To slow the rise in atmospheric carbon dioxide concentrations, Enhanced Silicate Weathering is emerging as a potentially significant Carbon Dioxide Removal technology. However, the biotic controls on rock weathering are not well understood, particularly for key soil faunal groups such as earthworms. Earthworms have shown to possibly enhance weathering, highlighting their potential to be introduced in controlled or engineered settings, such as reactors, to increase carbon sequestration. Here, we determined the potential for earthworms to thrive and to increase weathering rates in an artificial organo-mineral system simulating a bioreactor. We used two earthworm species (Aporrectodea caliginosa [Savigny] and Allolobophora chlorotica [Savigny]) at four densities (10, 20, 25 and 30 earthworms kg-1 organo-mineral mixture), four silicate rock types (two basanites, dunite and diabase) of two to three grain sizes (d50 between 0.026 and 1.536 mm), two sources of organic materials (straw and co-digestate), two amounts of biochar (0 and 100 g kg-1 organo-mineral mixture) and/or enzyme additions (laccase, urease and carbonic anhydrase at 20, 177 and 1955 units kg-1 organo-mineral mixture, respectively), three water irrigation rates (125, 250 and 375 mL day-1 kg-1 organo-mineral mixture) and three watering frequencies (one, two and five times day-1). The experiment was conducted in eight rounds, each one lasting eight weeks, yielding data for a total of 323 experimental units. We measured earthworm survival and activity, as well as several commonly used weathering indicators in the organo-mineral mixture and in the leachate, as total alkalinity, inorganic carbon, pH, electrical conductivity and major cations. Using random forest regression, we found that earthworm survival and activity mainly depended on variables influencing the structure and drainage potential of the organo-mineral mixture, such as the presence of straw and increasing percentages of coarse grain sizes. Furthermore, we concluded that the effect of earthworms on weathering indicators depended on whether they survived or died by the end of the experimental period. Surviving earthworms had a neutral or negative effect on weathering indicators, likely because the experimental duration was too short to detect an increase in inorganic carbon, or because there was an increase in organic rather than inorganic carbon in the organo-mineral mixture. In contrast, dead earthworms enhanced almost all weathering indicators considered, suggesting that microbial processes associated with decomposing earthworm bodies may play a role in enhancing weathering. Our results also emphasize that the role of earthworms in Enhanced Silicate Weathering within bioreactors might be overestimated if weathering indicators exclusively rely on changes in mineralogy and ions release to quantify earthworm effects on carbon sequestration through weathering.

The development of carbon dioxide removal methods, coupled with decreased CO2 emissions, is fundamental to achieving the targets outlined in the Paris Agreement limiting global warming to 1.5 °C. Here we are investigating the importance of the organic carbon feedstock to support silicate mineral weathering in small-scale flow through bioreactors and subsequent CO2 sequestration. Here, we combine two bacteria and two fungi, widely reported for their weathering potential, in simple flow through bioreactors (columns) consisting of forsterite and widely available, cheap organic carbon sources (wheat straw, bio-waste digestate of pig manure and biowaste, and manure compost), over six weeks. Compared to their corresponding abiotic controls, the inoculated straw and digestate columns release more total alkalinity (~2 times more) and produce greater dissolved and solid inorganic carbon (29% for straw and 13% for digestate), suggesting an increase in CO2 sequestration because of bio-enhanced silicate weathering. Microbial biomass is higher in the straw columns compared to the digestate and manure compost columns, with a phospholipid fatty acid derived total microbial biomass 10 x greater than the other biotic columns. Scanning Electron Microscopy imaging shows the most extensive colonisation and biofilm formation on the mineral surfaces in the straw columns. The biotic straw and digestate columns sequester 50 and 14 mg C more than their abiotic controls respectively, while there is no difference in the manure columns. The selection of organic carbon sources to support microbial communities in the flow through bioreactors controlls the silicate weathering rates and CO2 sequestration.

Enhanced weathering (EW) is an emerging carbon dioxide (CO2) removal technology that can contribute to climate change mitigation. This technology relies on accelerating the natural process of mineral weathering in soils by manipulating the abiotic variables that govern this process, in particular mineral grain size and exposure to acids dissolved in water. EW mainly aims at reducing atmospheric CO2 concentrations by enhancing inorganic carbon sequestration. Until now, knowledge of EW has been mainly gained through experiments that focused on the abiotic variables known for stimulating mineral weathering, thereby neglecting the potential influence of biotic components. While bacteria, fungi, and earthworms are known to increase mineral weathering rates, the use of soil organisms in the context of EW remains underexplored. This protocol describes the design and construction of an experimental setup developed to enhance mineral weathering rates through soil organisms while concurrently controlling abiotic conditions. The setup is designed to maximize weathering rates while maintaining soil organisms' activity. It consists of a large number of columns filled with rock powder and organic material, located in a climate chamber and with water applied via a downflow irrigation system. Columns are placed above a fridge containing jerrycans to collect the leachate. Representative results demonstrate that this setup is suitable to ensure the activity of soil organisms and quantify their effect on inorganic carbon sequestration. Challenges remain in minimizing leachate losses, ensuring homogeneous ventilation through the climate chamber, and avoiding flooding of the columns. With this setup, an innovative and promising approach is proposed to enhance mineral weathering rates through the activity of soil biota and disentangle the effect of biotic and abiotic factors as drivers of EW.

Diffusive gradients in thin films (DGTs) provide time-weighted solute concentrations in well-mixed fluvial environments. However, the DGT method has rarely been applied to relatively slowly moving groundwater systems, where the increased length of the diffusive boundary layer (DBL) must be accounted for, to give accurate concentration measurements. Because the DGT method pre-concentrates trace-level solutes (e.g., Cd2+) and averages out fluctuations in more dynamic contaminants (e.g., NO3-), it has potential advantages over conventional methods. This study evaluated the application of DGT methods for monitoring trace contaminants in groundwater, which are often present at levels below standard instrumental detection limits. We tested three different DGT deployment approaches using NO3- to estimate the DBL thickness: a static DGT suspension, a DGT shaker, and a pumped flow-cell unit coined the "universal DGT monitoring system" (UDMS). Unlike static and DGT shaker approaches, the UDMS reduced the DBL thickness (similar to 0.02 cm) to values within the range of well-mixed waters (DBL <= 0.03 cm). Comparable DBL thicknesses for NO3- and Cd2+ and correlations with grab sample measurements (R-2 = 0.99 and 0.98, respectively) demonstrate the suitability of DGTs combined with the UDMS for groundwater monitoring. This approach is particularly suitable for analysis of trace-level solutes including organic contaminants.

Concentrations determined using diffusive gradients in thin films (DGT) have been used to derive time-averaged loads in streams and rivers. However, DGT provide time-weighted average concentrations that assume the independence of concentration and flow. Additionally, dynamic and coordinated changes in temperature, flow, and concentration are potential sources of bias in concentration and load calculations. We modeled scenarios in which temperature and flow were correlated to varying degrees with concentration and evaluated the consequences for DGT concentration and load calculations. As the correlation between solution flow and concentration moved toward 1 and -1, the load determined by DGT either overestimated or underestimated the actual load by as much as 30%. In DGT-based load estimates, the degree of potential bias should be assessed, and the concentration-flow relation should be characterized. As the correlation of analyte concentration and temperature approached 1 and -1, the deviation of the concentration determined by DGT from the actual concentration increased. In most cases, this bias was < 2%; however, if the changes in concentration and temperature were large (∼10 mg L-1 and ∼10 °C), the bias exceeded 5%. Concentration and temperature are unlikely to be perfectly or strongly correlated or anti-correlated in natural systems and thus should not affect the accuracy of DGT concentration calculations in most circumstances. The more solution temperature, flow, and concentration were uncorrelated, the closer DGT-derived concentration and load were to the actual solution concentration and load.

Diffusive gradients in thin films (DGTs) have been established as useful tools for the determination of nitrate, phosphate, trace metals, and organic concentrations. General use of DGTs, however, is limited by the subsequent requirement for laboratory analysis. To increase the uptake of DGT as a tool for routine monitoring by nonspecialists, not researchers alone, methods for in-field analysis are required. Incorporation of color reagents into the binding layer, or as the binding layer, could enable the easy and accurate determination of analyte concentrations in-field. Here, we sought to develop a chitosan-stabilized silver nanoparticle (AuNP) suspension liquid-binding layer which developed color on exposure to nitrite, combined with an Fe(0)-impregnated poly-2-acrylamido-2-methyl-1-propanesulfonic acid/acrylamide copolymer hydrogel [Fe(0)-p(AMPS/AMA)] for the reduction of nitrate. The AuNP-chitosan suspension was housed in a 3D designed and printed DGT base, with a volume of 2 mL, for use with the standard DGT solution probe caps. A dialysis membrane with a molecular weight cutoff of <15 kDa was used, as part of the material diffusion layer, to ensure that the AuNP-chitosan did not diffuse through to the bulk solution. This synthesized AuNP-chitosan provided quantitative nitrite concentrations (0 to 1000 mg L–1) and masses (145 μg) in laboratory-based color development studies. An Fe(III)-impregnated poly-2-acrylamido-2-methyl-1-propanesulfonic acid/acrylamide copolymer hydrogel [Fe(III)-p(AMPS/AMA)] was developed (10% AMPS, and 90% AMA), which was treated with NaBH4 to form an Fe(0)-p(AMPS/AMA) hydrogel. The Fe(0)-p(AMPS/AMA) hydrogel quantitatively reduced nitrate to nitrite. The total nitrite mass produced was ∼110 μg, from nitrate. The diffusional characteristics of nitrite and nitrate through the Fe(III)-p(AMPS/AMA) and dialysis membrane were 1.40 × 10–5 and 1.40 × 10–5 and 5.05 × 10–6 and 5.15 × 10–6 cm2 s–1 at 25 °C respectively. The Fe(0)-hydrogel and AuNP-chitosan suspension operated successfully in laboratory tests individually; however, the combined AuNP-chitosan suspension and Fe(0)-hydrogel DGT did not provide quantitative nitrate concentrations. Further research is required to improve the reaction rate of the AuNP-chitosan nitrite-binding layer, to meet the requirement of rapid binding to operate as a DGT.

Diffusive Gradients in Thin-Films (DGT) have traditionally been used to measure time-weighted average concentration in water. We tested whether Br−-DGT in combination with the trace-dilution flow rate method, could be used as a new approach for measuring water flow rate. A novel bromide selective DGT based on the Purolite Bromide Plus anion exchange resin (Br−-DGT) was developed, which provided environmental bromide concentrations comparable to grab samples. The Br−-DGT provided quantitative bromide concentrations at a range of pH, competing ion concentrations, and in synthetic natural solution. The uptake efficiency was 95.7 ± 3.4%, and the elution efficiency was 95.5 ± 4.7%. The absorption maximum/saturation point of each binding disk was 0.684 ± 0.001 mg. Bromide adsorption to the binding layer was linear to 44.1% of the total binding capacity, 0.302 mg. The determined diffusion coefficient through the agarose cross-linked polyacrylamide (APA) hydrogels was 1.05 × 10−5 cm2 s−1 at 17.9 °C, temperature corrected to 25 °C was 1.29 × 10−5 cm2 s−1. DGT flow rates were between −14.7 and 6.50% of the flow independently monitored flow rate (weir). In comparison, grab sample flow rates diverged by 5.52 to 58.9% from the weir flow rate.

The increase in environmental nutrient availability as a result of human activities has necessitated the development of mitigation strategies for nutrient removal, such as nitrate. Current methods for determining the efficiency of different mitigation strategies required measurement of changes in nitrate concentrations, however, these methods can be expensive or do not account fully for the temporal variability of nitrate concentration. This study evaluated the utility of Diffusive Gradients in Thins-Films (DGT) for determining nitrate removal in two denitrifying bioreactors, and compared DGT performance to traditional approaches for determining performance, including high and low frequency water grab sampling. The binding layer was produced using the Purolite (R) A520E anion exchange resin. The uptake and elution efficiencies were 98.8% and 93.4% respectively. DGTs of three material diffusion layer thicknesses were placed in piezometers along longitudinal transects, to enable calculation of the diffusive boundary layer and provide replicates. These were removed after 16, 24 and 36 h, and the accumulated nitrate masses were extracted and quantified to calculate nitrate concentration. Concentrations were subsequently utilised to calculate nitrate removal rates in both bioreactors. Grab samples were taken at 30 and 60 min intervals over those periods, nitrate concentrations were also measured to determine nitrate removal. DGTs provided nitrate removal rates at bioreactor site one (controlled flow, wastewater treatment) of 14.83-30.75 g N m(-3) d(-1), and 1.22-3.63 g N m(-3) d(-1) at site two (variable flow, agricultural run-off). DGT determined nitrate concentrations and removal rates were in strong accordance with high frequency grab sampling, but data collection via DGTs was considerably easier. Utilising DGTs for the measurement of bioreactor performance overcame many of the challenges associated with high frequency grab sampling, and other methods, such as accounting for temporal variation in nitrate concentration and reduced analytical requirements. (C) 2019 Elsevier B.V. All rights reserved.