Data collected from both artificially and field (naturally) weathered biochar suggest that a potentially significant pathway of biochar disappearance is through physical breakdown of the biochar structure. Via scanning electron microscopy, we characterized this physical weathering that increased the number of structural fractures and yielded higher numbers of liberated biochar fragments. This was hypothesized to be due to the graphitic sheet expansion accompanying water sorption coupled with comminution. These fragments can be on the microscale and the nanoscale but are still carbon-rich particles with no detectable alteration in the oxygen:carbon ratio from that of the original biochar. However, these particles are now easily dissolved and could be moved by infiltration. There is a need to understand how to produce biochars that are resistant to physical degradation to maximize long-term biochar C sequestration potential within soil systems.

Ground and surface water contamination by herbicides applied to olive groves in Spain and other Mediterranean countries is demanding strategies to prevent and remediate the environmental problems repeatedly caused by such herbicides. In this study, six different organic cations (L-carnitine, spermine, hexadimethrine, tyramine, phenyltrimethylammonium, and hexadecyltrimethylammonium) were incorporated into Na-rich Wyoming montmorillonite (SWy-2) and Ca-rich Arizona montmorillonite (SAz-1) at two different loadings (50% and 100% of the cation exchange capacity of the clays) as a strategy to enhance the affinity of the clay minerals for three herbicides widely used in olive groves: terbuthylazine, diuron, and MCPA. The modified montmorillonites were characterized and tested as adsorbents of the herbicides through batch adsorption tests. At the experimental conditions used, some of the modified montmorillonites removed more than 95% of the herbicide initially present in aqueous solution, whereas the unmodified clays removed less than 15%. All three herbicides displayed very strong affinities for SAz-1 exchanged with hexadecyltrimethylammonium cations, particularly when these were incorporated at 100% of the cation exchange capacity of the clay mineral. Terbuthylazine and diuron also displayed very strong affinities for SWy-2 exchanged with L-carnitine and spermine, respectively. The chemical characteristics of the organic cation greatly influenced the adsorptive properties of the resultant organoclay. The herbicides were in general reversibly adsorbed by the modified clays. The results indicate that some of the tested modified clays could be suitable for the removal of the assayed herbicides from contaminated water and also as possible supports for the design of slow release formulations of such herbicides to attenuate their environmental impact when used in high-risk scenarios such as olive groves.

Sorption of four herbicides and a metabolite of indaziflam on a fresh macadamia nut biochar and biochars aged one or two years in soil was characterized. On fresh biochar, the sorption was terbuthylazine (K d = 595) > indaziflam (K d = 162) > MCPA (K d = 7.5) > fluoroethyldiaminotriazine (K d = 0.26) and nicosulfuron (K d = 0). Biochar surface area increased with aging attributed to the loss of a surface film. This was also manifested in a decline in water extractable organic carbon with aging. Correspondingly, an increase in the aromaticity was observed. The higher surface area and porosity in aged biochar increased sorption of indaziflam (K dBC-2yr = 237) and fluoroethyldiaminotriazine (K dBC-1yr = 1.2 and K dBC-2yr = 3.0), but interestingly decreased sorption of terbuthylazine (K dBC-1yr = 312 and K dBC-2yr = 221) and MCPA (K dBC-1yr = 2 and K dBC-2yr = 2). These results will facilitate development of biochars for specific remediation purposes.

Ground and surface water contamination caused by undesirable transport losses of herbicides commonly applied to olive (Olea Europaea) groves in Spain and other Mediterranean countries is demanding strategies to reduce the environmental impact of such herbicides. In this work, organoclay‐based formulations of a herbicide widely applied to olive groves, diuron [3‐(3,4‐dichlorophenyl)‐1,1‐dimethyl urea], were prepared by preadsorbing the herbicide on three organoclays. Then, the ability of the organoclay‐based formulations to reduce the herbicide release rate and leaching potential in two soils as compared to a standard commercial formulation of the herbicide was assessed. The organoclays were obtained by treating Wyoming montmorillonite (SWy‐2) and Arizona montmorillonite (SAz‐1) with an amount of spermine (SPERM) or hexadecyltrimethylammonium (HDTMA) cations equal to 100% of the cation exchange capacity of the montmorillonites. Batch release and column leaching tests indicated that the organoclay‐based formulations of diuron made the release of the herbicide slower and reduced its leaching potential in soils, as compared with a standard commercial formulation containing the herbicide in a readily available form. Soil incubation experiments revealed that the persistence of the herbicide applied to soil as organoclay‐based formulations was only slightly prolonged compared with the herbicide applied in a readily available form, and that most herbicide in the formulations was ultimately available for degradation. The use of organoclay‐based formulations is proposed as a strategy to reduce extensive transport losses associated with the application of diuron in high‐risk scenarios, such as Mediterranean olive groves.

Azimsulfuron is an acidic herbicide with a high water solubility which makes risk of groundwater contamination a concern. Various wood based biochars produced at different pyrolysis temperatures were characterized along with their sorption capacity for the herbicide azimsulfuron. In addition, we compared sorption on biochars with sorption on mineral sorbents such as clay minerals and iron oxides. In biochar formed at high temperatures (500 o C and 700 o C), FT-IR studies confirmed the increase in aromaticity. Scanning electron microscope (SEM) images of the biochars showed differences in the macroporous structure and lower size pores at higher temperatures. SSA (Specific Surface Area) of the biochars increased with pyrolysis temperature and, for all different biochars, this resulted in higher sorption of azimsulfuron. In the case of mineral sorbents, sorption is not related to SSA. Higher sorption is observed in a montmorillonite, of lower SSA, than in mixture of clay minerals with 30 % smectite (w/w). On the contrary as with the clays, sorption on the two iron oxyhydroxides increased with SSA. Desorption studies showed hysteresis. Leaching studies showed no effect on azimsulfuron retention on soil column amended with apple wood biochar, while a reduction of azimsulfuron in leachates in soil columns amended with the modified montmorillonite and alder wood biochar (500ºC). Total retention was shown for alder wood biochar.