Speciation Of Heavy Metals And Technologies To Reduce Its Bioavailability During Composting Of Pig Manure

The technology of composting is harmless and resourceful. Because of feedadditives in livestock production, pig manure usually contained higher concentrationsof heavy metals. Successive application of pig manure to agricultural lands will leadto accumulation of heavy metals in the soils and cause adverse effects on humanhealth through food chain. The knowledge of speciation of heavy metal duringcomposting and the technology to reduce heavy metal bioavailability are of concern.The present study aimed at evaluating the speciation of Cu and Zn during compostingof pig manure in different initial C/N ratio or with different amendments and to findout the effective way to reduce metal bioavailability. The parameters such as pH, OMwere measured in order to find the mechanism that influenced metal speciation duringcomposting. Distributions of heavy metals in humic substance were analysed in orderto investigate the influence of organic matter degradation in heavy metal speciation.The study could provide theoretical basis for harmless and resourceful treatment ofmanure.1) During composting, the total heavy metal concentration was increasedbecause of the concentration effect caused by organic matter degradation. At theinitial stage of composting, Cu was mainly in exchangeable, reducible and oxidizedfraction, while Zn was mainly in exchange and reducible fraction, the oxidized andresidual fraction of Zn was low. Composting could reduce the exchangeable andreducible fraction of Cu and increase the oxidized fraction of Cu. Thus thebioavailablity of Cu decreased during composting. The humification process duringcomposting could accelerate the formation of oxidized fraction of Cu and reduce Cubioavailability. Znic was in bioavailable form (exchangeable and reducible fractions)and was mainly in exchangeable fraction which had higher mobility. During composting, the bioavailability factor of Zn changed a little. The composting processaccelerated the transformation of exchangeable Zn to reducible Zn fractions and thusthe mobility of Zn decreased.2) After composting, the bioavailability factor of Cu in rice straw compost andleaf compost reduced32.6%and27.0%, respectively. Compared with leaf, using ricestraw as co-composting material was more suitable to immobilize Cu duringcomposting. The exchangeable Zn fraction reduced16.0%and7.0%, respectively, inrice straw compost and leaf compost, implied that rice straw was more favoured in thereduction in exchangeable Zn fraction.3) As compared with the compost which had an initial C/N ratio of15, thethermophilic phase was longer in the compost which had an initial C/N ratio of22or27, as well as the highest temperature. An initial C/N ratio of22or27was moresuitable to reduce the bioavailability of Cu during composting of pig manure with ricestraw. An initial C/N ratio near22could accelerate the organic matter degradation andenhance the transformation of exchangeable Zn to reducible Zn fraction.4) The bioavailability of Cu decreased depending on the rock phosphate or boronmud amendment level. During composting, addition of7.5%of rock phosphate orboron mud could reduce the bioavailability factor of Cu by47.2%and40.8%,respectively. The bioavailability factor of Zn changed a little and was not affected bythe rock phosphate or boron mud addition. Addition an amount of≦5.0%of rockphosphate could favour the transformation of exchangeable Zn to reducible Znfraction. Only addition of5.0%of boron mud could enhance the transformation ofexchangeable Zn to reducible Zn fraction. The rock phosphate or boron additionincreased the pH in the compost. Complexation of the metal ions on to inorganiccomponents and adsorption were also responsible for the speciation of Cu and Znduring composting of pig manure amended with rock phosphate or boron mud. Rockphosphate and boron mud were effective in immobilization heavy metals duringcomposting.5) The carbon extracted by H2O, KCl, Na4P2O7, NaOH and HNO3was accountedfor near20%of total carbon while the Cu and Zn extracted by these reagents were accounted for above95%of total content. An amount of32%-38%of Cu and81%-87%of Zn was in Na4P2O7extraction. The Cu complexed with the humic substanceextracted by Na4P2O7was not stable, during composting the organic complexed Cudecreased. The Cu bound with the humic substance extracted by NaOH was stableand the addition of roch phosphate or boron mud could increase the organic bound Cu.The organic complexed or organic bound Zn changed a little during composting. Mostof Cu and Zn were in fulvic acid (FA) fraction in humic substance. Duringcomposting, the distribution of Cu in humic acid (HA) increased while the HA-Znchanged little. The increase in HA content,HA/FA and pH was responsible for theincrease in HA-Cu. Rock phosphate and boron addition could favour the formation ofHA-Cu and thus enhance the Cu stability in the compost.6) Co-composting pig manure and rice straw with an initial C/N ratio near22,and addition an amount of5.0%phosphorus or boron mud could reduce thebioavailability of Cu and mobility of Zn. The use of industrial waste (boron mud) asamendment was a good way for the recycling of boron mud.