Removal mechanisms of cadmium and lead ions in contaminated water by stainless steel slag obtained from scrap metal recycling

Abstract

In this work, steel slag was shown to be a successful filter material for the removal of lead and cadmium from natural waters with a removal efficiency of >95 %, even under acidic conditions (pH 3.5). Overall, precipitation was found to be the main removal mechanism over adsorption with a tight relationship with the final matrix pH. Batch experiments showed fast removal kinetics with >98 % removal efficiency for lead and cadmium in 5 and 15 min, respectively. The removal kinetics followed a pseudo second-order reaction (R² = 0.999), and the K₂ was calculated to be 927 h⁻¹ and 204 h⁻¹ for lead and cadmium, respectively. A strong interference for the removal was observed with ethylenediaminetetraacetic acid (EDTA) but slightly with fulvic and humic acids, and non-significant with polymeric acid e.g. isosaccharinic acid. In addition, the column experiment showed spontaneous removal kinetics for lead and cadmium with a contact time of 3.6–36 seconds depending on loading flow-rates. Column experiments at similar liquid to solid ratios showed a high removal capacity with gradient over isocratic solution loading. The high removal efficiencies in combination with limited hazardous metals leaching led to the conclusion that slag is an appropriate filter material for treating contaminated waters. Moreover, the slag treated solutions with alkaline conditions showed reacted efficiently with CO₂ in flue gas. Consequently, the pH of the treated solution is neutralized making it easier to waste and this reaction may contribute to reducing CO₂ gas emissions.

Introduction

Cadmium and lead contamination are considered serious threats to the environment and human health due to their high toxicity and persistent characteristics [1]. They enter the food chain mainly through crop irrigation and soil contamination [2]. According to the World Health Organization (WHO), inorganic lead and cadmium compounds have been placed in Group 2A and are described as probable human carcinogens with a recommended maximum concentration of 10 μg L⁻¹ and 3 μg L⁻¹ in drinking water, respectively [3]. Exceedingly high lead and cadmium exposure can cause bone degeneration, liver damage, lung insufficiency, hypertension, and renal dysfunction in humans. Human exposure is mainly through consuming contaminated food and drinking water [4,5]. Industrial activities such as mining, electroplating, pigment manufacturing, metallurgy and agricultural activities increase the flux of heavy metals to local water bodies. More attention has been paid to reduce the concentrations of the dissolved metals in sewage, industrial, and mining wastes [6,7]. Several removal approaches have been proposed, among those adsorption and precipitation are considered to be the most suitable methods [[8], [9], [10], [11]]. Costs for treating contaminated water by synthesized materials are typically high, thus limiting their application on larger scales. Therefore, a robust, cheap and efficient method for removing the toxic metals is essential to mitigate the risk for society.

Currently, global crude steel production is around 1808 million tons per year (2018 estimate) [12]. Steel slag, a byproduct of steel manufacturing, makes up approximately 15 % by mass of the steel output, giving approx. 270 million tons produced per year [13,14]. Currently, use of the slag is limited. The majority is disposed of in landfills which occupy massive land areas and potentially pollute the environment [15]. Steel slag is mainly composed of the oxides of calcium, silicon, aluminum and iron, giving it very strong alkaline properties as well as a high adsorption and precipitation capacity. It has been reported that steel-making slag can be used as adsorbent for the removal of heavy metals and various contaminants [[16], [17], [18], [19]]. Few studies have reported the use of steel slag as adsorbent for treating lead and cadmium contamination [[20], [21], [22]]. However, steel slag is a complex system of oxides and typically has low specific surface area and poor pore structure caused by crystallization, limiting its adsorption efficiency [21]. A modified steel slag showed a sorption capacity for cadmium of 10 mg g⁻¹ (25 °C) [21] whereas other materials like iron oxide activated red mud was only 0.12 mg g⁻¹ [23]. The reported sorption capacities of lead by iron oxide nanoparticles and iron-coated filters were 36 mg g⁻¹ [24] and 1.2 mg g⁻¹ [25], respectively. A study on the removal of copper in contaminated water by steel slag indicated that the removal is a result of both adsorption and precipitation [26]. Adsorbents were shown to be effective for treating wastewater containing a low heavy metal concentration while precipitation with lime (CaO) has been found as one of the most effective method for treating extremely contaminated solutions (>1000 mg L⁻¹) [27].

In this study, the in situ removal of lead and cadmium by slag filled filtration columns was examined in order to assess the efficiency of using slag as a filter material for contaminated water treatment. The stainless steel slag in our study was obtained from scrap metal recycling which is considered environmentally friendly because it requires limited mining activities. Heavy metal removal efficiency by slag was tested under various conditions including pH, competition of ligands and liquid to solid ratios, and by the slag column with different solution loading flow-rates. Isotherm and kinetics for the heavy metal removal were investigated with these tests, and the contribution of adsorption and precipitation to the removal were examined. A natural water sample spiked with lead and cadmium was used to test the application of the slag material in heavy metals filtration. Ultimately, it was found that steel slag is a successful filter material for the removal of lead and cadmium from natural water with a removal efficiency of >95%.