Successful removal of glyphosate

In collaboration with researchers from the University of Northern British Columbia in Canada, Dominik Eder's team finally investigated the adsorption of glyphosate from groundwater. Remarkably, the new material was able to remove three times as much glyphosate in only 20% of the time as the currently best adsorbent.

Glyphosate Adsorption from Water Using Hierarchically Porous Metal–Organic Frameworks

Shaghayegh Naghdi, Emily Brown, Mohammad Zendehbad, Ann Duong, Wolfgang Ipsmiller, Santu Biswas, Maytal Caspary Toroker, Hossein Kazemian, Dominik Eder

First published: 23 February 2023

Abstract

The selective removal of one ligand in mixed-ligand MOFs upon thermolysis provides a powerful strategy to introduce additional mesopores without affecting the overall MOF structure. By varying the initial ligand ratio, MOFs of the MIL-125-Ti family with two distinct hierarchical pore architectures are synthesized, resembling either large cavities or branching fractures. The performance of the resulting hierarchically porous MOFs is evaluated toward the adsorptive removal of glyphosate (N-(phosphonomethyl)glycine) from water, and the adsorption kinetics and mechanism are examined. Due to their strong affinity for phosphoric groups, the numerous Ti–OH groups resulting from the selective ligand removal act as natural anchor points for effective glyphosate uptake. The relationships between contact duration, glyphosate concentration, and adsorbent dosage are investigated, and the impact of these parameters on the effectiveness of glyphosate removal from contaminated water samples is examined. The introduction of additional mesopores has increased the adsorption capacities by nearly 3 times with record values exceeding 440.9 mg g−1, which ranks these MOFs among the best-reported adsorbents.

1 Introduction

The interaction of the ligand N-(phosphonomethyl) glycine, also known as glyphosate (Gly), with both naturally occurring and anthropogenic species in the environment, is of significant interest due to its inclusion in organophosphorus herbicides in both agricultural and non-agricultural areas all over the world.

Glyphosate is a systemic herbicide and crop desiccant with a broad spectrum of activity. It is an organophosphorus chemical that works by blocking the plant enzyme 5-enolpyruvylshikimate-3-phosphate synthase. It is used to kill weeds, mainly annual broadleaf weeds, and grasses that compete with crops.

Glyphosate was first introduced by the US agrochemical corporation Monsanto, in the early 1970s. It currently accounts for 60% of worldwide broad-spectrum herbicide sales, with total global use of over 70 000 tons of technical acid each year.[1, 2] It is now the most extensively used herbicide in the world by volume, and it is manufactured and distributed by a variety of firms all over the world.[3]

There are growing worries about glyphosate's harmful effects on the environment due to its water solubility, high stability, and impairment of biological health.[4-9] Glyphosate can be introduced into the environment at various stages of its production and usage.[10, 11]

Incidents of glyphosate toxicity in humans have raised concerns about its health effects, including eye and skin irritation, contact dermatitis, eczema, cardiac and respiratory issues, and allergic responses. Buffin and Topsy provide a detailed assessment of glyphosate's acute hazardous effects in humans.[2]

There is no recommended value for glyphosate residue in drinking water; however, the EU standard for any pesticide in drinking water is 0.1 µg L−1.[12] This is, without a doubt, a significant challenge for a portable water treatment facility. In light of the rising reports of glyphosate in the aquatic environment, it has been claimed that the cost of building the required equipment to remove herbicides from drinking water may be £1.0 billion, with annual operating expenses of £50–100 million in the UK.[13-16] As a result, glyphosate-related water contamination must be strictly controlled. Although various traditional techniques, including the use of activated carbons, oxidation, ozonation, and photocatalytic degradation, may be used to remove glyphosate, an efficient and cost-effective approach is still desired.[17-19]

The removal of glyphosate from mineral surfaces and soils has been studied using biodegradation,[20] adsorption,[21] oxidation,[22] and photocatalysis degradation processes.[23] Some have concentrated on quantitative removal,[24-26] while others have attempted to comprehend the interactions at the molecular level.[27] Many researchers are interested in adsorption because of its advantages of convenience, low cost, and environmentally friendly operation.[28, 29]

Activated carbon, zeolite, and carbon-based composites are the most commonly reported adsorbents for glyphosate. However, their adsorption properties currently remain unimpressive, since the majority of them have poor selective recognition ability and adsorption capacity, limiting their use in organic pesticide removal.[8, 30-37] As a result, the parameters influencing the adsorption process between glyphosate and the adsorbent must be investigated.

Metal–organic frameworks (MOFs) are porous materials with record surface areas and tunable chemistries that render them highly promising candidates for the adsorptive elimination of toxic inorganic and organic substances as well as photocatalysis.[37-42] The absorption capacity of MOFs can be varied by adjusting pore size, shape, and chemistry through different synthetic techniques or post-synthetic modification.[43] Besides sufficient stability in aqueous solutions, suitable MOFs thus need appropriate pore size and connectivity that allow access to active adsorption sites for effective glyphosate adsorption.[44]

In our previous study, we developed a highly selective ligand removal technique (aka SeLiRe strategy), building on the work by Feng et al.,[46] which enables the construction of MOFs with dual porosity from their related mixed-ligand MOFs.[47] Through careful tuning of synthetic conditions and heating parameters, we were able to design MIL-125-Ti either with isolated cavity-based mesopores of uniform diameter or branching narrow fracture-type pores without noticeable collapse of the parent micropore structure. The process for removing ligands was investigated through comprehensive ex situ and in situ studies combined with Density Functional Theory (DFT) simulations. The ligand removal is very selective for aminoterephthalic acid (BDC-NH2), and it occurs in two phases, each of which may be finetuned by changing the temperature and duration.[47]

In this study, we investigate the effect of type, connectivity, and size of the added mesopores on the glyphosate adsorption. In addition, we also investigate single-ligands MIL-125-Ti and NH2-MIL-125-Ti as well as the corresponding pristine mixed-ligand MOFs prior to SeLiRe to identify the mechanism of glyphosate adsorption. Our results demonstrate that the introduction of large cavity-type mesopores significantly improves both the capacity and efficiency of glyphosate adsorption due to improved accessibility of the interior surface and increased number of Ti sites created by the SeLiRe process. Therefore, our study offers a fascinating example of how rationalized pore engineering can improve the adsorptive properties of MOFs for larger compounds.

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Taxonomy

• Pesticides
• Fertilizers and Pesticides
• Pesticides
• Heavy metals