Competitive Sorption Behavior of Arsenic, Selenium, Copper and Lead by Soil and Biosolid Nano- and Macro-Colloid Particles

Limited information exists on natural nanocolloid sorption behavior of As, Se, Cu and Pb in the environment. They are expected to have variable competitive sorption characteristics depending on size and composition and may transport elevated contaminant loads into surface and ground waters. A comprehensive characterization of their interactions with contaminants could provide a better understanding of the risks they pose to the environment. This study evaluated the sorption behavior of soil and biosolid nano- and macro-colloids with different mineralogical compositions for As, Se, Cu, and Pb contaminants. Single- and multi-contaminant Freundlich isotherms were us- ed to evaluate sorption affinity for the contaminants among the different colloid sizes and compositions. Sorption trends based on size indicated greater affinity for As and Cu by the smectitic and kaolinitic nanocolloids, greater affinity for Pb by the kaolinitic nanocolloids, and greater affinity for As, Se and Pb by bio-nanocolloids over corresponding macrocolloid fractions. Both, single- and multi-contaminant isotherms indicated sorption preferences for cation over anion contaminants, but with somewhat contrasting sequences depending on size and composition. Multi-contaminant isotherms generally predicted greater sorption affinities likely due to bridging effects, particularly for anionic contaminants. Surface properties such as zeta potentials, cation exchange capacity (CEC), surface area (SA), organic carbon (OC), and OC:SA significantly but variably affected sorption characteristics among the differing colloid sizes and compositions. Colloid zeta potential and pH shifts in the presence of different contaminant loads suggested prevalence of inner sphere bonding mechanisms for sorption of cation contaminants by mineral colloids and outer sphere sorption for cation and anion contaminants by bio-colloids.

Stability of Soil and Biosolid Nanocolloid and Macrocolloid Particles in the Absence and Presence of Arsenic, Selenium, Copper and Lead

Due to their enhanced stability and contaminant transport potential, environmental nanoparticles derived from soil and biosolid materials may pose a considerable risk to groundwater quality. Very little information exists on the stability and transportability of environmental or natural nanocolloids in the presence of As, Se, Pb and Cu contaminants, all of which are considered to represent substantial threats to human and animal populations through groundwater contamination. This study involved stability settling experiments of nanocolloids (NCs) (<100 nm) and macrocolloids (MCs) (100 - 2000 nm) fractionated from Bt horizons of three Kentucky soils and one biosolid waste material in water suspensions of 0, 2, and 10 mg·L-1 of As, Se, Pb and Cu. The results indicated greater stability in the mineral than the biosolid colloid fractions, and enhanced stability of NCs over corresponding MCs in the presence or absence of contaminants at low contaminant loads. At high contaminant loads nearly all colloids were unstable except for the bio-nanocolloids which still sustained considerable stability. At low contaminant loads, the MC fraction stability sequence was smectitic > mixed > kaolinitic > biosolid. Among the nano-fractions, the smectitic and kaolinitic colloids demonstrated lower stability than the MCs, but higher than those of the mixed and biosolid fractions. Physicochemical characterizations indicated that extensive organic carbon surface coatings and higher Al/Fe:Si ratios may have induced higher stability in the NC fractions, but their overall stability may also have been hindered in some cases by nano-aggregation phenomena.