Abstract
The amount of sludge produced in wastewater treatment plants (WWTPs) has increased over the years, and the methods used to reduce this waste, such as incineration, agricultural use, or disposal in landfills, cause problems of secondary pollution. For this reason, it is necessary to find sustainable and low-cost solutions to manage this waste. Additionally, emerging and priority pollutants are attracting attention from the scientific community as they can generate health problems due to inadequate removal in conventional WWTPs. In this work, a pharmaceutical industry sludge was used as a precursor in the synthesis of four activated carbons (ACs) using different activating agents (ZnCl2, FeCl3∙6H2O, Fe(NO3)3∙9H2O, and Fe(SO4)3∙H2O), to be used for the removal by adsorption of three neonicotinoid pesticides included in latest EU Watch List (Decision 2018/840): acetamiprid (ACT), thiamethoxam (THM), and imidacloprid (IMD). The prepared ACs showed micro–mesoporous properties, obtaining relatively slow adsorption kinetics to reach equilibrium, but despite this, high values of adsorption capacity (qe) were obtained. For example, for AC-ZnCl2 (SBET = 558 m2/g), high adsorption capacities of qe = 128.9, 126.8, and 166.1 mg/g for ACT, THM, and IMD, respectively, were found. In most cases, the adsorption isotherms showed a multilayer profile, indicating an important contribution of the mesoporosity of the activated carbons in the adsorption process.
Highlights
Sewage sludge is defined as the unavoidable waste generated in wastewater treatment processes and consists of a highly heterogeneous mixture of water and solids
These solids are the most relevant, as the greater the amount of organic matter present in the precursor, the higher the efficiency obtained in the synthesis of activated carbon [29]. Another noteworthy value is the Ca content (7.25%), as this is a value that stands out when compared to the rest of the metals present in the sludge. This may be due to the origin of the sludge and to the products used in the wastewater treatments applied in the industry, involving the use of calcium hydroxide, commonly known as slaked lime, which is the most-used calcium-based product in wastewater treatment plants (WWTPs)
The use of sewage sludge as a low-cost precursor for the preparation of activated carbons to removal neonicotinoid pesticides, e.g., acetamiprid, thiamethoxam, and imidacloprid, is a sustainable way to solve the serious problem of sludge management while being in accordance with the concept known as a circular economy
Summary
Sewage sludge is defined as the unavoidable waste generated in wastewater treatment processes and consists of a highly heterogeneous mixture of water and solids. The composition of the sludge varies considerably depending on its source, and it can be classified mainly into two types: if it comes from domestic wastewater, it is known as urban sludge, and if, on the other hand, it comes from industrial water, it is known as industrial sludge. This latter is notable for its high organic matter content and is generally the most suitable precursor to produce carbonaceous adsorbents [2,3]. A valorization route has been proposed within the concept of a circular economy; this route consists of using the sludge as a low-cost raw material for its transformation into high-added-value products such as activated carbons [1,7]—porous carbonaceous materials with adsorbent properties—which have proven to be highly effective in removing a wide range of organic pollutants from water [11,12,13]
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