Efficient phosphate recycling by adsorption on alkaline sludge biochar

Large amounts of septic tank sludges from sanitation facilities are either landfilled or illegally dumped into the natural environment, leading to environmental pollution and waste of resources. This issue calls for advanced methods to recycle septic tank sludges such as sustainable adsorbents to recycle phosphorus, e.g., in agriculture, in the context of the circular economy. Here, we hypothesized that alkaline septic tank sludge biochar could be an efficient adsorbent to recycle phosphate from wastewater. We first prepared raw biochar by pyrolysis of septic tank sludge at 500 °C. Then, we prepared alkaline biochar by pyrolysis at 800 °C of mixtures of potassium hydroxide (KOH) and raw biochar at 3/1, 4/1 and 5/1 mass ratios. We studied biochar properties by scanning electron microscopy, X-ray diffraction and Fourier transform infrared spectroscopy, and we quantified adsorption of phosphates by biochars. Results show that phosphate adsorption highly increases with KOH content, from 27.83 mg/g for the raw biochar to 42.51 mg/g for the 5/1 KOH-biochar. This trend is explained by the increase in biochar surface area from 64.214 m²/g for the raw biochar to 82.901 m²/g for the 5/1 KOH-biochar, and by the improvement of the structural properties and surface morphology of KOH-biochars. Overall, alkaline biochar appears as a promising adsorbent to recycle phosphates from wastewaters.