Heavy metals distribution and their bioavailability in earthworm assistant sludge treatment wetland

Sludge treatment wetlands (STWs) have been used for sludge treatment in recent years, however, heavy metals (HMs) are one of the limiting factors for the final sludge application, and the fate of HMs in STWs are still not well studied. Therefore, six STWs with two plant species and earthworm addition were investigated to evaluate their effects on HMs distribution and bioavailability. The results showed that plant uptake of HMs was insignificant with earthworm addition. Earthworm had the highest enrichment for Cd with bioaccumulation factors of 6.9-7.3. Moreover, earthworm had a positive effect to remove HMs in accumulated sludge. Meanwhile, the bioavailability of Cd in accumulated sludge was decreased by earthworm addition, with the acid-soluble fraction decreased from range 16.5-22.7% to range 7.2-10.1%. Furthermore, HMs mass balance in the STWs revealed that HMs were widely distributed in the accumulated sludge, leachate and others, while their (except Cd) accumulation in the plants and earthworm are less than 1%. The Cd toxicity in the accumulated sludge can be reduced by the addition of earthworm with enrichment of 3.6-8.2%. Overall, earthworm addition have positive effects on distribution and bioavailability of HMs in STWs.