Fate and behaviour of metformin and its transformation product guanylurea in soil-water systems

Abstract

Due to the widespread occurrence of diabetes, metformin (MET) has become one of the most prescribed medicines in the world. The human body excretes MET through urine and faeces as an unchanged compound and MET eventually ends up in wastewater treatment plants, where the compound is transformed into its bacterial dead-end product, guanylurea (GUA). Monitoring of surface, drinking and groundwaters, sediments and sludge has shown the persistence of these compounds in the environment. Recently, researchers have documented that MET at 40 μg/L, which has been detected in wastewater effluent, also caused endocrine disrupting effects in fish. Due to its high usage, persistence in the environmental media and chronic ecotoxicity, MET has been identified as an emerging contaminant, but there is a dearth of information about its fate in the environment. Globally, land-application of wastewater effluents and sludge has become a popular disposal method, thus, understanding the fate of MET and GUA in the soil environment is crucial to assessing their risk to the wider ecosystems. Forced degradation studies of MET and GUA indicated that combined hydrolysis and photolysis contributed to <30% degradation at neutral pH. Hydrolysis of MET was <16% for all pH range studied while hydrolysis of GUA resulted to 60% and 50% degradation in acidic (pH4) and basic (pH 9) solutions, respectively. Photolytic degradation was <5% for both compounds at all pH conditions. Batch sorption experiments revealed low to moderate sorption of MET and GUA in soils with their distribution coefficients ranging between 10.6 and 53.5 L/kg. The contrasting sorption behaviour of MET and GUA towards changes in pH and ionic strength suggests involvement of different sorption mechanisms for the two compounds. Electrostatic interactions and cation-exchange were the most significant mechanisms for MET and GUA sorption, respectively, at environmentally relevant pH range between 5.7 and 8.5. Biosolid-amended soils have also resulted in decreased MET sorption while GUA sorption significantly increased. Enrichment cultures demonstrated the capability of soil and sludge microbial communities to degrade MET and GUA to certain extent. In sludge cultures, 100% and 25% degradation of MET and GUA, respectively, was observed when supplied as single substrates while in soil cultures, 20% and 30% dissipation, respectively, was recorded. GUA was completely degraded in soil and sludge cultures with the addition of glucose which provided carbon and energy source to microbes. Out of the six kinetic models used (single first order, Michaelis-Menten, logistic, logarithmic, Quiroga-Rosales-Romero (QSR) and autocatalytic models), the QSR and autocatalytic models predicted the degradation profiles of GUA and MET, respectively, capturing the incomplete degradation of GUA and "autocatalytic" behaviour of MET. Soil incubation studies revealed a bi-phasic degradation pattern of both MET and GUA with an initial rapid degradation stage followed by a slow dissipation rate. Statistical indices and box-whisker plots showed that the double first-order in parallel and first-order two-compartment models best described the degradation patterns. The calculated half-lives (DT50) using both models were in the range of 2.7 to 15.5 days and 0.9 to 4 days for MET and GUA, respectively while 90% dissipation time (DT90) varied between 91 and 123 days and 44 and 137 days, respectively. Soil column experiments indicated high mobility of MET and GUA especially in biosolid-amended topsoils. Whilst biodegradation, which is affected by sorption, is a significant process affecting the attenuation of MET and GUA, their high mobility through soil leaching suggest that these compounds may still move into deeper soil profile. There is potential that both MET and GUA could also possibly reach the groundwater under conditions conducive to leaching such as during high rainfall events and storms. Experiments performed throughout this research led to a better understanding of the fate and behavior of MET and GUA in soil-water systems. Once MET- and GUA- contaminated wastewater enters the soil compartment, microbial degradation is the primary attenuation process that will contribute to their attenuation and dissipation in soil. However, biodegradation is influenced by the compound’s bioavailability, which is related to sorption and desorption, depending on the soil type and environmental conditions. Similarly, land application of biosolid may introduce MET and GUA to soil. Biosolid amendment changes soil properties greatly affecting their sorption and consequently, the rates of biodegradation.With biosolid amendment, desorption rates of both compounds increased which renders MET and GUA very mobile and they may potentially reach the groundwater.