > TL > EB > HW. The KF values are all greater than those observed for Yolo soil. These data demonstrate that the herbicides had a higher affinity for all bio-chars than for Yolo silt loam soil. Adding bio-char into soil has potential to significantly increase phenylurea herbicide sorption, which could impact the transmission, fate and effectiveness of herbicides. WS bio-char shows excellent sorptive properties among bio-chars. The relationship between initial concentration of herbicides in aqueous solution and the amount of herbicide adsorbed per unit mass of adsorbent also reflects the sorption capacity of adsorbents . The results indicate that the sorption capacity of WS bio-char to all three kinds of herbicides were the highest among bio-chars. The parameter qe in Freundlich model indicated the highest sorption capacity of WS bio-char among all the bio-chars for all three herbicides. Sorption efficiency of WS bio-char remained 100% for all initial concentrations of diuron and linuron and greater than 95% for monuron .