Effects of natural zeolite on speciation of heavy metals during agitated pile composting of water hyacinth

Water hyacinth (Eichhornia crassipes) is one of the worst weeds in the world. Composting is one of the most economical ways for the treatment and final disposal of water hyacinth, because it combines material recycling and biomass disposal. A major restriction of land application of water hyacinth compost is the possible high heavy metal concentration in the final product. Zeolites may be useful as metal scavengers in metal-rich water hyacinth compost. The speciation of heavy metals (Zn, Cu, Mn, Fe, Pb, Ni, Cd and Cr) was done according to Tessier’s sequential extraction method during water hyacinth composting mixed with cattle manure, sawdust and natural zeolite. The water hyacinth, cattle manure and sawdust were taken in the 6:3:1 ratio with 5, 10 and 15 % natural zeolite, respectively.

The temperature was measured in the range of 48.6–56.2 °C in control and zeolite treatments. Higher reduction of exchangeable (F1) and oxidizable (F4) fractions of Zn was observed about 72.8 and 79.6 %, respectively, in zeolite 1 treatment but higher reduction of carbonate (F2) and reducible (F3) fractions was observed about 70 and 37 %, respectively, in zeolite 2 treatment. The F1, F2, F3 and F4 fractions of Cu and Fe were reduced in the control and all zeolite treatments. The F1, F2, F3 and F4 fractions of Mn were reduced significantly in the all zeolite treatments in comparison to control. The bioavailability factor (BF) of Zn, Cu, Mn, Fe, Ni, Cd and Cr was reduced significantly in zeolite treatments in comparison to control. There was no significant change observed with BF of Pb in zeolite treatments. The total concentration of Pb was higher than Zn, Cu, Mn, Ni and Cr but its BF was lowest among the all these metals.

Addition of optimum percentage of natural zeolite was successful for enhancing degradation of organic matter and reducing bioavailability of heavy metals during water hyacinth composting mixed with cattle manure and sawdust. The bioavailable fractions of heavy metals were effectively correlated with pH and TOC in control and zeolite 1 treatment as compared to zeolite 2 and 3 treatments.