Table 4.

Overview of emerging waste by-products as amendments.

Waste by-product category and amendment		Source	Availability and general costs	Metal uptake	Benefits of using	Limitations and unintended consequences	Targeted metal(s)
Organic	Digestates (from anaerobic digester)	Anaerobic digestion of organic wastes (e.g., biosolids)	• Availability dependent on the location of anaerobic digesters	Presumed to be similar to biosolids due to the high organic matter content–sorption, complexation of the metal fractions associated with organic matter, metal oxides, or carbonates53	• High nutrient content	• Relatively new material and not well characterized; may contain metals	No studies were identified through the literature review that specifically investigated immobilization of metals in soil or sediment
			• Relatively new product, and costs are undetermined		• Source of OM	• Not regulated
						• Not routinely treated for pathogen reduction, although digestion temperatures are adequate to kill off pathogens
						• Potential odor issues
						• Possible leaching of phosphorous (P)
Organic	Biochar (also charcoal and activated C)	Pyrolysis or gasification of biomass (e.g., wood, straw, and tree bark)	Approximately 20+ biochar manufacturers across the United States162	Adsorption due to large surface areas	• High surface area and cation exchange capacity (CEC); most effective for organic compounds	• Low-temperature pyrolyzed biochars may degrade quickly and remobilize any sorbed metals	As, cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn)
					• C-rich	• Application of biochar increases OM and may either increase or decrease dissolved organic carbon (DOC); if it increases DOC, metals may mobilize (e.g., arsenic [As])
					• Attracts microbes and beneficial fungi
					• Has a long life in soils compared to fertilizers
Organic	Rice residues biochar (e.g., hulls/husks, straw, bran)	By-product from rice processing	• Major rice-producing states are Arkansas, California, Louisiana, Mississippi, Missouri, and Texas163	• Adsorption	• Husks have low solubility in water, good chemical stability, and structural strength as a result of a high silica content	See Biochar	Pb, Cd, Zn, Ni, and As
			• More than 19 billion pounds of rice are produced, but the amount of wastes generated were not determined	• Complexes and chelates are formed	• See Biochar
	Red gypsum (or titano-gypsum)	Titanium dioxide (TiO2) production (pigment industry)	Of 142,000 metric tons (MT) of raw material processed, 70,000 MT of red gypsum produced	Adsorption	• Rich in iron (Fe) oxides	Some variability exists due to raw material properties	As, Cu, Pb, Cd, Zn, and Ni
					• Useful as a soil amendment
Organic and liming/alkaline	Alperujo	Olive oil extraction processes	• Olive oil is produced in California, Arizona, Texas, Georgia, Florida, Oregon, and Hawaii164	Free ions can complex and co-mobilize with organic ligands during leaching events due to the addition of OM	• Addition of organic matter and potassium (K)	• Not widely available in the United States	As, Cd, Cu, Pb, Zn, and Mn
			• Annual amount of waste generated was not determined		• Provides nutrients (C, nitrogen [N], and P)	• May mobilize As in multicontaminant sites as a result of increases in soil pH
					• Lower levels of metals than biosolids or municipal solid waste (MSW) compost
					• Slow release of nutrients as evidenced by low mineralization rate165
Organic	Spent mushroom substrate (compost)	Mushroom crops	Approximately an equal ratio of spent mushroom substrate is generated per weight of mushrooms ready for consumption	Adsorption	• Addition of nutrients and OM	• May require additional processing after composting to obtain the desired and most effective particle size	Cd, Pb, Cr, and As
					• High pH buffering capacity due to the addition of lime during composting of the substrate	• Competing uses (e.g., crop production, reuse in the cultivation of mushrooms, animal and fish feed, pest management)
					• Generates relatively high volumes of waste
Organic	Silkworm excrement	Silkworm culture	• China leads the world production; approximately seven companies in the United States operate silkworm farms166	Sorption	Addition of nutrients and organic matter	Unknown at this time	Cd and Pb
			• Information was not obtained regarding annual quantities of excrement generated or cost
Organic	Vermicompost	Vermicomposting of various organic wastes such as vegetable or food waste using worms	• Large quantities available near large-scale vermicom-posting facilities	Adsorption by negatively charged functional groups	• Produces nutrient-rich humus	Potential metal contamination depending on feedstock (e.g., pig manure, sewage sludge, fly ash, and cow dung)	Cd, Pb, Zn, and Cu
			• Relatively low cost		• Addition of nutrients and organic matter
Organic	Palm oil waste by-products (e.g., palm kernel pie, boiler ash, empty fruit bunches)	Palm oil manufacturing	Palm oil by-products are not available in the United States; would need to be sourced internationally	Adsorption	Fertilizer	Unknown at this time	Cd and Zn
Organic and liming/alkaline	Sugarcane filter cake	Residue from sugarcane juice filtration	• Sugarcane is produced in Florida, Louisiana, Hawaii, and Texas, with Florida producing the most167	Adsorption	• Addition of P and organic matter	Unknown at this time	Cd and Zn
			• By-products are generally free		• Use of by-products would offset effects from field burning (where applicable)
Organic and liming/alkaline	Bagasse	By-product after crushing sugarcane or sorghum stalks to extract their juice	• See Sugarcane filter cake	Adsorption	Use of by-products would offset effects from field burning (where applicable)	• Potential for fugitive dust	Cd and Zn
			• Used as a fuel source for sugar mills, so its availability for other uses is limited			• Unpredictability in annual generation due to weather and economics
Organic and liming/alkaline	Sugar foam (SF)	By-product from sugar manufacturing	See sugarcane filter cake	• Adsorption	• Addition of CaCO3	• Potential for fugitive dust	As, Cd, thallium (Tl), Zn, Mn, Cu, aluminum (Al), and Fe
				• Formation of Al-hydroxy polymers	• Can increase pH	• Unpredictability in annual generation due to weather and economics
Organic and liming/alkaline	Sugar beet lime	Spent lime from the purification of sugar from sugar beets or sugarcane (lime is added to neutralize organic acids present in the plant material)	• Grown in 5 regions encompassing 11 states, primarily in the western United States167	• Adsorption	• Increases pH	• Potential for fugitive dust	Cu, Zn, Pb, and Cd
			• By-products are generally free	• Chelation, complexation with carboxyl groups	• High calcium (Ca), magnesium (Mg), and K	• Relatively high water content, which affects transportation costs
					• Can contain organic matter	• Unpredictability in annual generation due to weather and economics
					• Fine particle size
Liming/alkaline	Wood ash	Wood-fired utilities	• Available in small to moderate amounts from wood-fired utilities	Sorption	• Increases pH	• Highly variable content	No studies were identified through the literature review that specifically investigated the immobilization of metals in soil or sediment
			• Materials are generally free		• Source of Ca, Mg, and K	• May contain contaminants if other fuels (e.g., tires or waste oil) are co-combusted
						• May have dioxin and should be confirmed through testing
						• Lime equivalent will vary by burn temperature and age of material
Liming/alkaline	Seashell grit	Seafood processing	Available near coastlines with fishing or seafood processing operations; low costs	Adsorption	• pH increase and addition of Ca	None identified	As, Cu, and Zn
					• Used in horticulture and poultry industry
						Odor?
Liming/alkaline	Chitin, chitosan (as found in the exoskeletons of shellfish and crustaceans, as well as lobster, mushrooms, and bacteria)	Seafood processing	• Seasonally available near coastlines with fishing or seafood processing (canning) industries	Adsorption	• Source of Ca from CaCO3	None identified	Al, As, Cd, Cr, Cu, Fe, mercury (Hg), manganese (Mn), Ni, and Zn
			• Presumed low costs		• Slow release of N, P, and Mg
					• May contain some K and chitin
					• Second most abundant natural biopolymer (after cellulose) and can be assumed to be of plentiful supply
						Odor
Mineral/inorganic	Hydroxyapatite (phosphate in the form of apatite from fish bones)	Seafood processing	Available near coastlines with fishing or seafood processing operations; presumed low costs	Precipitation, solid solution	• Addition of P to the soil	None identified	Pb in soil, numerous other metals in aqueous solution
					• Used as a fertilizer in land application and horticulture
						Odor
Mineral/inorganic	Bone char, bonemeal, bonemeal biochar (finely ground, poorly crystalline apatite, Ca10(PO4)6OH2)	Meat processing (e.g., cow bones)	Readily available near animal farms	• Adsorption	Free of metal and other types of contaminants	See Biochar	Cd, Pb, and Zn168
				• Formation of metal phosphates