Organic amendments temporarily change arsenic speciation and bioaccessibility in a lead and arsenic co-contaminated urban soil

Organic amendments often reduce the bioaccessibility of soil lead (Pb) but not that of soil arsenic (As). The effect of Pb on As bioaccessibility is rarely studied in co-contaminated soils. In a field study, we assessed the effect of mushroom compost, leaf compost, noncomposted biosolids, and composted biosolids amendments on As speciation in a co-contaminated (As and Pb) soil at 7, 349, and 642 days after amending soils and the change of As speciation during an in vitro bioaccessibility extraction (gastric solution, pH 2.5) using bulk X-ray absorption near-edge structure spectroscopy. Soil was contaminated by coal combustion and other diffuse sources and had low As bioaccessibility (7%-12%). Unamended soil had As(III) sorbed onto pyrite (As(III)-pyrite; similar to 60%) and As(V) adsorbed onto Fe oxy(hydr)oxides (As(V)-Fh; similar to 40%). In amended soils, except in composted biosolids-amended soils, at 7 days, As(V)-Fh decreased to 15%-26% and redistributed into As(III)-Fh and/or As(III)-pyrite. This transformation was most pronounced in mushroom compost amended soil resulting in a significant (46%) increase of As bioaccessibility compared to the unamended soil. Composted biosolids-amended soils had relatively stable As(V)-Fh. Lead arsenate formed during the in vitro extraction in amended soils, except in composted biosolids-amended soils. Arsenic speciation and bioaccessibility were similar in 349- and 642-day in all the amended and unamended soils. Reduction of As(V)-Fh to As(III) forms in the short term in three of the amended soils showed the potential to increase As bioaccessibility. The formation of stable lead arsenate during the in vitro extraction would counteract the short-term increase of As bioaccessibility in those amended soils. Sources of soil arsenic (As) and lead (Pb) were coal burning and other diffuse sources. Soil As speciation: 60% As(III) sorbed onto pyrite and 40% As(V) adsorbed onto Fe oxy(hydr)oxides. Three amendments temporarily reduced sorbed As(V) into As(III) forms. Lead arsenate formed during the in vitro bioaccessibility extraction. Lead may counteract temporarily increased As bioaccessibility in amended soils. Lead and arsenic in soils can be transferred to humans by ingesting soil accidentally, especially when children play outdoors. Organic amendments such as compost change the chemical forms of arsenic in the field and affect arsenic release from ingested soils within the digestive system. We assessed four organic amendments on their effects on chemical form of soil arsenic in the field and in a simulated gastric system. Within 7 days, three of the amendments temporarily changed the chemical form of arsenic in the field to more bioavailable forms. We could not tell the exact time duration that the temporary change of As form persisted in soils, but we have evidence that it was less than 1 year. Arsenic and lead in the amended soils reacted and formed an insoluble precipitate, called lead arsenate. This transformation would reduce the negative effect of both soil arsenic and lead on humans. The use of organic amendments is beneficial for urban agriculture.