## 1 Introduction  
The sediment organic matter (SOM) includes particulate (POM) and dissolved (DOM) organic matter [1, 2, 3, 4, 5]. The distribution of heavy metals in DOM/POM affects heavy metals toxicity to microorganisms and the safety of ecology that has received extensive attention worldwide [6, 7, 8, 9]. The binding affinity between heavy metals and organic matter is affected by the quality and quantity of organic matter, and the type and total concentration of metals [2, 6].  
Sediment POM and DOM is complex organic mixture; the constituents include monosaccharide, protein, humic acid, and humin [2; 3]. The chemical composition and structure of SOM are affected by its sources and environmental processes such as the exchange of POM and DOM as well as biogeochemical processes in an anoxic condition [2; 3; 10]. Biochemical processes, such as hydrolysis and bio-oxidation, affect the exchange and chemical composition and structure of DOM and POM [2; 5]. Moreover, the exchange of DOM and POM alter heavy metals distribution and affinity to DOM/POM [5; 11].  
Heavy metals bound to DOM/POM may decrease their reactivity and resistance in the environment [6; 9; 12; 13; 14]. The metal bound with various molecular weight OM can be used to understand the exchange process of DOM and POM. The [Me]/[DOC] ratios, \(\mu\)mol/g-C, have been used to understand the distribution, bioavailability, and transport potential of metals binding with DOM/POM. The high ratio suggests heavy metals strongly associate with organic matter [6; 12; 15; 16; 17].  
In aquatic environments, [Ni]/[DOC] ratios showed significant variation with the values, which ranged from 1.6 to 344.1 \(\mu\)mol/g-C [12; 13; 18; 19; 20; 21; 22; 23]. The metal and organic matter binding affinity has been affected by many factors such as wastewater treatment processes [18], DOM chemical composition and structure [6; 15; 16; 17; 22], DOM molecular weight [18; 20; 21] and an anthropogenic impact [12; 17]. The OM chemical properties are particularly important factors that influence the metal-organic matter binding affinity [12; 15; 16; 17; 22].  
A widely used analysis methods in previous and our research, the optical method, is a fast, sensitive, non-destructive method that examines OM chemical composition and structures [24; 25; 26; 27; 28; 29]. Wastewater discharged from the food industry contains high levels and various compositions of organic matter [30; 31]. In one study, the deposited POM underwent a biochemical process and the exchange between POM and DOM in an anoxic environment altered the distribution and binding behavior of organic matter and heavy metals [2; 5]. In another studies, the wider range molecular weight of DOM and POM had different chemical compositions and structure as well as different metal binding affinities [6; 21; 22]. Therefore, the [Ni]/[DOC] ratios of size-fractionated DOM and POM are important to understand the Ni binding affinity affected by the chemical properties of DOM/POM as well as the metal distribution in size-fractionated DOM/POM [18; 20; 21; 22].  
This study investigated the Ni binding affinity with size-fractionated DOM and POM in the downstream sediment of a food industrial park. The detailed experimental procedures included: (1) Sediment was taken downstream from a food industrial park and separated into a liquid DOM (\(<\)0.45 \(\mu\)m) and particulate POM. (2) The POM was extracted with a NaOH solution, which filtrate (\(<\)0.45 \(\mu\)m) was designed as AEOM. (3) DOM and AEOM were sequentially separated into five different molecular weight solutions, which measured the DOC and Ni concentrations as well as UV-Vis and fluorescent spectra. (4) The Ni and organic carbon mass fractions and [Ni]/[DOC] ratios in fractioned DOM and AEOM were investigated, and the selected optical indicators were used to examine correlation with [Ni]/[DOC] ratios to find the dominate factors of Ni binding affinity to DOM/AEOM.