Release behavior of soluble salts in MSWI bottom ash used as road basement materials under continuous rainfall conditions

The bottom ash is increasingly used as a substitute aggregate material in road construction in China,and road salting is the major salt source in groundwater.Continuous rainfall releases soluble salts from the bottom ash subgrade into the surrounding soil and groundwater,resulting in potential hazards.Different methods were employed to simulate and collect runoff water during rainfall events,including batch leaching test,dynamic leaching test and constant head test,to assess environmental impact of bottom ash as road basement materials under continuous rainfall conditions.This study simulated the seepage of bottom ash backfill roads under different rainfall intensities,rainfall times,and rainfall pH values.A comprehensive sampling and laboratory testing program was undertaken to characterize the environmental impact of soluble salts from bottom ash.The obtained results reveal that the leaching concentrations of Cl^(−)and SO_(4)^(2−)exceed the limit specified in the class V standard of surface water,which are 2.06–2.17 times and 1.08–1.25 times,respectively.By examining the long-term environmental influence under the condition of continuous rainfall,the leaching of Cl^(−)mainly occurs in the early leaching stage,and the maximum leaching concentration reaches 19,700 mg/L.The release concentration of Cl^(−)begins to be lower than the class V standard of surface water when continuous rainfall approaches the total rainfall for 13 months.The cumulative release of Cl^(−)in the bottom ash is 2.8–5.4 mg/g.Both rainfall intensity and rain pH affect the release of Cl^(−).The obtained results derived from the constant head tests indicate that stagnant water caused by rainfall deteriorates the release of soluble salt into the groundwater in only 1 day,especially at the early stage of 12 h.This work provides some basic information about how to minimize damage to the surrounding environment caused by the leaching of salt in bottom ash.

Quantification and distribution of extractable metals of MSWI bottom ash in view of its valorization in China

Metal recovery from bottom ash was deemed to be significant to achieve a higher stability of bottom ash and recycle valuable extractable metals.In China,the existing rugged industrial production ignores the actual metal distribution and thus fails to exploit the utilization potential of recoverable metals in bottom ash.Based on these findings,this work was proposed to obtain a comprehensive and in-depth study on the recoverability of metals in bottom ash.First,the particle size distribution and elemental composition of the bottom ash were analyzed.Then,complete information on the recoverable metals in bottom ash fractions with different sizes was obtained by washing,sorting,crushing,density separation and XRF(X Ray Fluorescence)analysis.The results showed that the smaller than 5 mm fraction accounted for up to 60%of the bottom ash,and the 5–20 mm fractions accounted for about 15%.The material characterization revealed that the contents of recoverable Fe,stainless steel,Al and Cu in bottom ash were averagely 9.01%,0.136%,0.78%and 0.08%,respectively.About 50%of Fe,68%of Al,61%of Cu,and 22%of stainless steel were distributed in smaller than 10 mm fraction.Particularly,Fe was evenly distributed among 0–2 mm,2–5 mm,5–10 mm fractions,and the content was between 5.41%and 7.5%.Non-magnetic stainless steel was mainly distributed in 20–40 mm and larger than 40 mm fractions.The highest share of Al was present in the fractions between 5 mm and 20 mm,accounting for 48%of the total aluminum.About 45.6%of the Cu was enriched in the 5–10 mm fraction.However,the Zn content was less than 0.01%.This work provides an in-depth understanding and information on metal recovery as well as promisingly guide ash utilization.