A Study On The Occurrence Modes Of Trace Elements In Agricultural Crop Residues And Their Enrichments In Bio-availability In Bio-chars

Trace elements in agricultural crop residues and their bio-chars have seldom been studied with respect to their modes of occurrence,volatilities during pyrolysis and bioavailability.The objective of this work is to address these issues by determining the concentrations of major and trace elements in five crop residues(peanut straw,cotton straw,corn stalk,sorghum stalk and reed)and in the bio-chars,and by leaching of the biomasses and bio-chars using different approaches.Many trace elements in the five biomasses from the same village were found to be correlated to specific major elements in the concentrations.The results show that Li,V,Co,Ni,Ga,As,Se,Sr,Ba and Ca show a high correlation,among which V,Co,Ni,Ga and Fe are also significantly related;Be,Cr and Al,Cu,Zn,Cd and K,as well as Mn and Mg also have strong correlations.By analysis of the concentration of trace elements in different fiber components in three crop residues(PN,CT,CN),it was found that neutral extractives(NE)were a dominant host of major elements K,Ca and Mg and trace elements Mn,Co,Cu,Zn,As,Se,Sr,Cd,Ba and Pb.Fe,Li,Be,V,Cr,Ni and Ga also exhibited the main association with NE,but with a relatively distinct association with hemicellulose and lignin.Cellulose enclosed lesser trace elements.Three crop residues(PN,CT,CN)were pyrolyzed with a heating rate of 10?/min at four final temperatures(250?,350?.450?,and 550?).The volatilities of trace elements during the slow pyrolysis were increased with the increase of temperature.Only Li,Be,Ga,As,Se,and Cd showed higher release rate at 250?,especially in CN.The remaining trace elements hardly volatilized at 250?.Ni,Cu,Sr and Ba in peanut straw did begin to volatilize until 450?.At the same time,the volatility of some trace elements(Be,V,Co,Cu,Zn,Ga,As,Sr,Pb)and their water-soluble forms showed a significant positive correlation in the three agricultural crop residues.During the slow pyrolysis of the three biomasses at 550?,K shows the highest enrichment factors(2.9-3.3)for three crop residues.Meanwhile most trace elements except for highly volatile Se and Cd showed the enrichment factors ranging in 1.6-2.9,resulting in their higher concentrations in the bio-chars.The bio-chars derived from PN and CT were enriched with soluble K,while Mg,Ca,and Fe are almost insoluble in water.Many trace elements in the bio-chars had lower solubility in water but showed increases in the dissolution into the ion-exchangeable or chelating solution,differing from the observation that most of the trace elements(except Li and Zn)in the parent biomasses were water-soluble.Trace elements in the crop residues and the bio-chars showed different dissolutions in citric acid,depending on crop residues.Most of the trace elements in PN and CN showed higher dissolution than those in their bio-chars,while those in the bio-char derived from CT had higher dissolution.Overall,the bio-chars originated from crop residues could provide many bioavailable elemental nutrients as the essential elements for plants if used as an amendment of soils.