Passivating effect of biochar and humic acid materials on heavy metals during composting of pig manure

With the rapid development of animal husbandry in China, proportion of large-scale farms is increasing. At the same time, animal manure has largely produced. The soil heavy metal pollution caused by use of manure as organic fertilizer, has seriously affected the quality of the soil and agricultural products. Some of the passivation materials can play a role to immobilize heavy metals. But, issues on use of the passivation material exist, those such as relatively high cost, difficult to obtain, low passivation rate. To better use animal manure as a resource and reduce its environmental impact, and to explore different passivation materials on the influence of passivation effect of heavy metals during manure composting process, a series of composting tests were conducted to study the effect of passivating agents including biochar (wood biochar, corn stalks charcoal, peanut shell biochar) and humic acid (FJ biological humic acid, JBW biological humic acid, peat) on the status of heavy metals through composting. Seven manure mixtures (including a blank) were composted over a 30-day period in an aerobic composting tank using the same source of pig manure but with different bulking agents. Parameters monitored over this period included temperature, pH, electric conductivity (EC) and germination index (GI) to evaluation the compost maturity. The content of heavy metals in various forms before and after the composting was also determined. The results showed that peanut shell biochar (F3), corn stalks charcoal (F2), JBW biological humic acid (F5) and wood biochar (F1) were the best passivation for Cu, Pb, Zn and Cd, respectively when exchangeable heavy metals were used as an indicator. The passivation effect on exchangeable heavy metals (Cu, Pb, Zn and Cd) as compared with the control was reduced 65.79%, 57.2%, 64.94%, 94.67%, respectively. The effect of these four passivation treatments on exchangeable heavy metals was significantly better than the control treatment (p<0.05). On the other hand, the treatment of peanut shell biochar as a passivation material (F3) did not reach the standard of compost maturity. The value of highest temperature in the compost file of peanut shell biochar treatment (F3) was 45.14°C, and at the end of the composting, the value of pH was 5.41, EC was 9.48 mS/cm, GI was 0.47%. The low germination index indicated that immature compost may contain phytotoxic materials preventing seed from germination. In this experiment, JBW biological humic acid treatment (F5) not only showed better passivation effect on Zn, but also exhibited good passivation effect on Cu, Pb, Cd (47.78%, 47.54%, 87.36%, respectively). In all, taking into account of the effects of compost maturity, compost quality, and different heavy metals, wood biochar, corn stalks charcoal, and JBW biological humic acid can be used as heavy metal passivation material for pig manure composting. The use of these passivation materials created conditions for use of the manure as a resource. This will help to reduce the environmental risks of heavy metal pollution when using the manure as a source of organic fertilizer.