Effect heating dwell time has on the retention of heavy metals in the structure of lightweight aggregates manufactured from wastes.

The main objective of this paper was to study how effective thermal treatment is in the retention of different heavy metals (HMs) within the structure of artificial lightweight aggregates (LWAs). These LWAs were manufactured by washing aggregate sludge and sewage sludge. The consequence of increasing the heating dwell time whilst manufacturing these LWAs was also determined. Partitioning of the HMs (Cr, Ni, Cu, Zn, Cd and Pb) was studied by means of the optimized BCR sequential extraction procedure. Then, the leaching ratio (LRx,y) was calculated. Thermal treatment was totally effective for immobilizing most of the elements studied except for a part of the non-residual Zn and Cd fraction which could volatilize, and the fractions of Pb which were water- and acid-soluble, weakly adsorbed, exchangeable, and oxidable. These were more highly concentrated in the LWAs than in the initial waste mixture. The effect of increasing heating dwell time on the retention of heavy metals in the LWAs depended on both the chemical element studied and the heating dwell time. This study is very important since certain rises in the heating dwell time caused a decrease in retention of some specific heavy metals in the LWAs. ABBREVIATIONS: BCR-SEP: optimized BCR sequential extraction procedure; b.d.l: below the detection limit; F1: weakly adsorbed, exchangeable and water- and acid- soluble fraction; F2: reducible fraction; F3: oxidable fraction; F4: residual fraction; HM: heavy metal; ICP-MS: inductively coupled plasma-mass spectroscopy; LOI: loss on ignition; LWA: lightweight aggregate; LWA-5: lightweight aggregate sintered for 5 min; LWA-10: lightweight aggregate sintered for 10 min; LWA-20: lightweight aggregate sintered for 20 min; LWA-30: lightweight aggregate sintered for 30 min; LRx,y: leaching ratio of the element x in the fraction y; n.e: not established; S: compressive strength; SS: sewage sludge; WA24h: water absorption after 24 hours; WAS: washing aggregate sludge; W75S25: mixture of 75% (wt) of the dried washing aggregate sludge and 25% (wt) of the dried sewage sludge; ρb: loose bulk density; ρd: dry particle density; ∑1 + 2 + 3: non-residual fraction; ∑1 + 2 + 3 + 4: total concentration; ∑2 + 3: reducible and oxidable fractions.