Cost-effective and eco-friendly superadsorbent derived from natural calcium-rich clay for ultra-efficient phosphate removal in diverse waters

Abstract

High-performance phosphate adsorbents integrated with the advantages of excellent adsorption performance, low cost, strong adaptability, and environmental friendliness have always been desirable, because the existing rare earth-free adsorbents were confined to poor adsorption ability and the rare earth-based adsorbents were confined to high cost and waste of rare earth resources. Here, in line with the concept of “from nature, for purifying nature”, a novel cost-effective and robust phosphate adsorbent was synthesized from natural calcium-rich clay (a very abundant and inexpensive raw material) by coupling the hydrothermal reaction with a simple pyrolysis process. The natural calcium-rich clay was firstly reacted with sodium silicate to form silicate precursor, which was then calcined to obtain superadsorbent with a number of available sites for capturing phosphates. The experimental adsorption capacity of the superadsorbent for phosphate is as high as 372.57 mg/g, which is 24.27 times that of raw clay, and is obviously superior to most other adsorbents. A satisfactory adsorption capacity was still attained in a broad pH range of 4 ∼ 12. After adsorption with 3 g/L of the superadsorbent, 99.99% of the phosphate in the initial phosphate solution (200 mg/L) was removed. After adsorption with 4 g/L of the adsorbent, the phosphate in natural water (e.g., Yellow river water, seawater, Yangtze river water, and Tap water) can be almost completely removed, and the total P concentration in the actual phosphate solution can be reduced to below 0.01 mg/L, which is significantly better than the commercially available phosphorus adsorbents. The in-depth structural analysis of the adsorbent after adsorption revealed that the adsorption of phosphate on the superadsorbent was mainly driven by chemical complexation interactions.